diff --git a/include/swift/AST/DiagnosticsSIL.def b/include/swift/AST/DiagnosticsSIL.def
index 4e5e1633db597..943e67e29be8f 100644
--- a/include/swift/AST/DiagnosticsSIL.def
+++ b/include/swift/AST/DiagnosticsSIL.def
@@ -315,12 +315,12 @@ ERROR(missing_never_call_closure,none,
       (Type))
 
 ERROR(missing_return_decl,none,
-      "missing return in %1 expected to return %0",
-      (Type, DescriptiveDeclKind))
+      "missing return in %kindonly1 expected to return %0",
+      (Type, const AbstractFunctionDecl *))
 ERROR(missing_never_call_decl,none,
-      "%1 with uninhabited return type %0 is missing "
+      "%kindonly1 with uninhabited return type %0 is missing "
       "call to another never-returning function on all paths",
-      (Type, DescriptiveDeclKind))
+      (Type, const AbstractFunctionDecl *))
 
 NOTE(missing_return_last_expr_note,none,
     "did you mean to return the last expression?", ())
@@ -701,8 +701,9 @@ WARNING(warning_int_to_fp_inexact, none,
 
 // Flow-isolation diagnostics
 ERROR(isolated_after_nonisolated, none,
-      "cannot access %1 %2 here in %select{nonisolated initializer|deinitializer}0",
-      (bool, DescriptiveDeclKind, DeclName))
+      "cannot access %kind1 here in "
+      "%select{nonisolated initializer|deinitializer}0",
+      (bool, const ValueDecl *))
 NOTE(nonisolated_blame, none, "after %1%2 %3, "
      "only nonisolated properties of 'self' can be accessed from "
      "%select{this init|a deinit}0", (bool, StringRef, StringRef, DeclName))
@@ -982,23 +983,23 @@ GROUPED_ERROR(regionbasedisolation_type_send_yields_race, SendingRisksDataRace,
       (Type))
 NOTE(regionbasedisolation_type_use_after_send, none,
       "sending value of non-Sendable type %0 to %1 callee risks causing data races between %1 and local %2 uses",
-      (Type, ActorIsolation, ActorIsolation))
+      (Type, StringRef, StringRef))
 NOTE(regionbasedisolation_type_use_after_send_callee, none,
-     "sending value of non-Sendable type %0 to %1 %2 %3 risks causing data "
-     "races between %1 and local %4 uses",
-     (Type, ActorIsolation, DescriptiveDeclKind, DeclName, ActorIsolation))
+     "sending value of non-Sendable type %0 to %1 %kind2 risks causing data "
+     "races between %1 and local %3 uses",
+     (Type, StringRef, const ValueDecl *, StringRef))
 
 NOTE(regionbasedisolation_named_info_send_yields_race, none,
-     "sending %1%0 to %2 callee risks causing data races between %2 and local %3 uses",
-     (Identifier, StringRef, ActorIsolation, ActorIsolation))
+     "sending %select{%2 |}0%1 to %3 callee risks causing data races between %3 and local %4 uses",
+     (bool, Identifier, StringRef, StringRef, StringRef))
 NOTE(regionbasedisolation_named_info_send_yields_race_callee, none,
-     "sending %1%0 to %2 %3 %4 risks causing data races between %2 and local %5 uses",
-     (Identifier, StringRef, ActorIsolation, DescriptiveDeclKind, DeclName, ActorIsolation))
+     "sending %select{%2 |}0%1 to %3 %kind4 risks causing data races between %3 and local %5 uses",
+     (bool, Identifier, StringRef, StringRef, const ValueDecl *, StringRef))
 
 // Use after send closure.
 NOTE(regionbasedisolation_type_isolated_capture_yields_race, none,
       "sending value of non-Sendable type %0 to %1 closure due to closure capture risks causing races in between %1 and %2 uses",
-      (Type, ActorIsolation, ActorIsolation))
+      (Type, StringRef, StringRef))
 
 // Value captured in async let and reused.
 NOTE(regionbasedisolation_named_nonisolated_asynclet_name, none,
@@ -1009,33 +1010,33 @@ NOTE(regionbasedisolation_named_value_used_after_explicit_sending, none,
       "%0 used after being passed as a 'sending' parameter; Later uses could race",
       (Identifier))
 NOTE(regionbasedisolation_named_isolated_closure_yields_race, none,
-      "%0%1 is captured by a %2 closure. %2 uses in closure may race against later %3 uses",
-      (StringRef, Identifier, ActorIsolation, ActorIsolation))
+      "%select{%1 |}0%2 is captured by a %3 closure. %3 uses in closure may race against later %4 uses",
+      (bool, StringRef, Identifier, StringRef, StringRef))
 
 NOTE(regionbasedisolation_typed_use_after_sending, none,
       "Passing value of non-Sendable type %0 as a 'sending' argument risks causing races in between local and caller code",
       (Type))
 NOTE(regionbasedisolation_typed_use_after_sending_callee, none,
-      "Passing value of non-Sendable type %0 as a 'sending' argument to %1 %2 risks causing races in between local and caller code",
-      (Type, DescriptiveDeclKind, DeclName))
+      "Passing value of non-Sendable type %0 as a 'sending' argument to %kind1 risks causing races in between local and caller code",
+      (Type, const ValueDecl *))
 
 //===
 // Sending Never Sendable Emitter
 
 NOTE(regionbasedisolation_named_send_never_sendable, none,
-      "sending %1%0 to %2 callee risks causing data races between %2 and %3 uses",
-      (Identifier, StringRef, ActorIsolation, StringRef))
+      "sending %select{%2 |}0%1 to %3 callee risks causing data races between %3 and %4 uses",
+      (bool, Identifier, StringRef, StringRef, StringRef))
 NOTE(regionbasedisolation_named_send_never_sendable_callee, none,
-      "sending %1%0 to %2 %3 %4 risks causing data races between %2 and %5 uses",
-      (Identifier, StringRef, ActorIsolation, DescriptiveDeclKind, DeclName, StringRef))
+      "sending %select{%2 |}0%1 to %3 %kind4 risks causing data races between %3 and %5 uses",
+      (bool, Identifier, StringRef, StringRef, const ValueDecl *, StringRef))
 
 NOTE(regionbasedisolation_named_send_into_sending_param, none,
-     "%0%1 is passed as a 'sending' parameter; Uses in callee may race with "
-     "later %0uses",
-     (StringRef, Identifier))
+     "%select{%1 |}0%2 is passed as a 'sending' parameter; Uses in callee may race with "
+     "later %1 uses",
+     (bool, StringRef, Identifier))
 NOTE(regionbasedisolation_named_nosend_send_into_result, none,
-     "%0%1 cannot be a 'sending' result. %2 uses may race with caller uses",
-     (StringRef, Identifier, StringRef))
+     "%select{%1 |}0%2 cannot be a 'sending' result. %3 uses may race with caller uses",
+     (bool, StringRef, Identifier, StringRef))
 NOTE(regionbasedisolation_typed_tns_passed_to_sending, none,
      "Passing %0 value of non-Sendable type %1 as a 'sending' parameter risks "
      "causing races inbetween %0 uses and uses reachable from the callee",
@@ -1051,8 +1052,8 @@ NOTE(regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_value_
      "closure captures %0 which is accessible to code in the current task",
      (DeclName))
 NOTE(regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_boxed_value_task_isolated, none,
-     "closure captures reference to mutable %1 %0 which is accessible to code in the current task",
-     (DeclName, DescriptiveDeclKind))
+     "closure captures reference to mutable %kind0 which is accessible to code in the current task",
+     (const ValueDecl *))
 NOTE(regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_value_region, none,
      "closure captures %1 which is accessible to %0 code",
      (StringRef, DeclName))
@@ -1068,18 +1069,18 @@ NOTE(regionbasedisolation_closure_captures_actor, none,
      (DeclName, StringRef))
 
 NOTE(regionbasedisolation_typed_tns_passed_to_sending_callee, none,
-     "Passing %0 value of non-Sendable type %1 as a 'sending' parameter to %2 %3 risks causing races inbetween %0 uses and uses reachable from %3",
-     (StringRef, Type, DescriptiveDeclKind, DeclName))
+     "Passing %0 value of non-Sendable type %1 as a 'sending' parameter to %kind2 risks causing races inbetween %0 uses and uses reachable from %2",
+     (StringRef, Type, const ValueDecl *))
 
 NOTE(regionbasedisolation_named_send_nt_asynclet_capture, none,
       "sending %1 %0 into async let risks causing data races between nonisolated and %1 uses",
       (Identifier, StringRef))
 NOTE(regionbasedisolation_typed_sendneversendable_via_arg, none,
       "sending %0 value of non-Sendable type %1 to %2 callee risks causing races in between %0 and %2 uses",
-      (StringRef, Type, ActorIsolation))
+      (StringRef, Type, StringRef))
 NOTE(regionbasedisolation_typed_sendneversendable_via_arg_callee, none,
-      "sending %0 value of non-Sendable type %1 to %2 %3 %4 risks causing races in between %0 and %2 uses",
-      (StringRef, Type, ActorIsolation, DescriptiveDeclKind, DeclName))
+      "sending %0 value of non-Sendable type %1 to %2 %kind3 risks causing races in between %0 and %2 uses",
+      (StringRef, Type, StringRef, const ValueDecl *))
 
 NOTE(regionbasedisolation_isolated_conformance_introduced, none,
      "isolated conformance to %kind0 can be introduced here",
@@ -1097,10 +1098,10 @@ NOTE(regionbasedisolation_inout_sending_must_be_reinitialized, none,
      "'inout sending' parameter must be reinitialized before function exit with a non-actor isolated value",
      ())
 ERROR(regionbasedisolation_inout_sending_cannot_be_actor_isolated, none,
-      "'inout sending' parameter %0 cannot be %1at end of function",
+      "'inout sending' parameter %0 cannot be %1 at end of function",
       (Identifier, StringRef))
 NOTE(regionbasedisolation_inout_sending_cannot_be_actor_isolated_note, none,
-      "%1%0 risks causing races in between %1uses and caller uses since caller assumes value is not actor isolated",
+      "%1 %0 risks causing races in between %1 uses and caller uses since caller assumes value is not actor isolated",
       (Identifier, StringRef))
 
 //===
@@ -1125,10 +1126,10 @@ NOTE(regionbasedisolation_out_sending_cannot_be_actor_isolated_note_named, none,
 // Example: returning main-actor isolated result to a custom-actor isolated context risks causing data races
 ERROR(rbi_isolation_crossing_result, none,
       "non-Sendable %0-typed result can not be returned from %1 %kind2 to %3 context",
-      (Type, ActorIsolation, const ValueDecl *, ActorIsolation))
+      (Type, StringRef, const ValueDecl *, StringRef))
 ERROR(rbi_isolation_crossing_result_no_decl, none,
       "non-Sendable %0-typed result can not be returned from %1 function to %2 context",
-      (Type, ActorIsolation, ActorIsolation))
+      (Type, StringRef, StringRef))
 NOTE(rbi_non_sendable_nominal,none,
      "%kind0 does not conform to the 'Sendable' protocol",
       (const ValueDecl *))
diff --git a/include/swift/AST/Identifier.h b/include/swift/AST/Identifier.h
index 4724b5b202e3c..ee6a4c0a8b8d8 100644
--- a/include/swift/AST/Identifier.h
+++ b/include/swift/AST/Identifier.h
@@ -230,7 +230,7 @@ class Identifier {
 private:
   bool isOperatorSlow() const;
 };
-  
+
 class DeclName;
 class DeclNameRef;
 class ObjCSelector;
diff --git a/include/swift/SILOptimizer/Analysis/RegionAnalysis.h b/include/swift/SILOptimizer/Analysis/RegionAnalysis.h
index 7b81e1a3356df..eefdf5f47bbf9 100644
--- a/include/swift/SILOptimizer/Analysis/RegionAnalysis.h
+++ b/include/swift/SILOptimizer/Analysis/RegionAnalysis.h
@@ -179,16 +179,16 @@ class regionanalysisimpl::TrackableValueState {
 
   void removeFlag(TrackableValueFlag flag) { flagSet -= flag; }
 
-  void print(llvm::raw_ostream &os) const {
+  void print(SILFunction *fn, llvm::raw_ostream &os) const {
     os << "TrackableValueState[id: " << id
        << "][is_no_alias: " << (isNoAlias() ? "yes" : "no")
        << "][is_sendable: " << (isSendable() ? "yes" : "no")
        << "][region_value_kind: ";
-    getIsolationRegionInfo().printForOneLineLogging(os);
+    getIsolationRegionInfo().printForOneLineLogging(fn, os);
     os << "].";
   }
 
-  SWIFT_DEBUG_DUMP { print(llvm::dbgs()); }
+  SWIFT_DEBUG_DUMPER(dump(SILFunction *fn)) { print(fn, llvm::dbgs()); }
 
 private:
   bool hasIsolationRegionInfo() const { return bool(regionInfo); }
@@ -249,26 +249,26 @@ class regionanalysisimpl::TrackableValue {
   /// parameter.
   bool isSendingParameter() const;
 
-  void printIsolationInfo(SmallString<64> &outString) const {
+  void printIsolationInfo(SILFunction *fn, SmallString<64> &outString) const {
     llvm::raw_svector_ostream os(outString);
-    getIsolationRegionInfo().printForDiagnostics(os);
+    getIsolationRegionInfo().printForDiagnostics(fn, os);
   }
 
-  void print(llvm::raw_ostream &os) const {
+  void print(SILFunction *fn, llvm::raw_ostream &os) const {
     os << "TrackableValue. State: ";
-    valueState.print(os);
+    valueState.print(fn, os);
     os << "\n    Rep Value: ";
     getRepresentative().print(os);
   }
 
-  void printVerbose(llvm::raw_ostream &os) const {
+  void printVerbose(SILFunction *fn, llvm::raw_ostream &os) const {
     os << "TrackableValue. State: ";
-    valueState.print(os);
+    valueState.print(fn, os);
     os << "\n    Rep Value: " << getRepresentative();
   }
 
-  SWIFT_DEBUG_DUMP {
-    print(llvm::dbgs());
+  SWIFT_DEBUG_DUMPER(dump(SILFunction *fn)) {
+    print(fn, llvm::dbgs());
     llvm::dbgs() << '\n';
   }
 };
@@ -288,8 +288,8 @@ struct regionanalysisimpl::TrackableValueLookupResult {
   /// TrackableValue.
   std::optional<TrackableValue> base;
 
-  void print(llvm::raw_ostream &os) const;
-  SWIFT_DEBUG_DUMP { print(llvm::dbgs()); }
+  void print(SILFunction *fn, llvm::raw_ostream &os) const;
+  SWIFT_DEBUG_DUMPER(dumper(SILFunction *fn)) { print(fn, llvm::dbgs()); }
 };
 
 class RegionAnalysis;
@@ -390,6 +390,8 @@ class RegionAnalysisValueMap {
     return getUnderlyingTrackedValue(value).value;
   }
 
+  SILFunction *getFunction() const { return fn; }
+
   /// Returns the value for this instruction if it isn't a fake "represenative
   /// value" to inject actor isolatedness. Asserts in such a case.
   SILValue getRepresentative(Element trackableValueID) const;
diff --git a/include/swift/SILOptimizer/Utils/PartitionUtils.h b/include/swift/SILOptimizer/Utils/PartitionUtils.h
index d0f99cd2d5cc6..dfddfb537a4e3 100644
--- a/include/swift/SILOptimizer/Utils/PartitionUtils.h
+++ b/include/swift/SILOptimizer/Utils/PartitionUtils.h
@@ -1598,9 +1598,32 @@ struct PartitionOpEvaluator {
   }
 
 private:
-  bool isConvertFunctionFromSendableType(SILValue equivalenceClassRep) const {
+  /// To work around not having isolation in interface types, the type checker
+  /// inserts casts and other AST nodes that are used to enrich the AST with
+  /// isolation information. This results in Sendable functions being
+  /// wrapped/converted/etc in ways that hide the Sendability. This helper looks
+  /// through these conversions/wrappers/thunks to see if the original
+  /// underlying function is Sendable.
+  ///
+  /// The two ways this can happen is that we either get an actual function_ref
+  /// that is Sendable or we get a convert function with a Sendable operand.
+  bool isHiddenSendableFunctionType(SILValue equivalenceClassRep) const {
     SILValue valueToTest = equivalenceClassRep;
     while (true) {
+      if (auto *pai = dyn_cast<PartialApplyInst>(valueToTest)) {
+        if (auto *calleeFunction = pai->getCalleeFunction()) {
+          if (pai->getNumArguments() >= 1 &&
+              pai->getArgument(0)->getType().isFunction() &&
+              calleeFunction->isThunk()) {
+            valueToTest = pai->getArgument(0);
+            continue;
+          }
+
+          if (calleeFunction->getLoweredFunctionType()->isSendable())
+            return true;
+        }
+      }
+
       if (auto *i = dyn_cast<ThinToThickFunctionInst>(valueToTest)) {
         valueToTest = i->getOperand();
         continue;
@@ -1612,6 +1635,9 @@ struct PartitionOpEvaluator {
       break;
     }
 
+    if (auto *fn = dyn_cast<FunctionRefInst>(valueToTest))
+      return fn->getReferencedFunction()->getLoweredFunctionType()->isSendable();
+
     auto *cvi = dyn_cast<ConvertFunctionInst>(valueToTest);
     if (!cvi)
       return false;
@@ -1644,7 +1670,7 @@ struct PartitionOpEvaluator {
 
         // See if we have a convert function from a `@Sendable` type. In this
         // case, we want to squelch the error.
-        if (isConvertFunctionFromSendableType(equivalenceClassRep))
+        if (isHiddenSendableFunctionType(equivalenceClassRep))
           return;
       }
 
@@ -1689,7 +1715,7 @@ struct PartitionOpEvaluator {
 
         // See if we have a convert function from a `@Sendable` type. In this
         // case, we want to squelch the error.
-        if (isConvertFunctionFromSendableType(equivalenceClassRep))
+        if (isHiddenSendableFunctionType(equivalenceClassRep))
           return;
       }
     }
diff --git a/include/swift/SILOptimizer/Utils/SILIsolationInfo.h b/include/swift/SILOptimizer/Utils/SILIsolationInfo.h
index c6f15400efe26..a57a50c9a950a 100644
--- a/include/swift/SILOptimizer/Utils/SILIsolationInfo.h
+++ b/include/swift/SILOptimizer/Utils/SILIsolationInfo.h
@@ -185,6 +185,10 @@ class SILIsolationInfo {
     /// parameter and should be allowed to merge into a self parameter.
     UnappliedIsolatedAnyParameter = 0x2,
 
+    /// If set, this was a TaskIsolated value from a nonisolated(nonsending)
+    /// parameter.
+    NonisolatedNonsendingTaskIsolated = 0x4,
+
     /// The maximum number of bits used by a Flag.
     MaxNumBits = 3,
   };
@@ -317,6 +321,25 @@ class SILIsolationInfo {
     return result;
   }
 
+  bool isNonisolatedNonsendingTaskIsolated() const {
+    return getOptions().contains(Flag::NonisolatedNonsendingTaskIsolated);
+  }
+
+  SILIsolationInfo
+  withNonisolatedNonsendingTaskIsolated(bool newValue = true) const {
+    assert(*this && "Cannot be unknown");
+    assert(isTaskIsolated() && "Can only be task isolated");
+    auto self = *this;
+    if (newValue) {
+      self.options =
+          (self.getOptions() | Flag::NonisolatedNonsendingTaskIsolated).toRaw();
+    } else {
+      self.options = self.getOptions().toRaw() &
+                     ~Options(Flag::NonisolatedNonsendingTaskIsolated).toRaw();
+    }
+    return self;
+  }
+
   /// Returns true if this actor isolation is derived from an unapplied
   /// isolation parameter. When merging, we allow for this to be merged with a
   /// more specific isolation kind.
@@ -337,16 +360,16 @@ class SILIsolationInfo {
     return self;
   }
 
-  void print(llvm::raw_ostream &os) const;
+  void print(SILFunction *fn, llvm::raw_ostream &os) const;
 
   /// Print a textual representation of the text info that is meant to be
   /// included in other logging output for types that compose with
   /// SILIsolationInfo. As a result, we only print state that can fit on
   /// one line.
-  void printForOneLineLogging(llvm::raw_ostream &os) const;
+  void printForOneLineLogging(SILFunction *fn, llvm::raw_ostream &os) const;
 
-  SWIFT_DEBUG_DUMP {
-    print(llvm::dbgs());
+  SWIFT_DEBUG_DUMPER(dump(SILFunction *fn)) {
+    print(fn, llvm::dbgs());
     llvm::dbgs() << '\n';
   }
 
@@ -355,12 +378,20 @@ class SILIsolationInfo {
   ///
   /// We do this programatically since task-isolated code needs a very different
   /// form of diagnostic than other cases.
-  void printForCodeDiagnostic(llvm::raw_ostream &os) const;
+  void printForCodeDiagnostic(SILFunction *fn, llvm::raw_ostream &os) const;
+
+  /// Overload of printForCodeDiagnostics that returns an interned StringRef
+  /// owned by the AST.
+  StringRef printForCodeDiagnostic(SILFunction *fn) const;
 
-  void printForDiagnostics(llvm::raw_ostream &os) const;
+  void printForDiagnostics(SILFunction *fn, llvm::raw_ostream &os) const;
 
-  SWIFT_DEBUG_DUMPER(dumpForDiagnostics()) {
-    printForDiagnostics(llvm::dbgs());
+  /// Overload of printForDiagnostics that returns an interned StringRef owned
+  /// by the AST.
+  StringRef printForDiagnostics(SILFunction *fn) const;
+
+  SWIFT_DEBUG_DUMPER(dumpForDiagnostics(SILFunction *fn)) {
+    printForDiagnostics(fn, llvm::dbgs());
     llvm::dbgs() << '\n';
   }
 
@@ -549,6 +580,19 @@ class SILIsolationInfo {
   /// that the isolation and the isolated value match.
   bool isEqual(const SILIsolationInfo &other) const;
 
+  /// A helper function that prints ActorIsolation like we normally do except
+  /// that it prints nonisolated(nonsending) as nonisolated. This is needed in
+  /// certain cases when talking about use-after-free uses in send non sendable.
+  static void printActorIsolationForDiagnostics(
+      SILFunction *fn, ActorIsolation iso, llvm::raw_ostream &os,
+      StringRef openingQuotationMark = "'", bool asNoun = false);
+
+  /// Overload for printActorIsolationForDiagnostics that produces a StringRef.
+  static StringRef
+  printActorIsolationForDiagnostics(SILFunction *fn, ActorIsolation iso,
+                                    StringRef openingQuotationMark = "'",
+                                    bool asNoun = false);
+
   void Profile(llvm::FoldingSetNodeID &id) const;
 
 private:
@@ -612,43 +656,33 @@ class SILDynamicMergedIsolationInfo {
         SILIsolationInfo::getDisconnected(isUnsafeNonIsolated));
   }
 
-  SWIFT_DEBUG_DUMP { innerInfo.dump(); }
+  SWIFT_DEBUG_DUMPER(dump(SILFunction *fn)) { innerInfo.dump(fn); }
 
-  void printForDiagnostics(llvm::raw_ostream &os) const {
-    innerInfo.printForDiagnostics(os);
+  void printForDiagnostics(SILFunction *fn, llvm::raw_ostream &os) const {
+    innerInfo.printForDiagnostics(fn, os);
   }
 
-  SWIFT_DEBUG_DUMPER(dumpForDiagnostics()) {
-    innerInfo.dumpForDiagnostics();
+  StringRef printForDiagnostics(SILFunction *fn) const {
+    return innerInfo.printForDiagnostics(fn);
   }
 
-  void printForCodeDiagnostic(llvm::raw_ostream &os) const {
-    innerInfo.printForCodeDiagnostic(os);
+  SWIFT_DEBUG_DUMPER(dumpForDiagnostics(SILFunction *fn)) {
+    innerInfo.dumpForDiagnostics(fn);
   }
 
-  void printForOneLineLogging(llvm::raw_ostream &os) const {
-    innerInfo.printForOneLineLogging(os);
+  void printForCodeDiagnostic(SILFunction *fn, llvm::raw_ostream &os) const {
+    innerInfo.printForCodeDiagnostic(fn, os);
   }
-};
 
-} // namespace swift
-
-namespace llvm {
-
-inline llvm::raw_ostream &
-operator<<(llvm::raw_ostream &os,
-           const swift::SILIsolationInfo &isolationInfo) {
-  isolationInfo.printForOneLineLogging(os);
-  return os;
-}
+  StringRef printForCodeDiagnostic(SILFunction *fn) const {
+    return innerInfo.printForCodeDiagnostic(fn);
+  }
 
-inline llvm::raw_ostream &
-operator<<(llvm::raw_ostream &os,
-           const swift::SILDynamicMergedIsolationInfo &isolationInfo) {
-  isolationInfo.printForOneLineLogging(os);
-  return os;
-}
+  void printForOneLineLogging(SILFunction *fn, llvm::raw_ostream &os) const {
+    innerInfo.printForOneLineLogging(fn, os);
+  }
+};
 
-} // namespace llvm
+} // namespace swift
 
 #endif
diff --git a/lib/SILGen/SILGenExpr.cpp b/lib/SILGen/SILGenExpr.cpp
index f33cc526bd2d8..15686b18ac236 100644
--- a/lib/SILGen/SILGenExpr.cpp
+++ b/lib/SILGen/SILGenExpr.cpp
@@ -497,6 +497,10 @@ namespace {
     RValue
     emitFunctionCvtFromExecutionCallerToGlobalActor(FunctionConversionExpr *E,
                                                     SGFContext C);
+
+    RValue emitFunctionCvtForNonisolatedNonsendingClosureExpr(
+        FunctionConversionExpr *E, SGFContext C);
+
     RValue visitActorIsolationErasureExpr(ActorIsolationErasureExpr *E,
                                           SGFContext C);
     RValue visitExtractFunctionIsolationExpr(ExtractFunctionIsolationExpr *E,
@@ -2033,6 +2037,44 @@ RValueEmitter::emitFunctionCvtToExecutionCaller(FunctionConversionExpr *e,
   return RValue(SGF, e, destType, result);
 }
 
+RValue RValueEmitter::emitFunctionCvtForNonisolatedNonsendingClosureExpr(
+    FunctionConversionExpr *E, SGFContext C) {
+  // The specific AST pattern for this looks as follows:
+  //
+  //   (function_conversion_expr type="nonisolated(nonsending) () async -> Void"
+  //      (closure_expr type="() async -> ()" isolated_to_caller_isolation))
+  CanAnyFunctionType destType =
+      cast<FunctionType>(E->getType()->getCanonicalType());
+  auto subExpr = E->getSubExpr()->getSemanticsProvidingExpr();
+
+  // If we do not have a closure or if that closure is not caller isolation
+  // inheriting, bail.
+  auto *closureExpr = dyn_cast<ClosureExpr>(subExpr);
+  if (!closureExpr ||
+      !closureExpr->getActorIsolation().isCallerIsolationInheriting())
+    return RValue();
+
+  // Then grab our closure type... make sure it is non isolated and then make
+  // sure it is the same as our destType but with nonisolated.
+  CanAnyFunctionType closureType =
+      cast<FunctionType>(closureExpr->getType()->getCanonicalType());
+  if (!closureType->getIsolation().isNonIsolated() ||
+      closureType !=
+          destType->withIsolation(FunctionTypeIsolation::forNonIsolated())
+              ->getCanonicalType())
+    return RValue();
+
+  // NOTE: This is a partial inline of getClosureTypeInfo. We do this so we have
+  // more control and make this change less viral in the compiler for 6.2.
+  auto newExtInfo = closureType->getExtInfo().withIsolation(
+      FunctionTypeIsolation::forNonIsolatedCaller());
+  closureType = closureType.withExtInfo(newExtInfo);
+  auto info = SGF.getFunctionTypeInfo(closureType);
+
+  auto closure = emitClosureReference(closureExpr, info);
+  return RValue(SGF, closureExpr, destType, closure);
+}
+
 RValue RValueEmitter::emitFunctionCvtFromExecutionCallerToGlobalActor(
     FunctionConversionExpr *e, SGFContext C) {
   // We are pattern matching a conversion sequence like the following:
@@ -2144,6 +2186,28 @@ RValue RValueEmitter::visitFunctionConversionExpr(FunctionConversionExpr *e,
   // TODO: Move this up when we can emit closures directly with C calling
   // convention.
   auto subExpr = e->getSubExpr()->getSemanticsProvidingExpr();
+
+  // Before we go any further into emitting the convert function expr, see if
+  // our SubExpr is a ClosureExpr with the exact same type as our
+  // FunctionConversionExpr except with the FunctionConversionExpr adding
+  // nonisolated(nonsending). Then see if the ClosureExpr itself (even though it
+  // is not nonisolated(nonsending) typed is considered to have
+  // nonisolated(nonsending) isolation. In such a case, emit the closure
+  // directly. We are going to handle it especially in closure emission to work
+  // around the missing information in the type.
+  //
+  // DISCUSSION: We need to do this here since in the Expression TypeChecker we
+  // do not have access to capture information when we would normally want to
+  // mark the closure type as being nonisolated(nonsending). As a result, we
+  // cannot know if the nonisolated(nonsending) should be overridden by for
+  // example an actor that is captured by the closure. So to work around this in
+  // Sema, we still mark the ClosureExpr as having the appropriate isolation
+  // even though its type does not have it... and then we work around this here
+  // and also in getClosureTypeInfo.
+  if (destType->getIsolation().isNonIsolatedCaller())
+    if (auto rv = emitFunctionCvtForNonisolatedNonsendingClosureExpr(e, C))
+      return rv;
+
   // Look through `as` type ascriptions that don't induce bridging too.
   while (auto subCoerce = dyn_cast<CoerceExpr>(subExpr)) {
     // Coercions that introduce bridging aren't simple type ascriptions.
diff --git a/lib/SILOptimizer/Analysis/RegionAnalysis.cpp b/lib/SILOptimizer/Analysis/RegionAnalysis.cpp
index 79be3cf57cb2e..1d19efe5c1144 100644
--- a/lib/SILOptimizer/Analysis/RegionAnalysis.cpp
+++ b/lib/SILOptimizer/Analysis/RegionAnalysis.cpp
@@ -820,7 +820,7 @@ void RegionAnalysisValueMap::print(llvm::raw_ostream &os) const {
   for (auto p : temp) {
     os << "%%" << p.first << ": ";
     auto value = getValueForId(Element(p.first));
-    value->print(os);
+    value->print(getFunction(), os);
   }
 #endif
 }
@@ -1127,12 +1127,13 @@ bool TrackableValue::isSendingParameter() const {
 //                      MARK: TrackableValueLookupResult
 //===----------------------------------------------------------------------===//
 
-void TrackableValueLookupResult::print(llvm::raw_ostream &os) const {
+void TrackableValueLookupResult::print(SILFunction *fn,
+                                       llvm::raw_ostream &os) const {
   os << "Value:\n";
-  value.print(os);
+  value.print(fn, os);
   if (base) {
     os << "Base:\n";
-    base->print(os);
+    base->print(fn, os);
   }
 }
 
@@ -1618,6 +1619,11 @@ struct PartitionOpBuilder {
   /// generating partition ops.
   SmallVector<PartitionOp, 8> currentInstPartitionOps;
 
+  SILFunction *getFunction() const {
+    assert(currentInst);
+    return currentInst->getFunction();
+  }
+
   void reset(SILInstruction *inst) {
     currentInst = inst;
     currentInstPartitionOps.clear();
@@ -1978,7 +1984,8 @@ class PartitionOpTranslator {
                 isNonSendableType(val->getType())) {
               auto trackVal = getTrackableValue(val, true);
               (void)trackVal;
-              REGIONBASEDISOLATION_LOG(trackVal.print(llvm::dbgs()));
+              REGIONBASEDISOLATION_LOG(
+                  trackVal.print(val->getFunction(), llvm::dbgs()));
               continue;
             }
             if (auto *pbi = dyn_cast<ProjectBoxInst>(val)) {
@@ -2055,7 +2062,7 @@ class PartitionOpTranslator {
         // send list and to the region join list.
         REGIONBASEDISOLATION_LOG(
             llvm::dbgs() << "    %%" << value.getID() << ": ";
-            value.print(llvm::dbgs()); llvm::dbgs() << *arg);
+            value.print(function, llvm::dbgs()); llvm::dbgs() << *arg);
         nonSendableJoinedIndices.push_back(value.getID());
       } else {
         REGIONBASEDISOLATION_LOG(llvm::dbgs() << "    Sendable: " << *arg);
@@ -2228,14 +2235,15 @@ class PartitionOpTranslator {
           REGIONBASEDISOLATION_LOG(
               llvm::dbgs() << "Merge Failure!\n"
                            << "Original Info: ";
-              if (originalMergedInfo)
-                  originalMergedInfo->printForDiagnostics(llvm::dbgs());
+              if (originalMergedInfo) originalMergedInfo->printForDiagnostics(
+                  builder.getFunction(), llvm::dbgs());
               else llvm::dbgs() << "nil";
               llvm::dbgs() << "\nValue Rep: "
                            << value.getRepresentative().getValue();
               llvm::dbgs() << "Original Src: " << src;
               llvm::dbgs() << "Value Info: ";
-              value.getIsolationRegionInfo().printForDiagnostics(llvm::dbgs());
+              value.getIsolationRegionInfo().printForDiagnostics(
+                  builder.getFunction(), llvm::dbgs());
               llvm::dbgs() << "\n");
           builder.addUnknownPatternError(src);
           continue;
@@ -3220,7 +3228,7 @@ void PartitionOpBuilder::print(llvm::raw_ostream &os) const {
     auto trackableValue = translator->getValueForId(opArg);
     assert(trackableValue);
     llvm::dbgs() << "State: %%" << opArg << ". ";
-    trackableValue->getValueState().print(llvm::dbgs());
+    trackableValue->getValueState().print(getFunction(), llvm::dbgs());
     llvm::dbgs() << "\n             Rep Value: "
                  << trackableValue->getRepresentative();
     if (auto value = trackableValue->getRepresentative().maybeGetValue()) {
@@ -4310,7 +4318,7 @@ static FunctionTest
                               RegionAnalysisValueMap valueMap(&function);
                               auto value = arguments.takeValue();
                               auto trackableValue = valueMap.getTrackableValue(value);
-                              trackableValue.print(llvm::outs());
+                              trackableValue.print(&function, llvm::outs());
                               llvm::outs() << '\n';
                             });
 
diff --git a/lib/SILOptimizer/Mandatory/DataflowDiagnostics.cpp b/lib/SILOptimizer/Mandatory/DataflowDiagnostics.cpp
index 5e161eb690350..ea8397a6fd295 100644
--- a/lib/SILOptimizer/Mandatory/DataflowDiagnostics.cpp
+++ b/lib/SILOptimizer/Mandatory/DataflowDiagnostics.cpp
@@ -69,10 +69,9 @@ static void diagnoseMissingReturn(const UnreachableInst *UI,
           Context.Diags.diagnose(expr->getStartLoc(),
                                  diag::missing_return_closure, ResTy);
         } else {
-          auto *DC = FLoc.getAsDeclContext();
-          assert(DC && DC->getAsDecl() && "not a declaration?");
+          auto *FD = FLoc.castToASTNode<AbstractFunctionDecl>();
           Context.Diags.diagnose(expr->getStartLoc(), diag::missing_return_decl,
-                                 ResTy, DC->getAsDecl()->getDescriptiveKind());
+                                 ResTy, FD);
         }
         Context.Diags
             .diagnose(expr->getStartLoc(), diag::missing_return_last_expr_note)
@@ -89,12 +88,10 @@ static void diagnoseMissingReturn(const UnreachableInst *UI,
                                : diag::missing_return_closure;
     diagnose(Context, L.getEndSourceLoc(), diagID, ResTy);
   } else {
-    auto *DC = FLoc.getAsDeclContext();
-    assert(DC && DC->getAsDecl() && "not a declaration?");
+    auto *FD = FLoc.castToASTNode<AbstractFunctionDecl>();
     auto diagID = isNoReturnFn ? diag::missing_never_call_decl
                                : diag::missing_return_decl;
-    diagnose(Context, L.getEndSourceLoc(), diagID, ResTy,
-             DC->getAsDecl()->getDescriptiveKind());
+    diagnose(Context, L.getEndSourceLoc(), diagID, ResTy, FD);
   }
 }
 
diff --git a/lib/SILOptimizer/Mandatory/FlowIsolation.cpp b/lib/SILOptimizer/Mandatory/FlowIsolation.cpp
index 26cd098062d61..41bd158bf2070 100644
--- a/lib/SILOptimizer/Mandatory/FlowIsolation.cpp
+++ b/lib/SILOptimizer/Mandatory/FlowIsolation.cpp
@@ -484,9 +484,9 @@ void Info::diagnoseAll(AnalysisInfo &info, bool forDeinit,
     VarDecl *var = cast<RefElementAddrInst>(use)->getField();
 
     diag.diagnose(illegalLoc.getSourceLoc(), diag::isolated_after_nonisolated,
-                  forDeinit, var->getDescriptiveKind(), var->getName())
-      .highlight(illegalLoc.getSourceRange())
-      .warnUntilSwiftVersion(6);
+                  forDeinit, var)
+        .highlight(illegalLoc.getSourceRange())
+        .warnUntilSwiftVersion(6);
 
     // after <verb><adjective> <subject>, ... can't use self anymore, etc ...
     //   example:
diff --git a/lib/SILOptimizer/Mandatory/SendNonSendable.cpp b/lib/SILOptimizer/Mandatory/SendNonSendable.cpp
index 8a34796fe6d95..1f1fdf9e1a253 100644
--- a/lib/SILOptimizer/Mandatory/SendNonSendable.cpp
+++ b/lib/SILOptimizer/Mandatory/SendNonSendable.cpp
@@ -149,17 +149,16 @@ static std::optional<SILDeclRef> getDeclRefForCallee(SILInstruction *inst) {
   }
 }
 
-static std::optional<std::pair<DescriptiveDeclKind, DeclName>>
-getSendingApplyCalleeInfo(SILInstruction *inst) {
+static std::optional<ValueDecl *> getSendingApplyCallee(SILInstruction *inst) {
   auto declRef = getDeclRefForCallee(inst);
   if (!declRef)
     return {};
 
   auto *decl = declRef->getDecl();
-  if (!decl || !decl->hasName())
+  if (!decl)
     return {};
 
-  return {{decl->getDescriptiveKind(), decl->getName()}};
+  return decl;
 }
 
 static Expr *inferArgumentExprFromApplyExpr(ApplyExpr *sourceApply,
@@ -672,10 +671,9 @@ class UseAfterSendDiagnosticEmitter {
         getFunction());
   }
 
-  /// If we can find a callee decl name, return that. None otherwise.
-  std::optional<std::pair<DescriptiveDeclKind, DeclName>>
-  getSendingCalleeInfo() const {
-    return getSendingApplyCalleeInfo(sendingOp->getUser());
+  /// Attempts to retrieve and return the callee declaration.
+  std::optional<const ValueDecl *> getSendingCallee() const {
+    return getSendingApplyCallee(sendingOp->getUser());
   }
 
   void
@@ -688,26 +686,25 @@ class UseAfterSendDiagnosticEmitter {
         .limitBehaviorIf(getBehaviorLimit());
 
     // Then emit the note with greater context.
-    SmallString<64> descriptiveKindStr;
-    {
-      if (!namedValuesIsolationInfo.isDisconnected()) {
-        llvm::raw_svector_ostream os(descriptiveKindStr);
-        namedValuesIsolationInfo.printForDiagnostics(os);
-        os << ' ';
-      }
-    }
-
-    if (auto calleeInfo = getSendingCalleeInfo()) {
+    auto descriptiveKindStr =
+        namedValuesIsolationInfo.printForDiagnostics(getFunction());
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCalleeIsolation());
+    auto callerIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCallerIsolation());
+
+    bool isDisconnected = namedValuesIsolationInfo.isDisconnected();
+    if (auto callee = getSendingCallee()) {
       diagnoseNote(
           loc, diag::regionbasedisolation_named_info_send_yields_race_callee,
-          name, descriptiveKindStr, isolationCrossing.getCalleeIsolation(),
-          calleeInfo->first, calleeInfo->second,
-          isolationCrossing.getCallerIsolation());
+          isDisconnected, name, descriptiveKindStr, calleeIsolationStr,
+          callee.value(), callerIsolationStr);
     } else {
       diagnoseNote(loc, diag::regionbasedisolation_named_info_send_yields_race,
-                   name, descriptiveKindStr,
-                   isolationCrossing.getCalleeIsolation(),
-                   isolationCrossing.getCallerIsolation());
+                   isDisconnected, name, descriptiveKindStr, calleeIsolationStr,
+                   callerIsolationStr);
     }
     emitRequireInstDiagnostics();
   }
@@ -716,27 +713,27 @@ class UseAfterSendDiagnosticEmitter {
   emitNamedIsolationCrossingError(SILLocation loc, Identifier name,
                                   SILIsolationInfo namedValuesIsolationInfo,
                                   ApplyIsolationCrossing isolationCrossing,
-                                  DeclName calleeDeclName,
-                                  DescriptiveDeclKind calleeDeclKind) {
+                                  const ValueDecl *callee) {
     // Emit the short error.
     diagnoseError(loc, diag::regionbasedisolation_named_send_yields_race, name)
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
 
     // Then emit the note with greater context.
-    SmallString<64> descriptiveKindStr;
-    {
-      if (!namedValuesIsolationInfo.isDisconnected()) {
-        llvm::raw_svector_ostream os(descriptiveKindStr);
-        namedValuesIsolationInfo.printForDiagnostics(os);
-        os << ' ';
-      }
-    }
+    auto descriptiveKindStr =
+        namedValuesIsolationInfo.printForDiagnostics(getFunction());
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCalleeIsolation());
+    auto callerIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCallerIsolation());
+    bool isDisconnected = namedValuesIsolationInfo.isDisconnected();
 
-    diagnoseNote(
-        loc, diag::regionbasedisolation_named_info_send_yields_race_callee,
-        name, descriptiveKindStr, isolationCrossing.getCalleeIsolation(),
-        calleeDeclKind, calleeDeclName, isolationCrossing.getCallerIsolation());
+    diagnoseNote(loc,
+                 diag::regionbasedisolation_named_info_send_yields_race_callee,
+                 isDisconnected, name, descriptiveKindStr, calleeIsolationStr,
+                 callee, callerIsolationStr);
     emitRequireInstDiagnostics();
   }
 
@@ -759,15 +756,20 @@ class UseAfterSendDiagnosticEmitter {
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
 
-    if (auto calleeInfo = getSendingCalleeInfo()) {
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCalleeIsolation());
+    auto callerIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCallerIsolation());
+
+    if (auto callee = getSendingCallee()) {
       diagnoseNote(loc, diag::regionbasedisolation_type_use_after_send_callee,
-                   inferredType, isolationCrossing.getCalleeIsolation(),
-                   calleeInfo->first, calleeInfo->second,
-                   isolationCrossing.getCallerIsolation());
+                   inferredType, calleeIsolationStr, callee.value(),
+                   callerIsolationStr);
     } else {
       diagnoseNote(loc, diag::regionbasedisolation_type_use_after_send,
-                   inferredType, isolationCrossing.getCalleeIsolation(),
-                   isolationCrossing.getCallerIsolation());
+                   inferredType, calleeIsolationStr, callerIsolationStr);
     }
     emitRequireInstDiagnostics();
   }
@@ -793,10 +795,10 @@ class UseAfterSendDiagnosticEmitter {
                   inferredType)
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
-    if (auto calleeInfo = getSendingCalleeInfo()) {
+    if (auto callee = getSendingCallee()) {
       diagnoseNote(loc,
                    diag::regionbasedisolation_typed_use_after_sending_callee,
-                   inferredType, calleeInfo->first, calleeInfo->second);
+                   inferredType, callee.value());
     } else {
       diagnoseNote(loc, diag::regionbasedisolation_typed_use_after_sending,
                    inferredType);
@@ -813,19 +815,19 @@ class UseAfterSendDiagnosticEmitter {
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
 
-    SmallString<64> descriptiveKindStr;
-    {
-      if (!namedValuesIsolationInfo.isDisconnected()) {
-        llvm::raw_svector_ostream os(descriptiveKindStr);
-        namedValuesIsolationInfo.printForDiagnostics(os);
-        os << ' ';
-      }
-    }
-
-    diagnoseNote(
-        loc, diag::regionbasedisolation_named_isolated_closure_yields_race,
-        descriptiveKindStr, name, isolationCrossing.getCalleeIsolation(),
-        isolationCrossing.getCallerIsolation())
+    auto descriptiveKindStr =
+        namedValuesIsolationInfo.printForDiagnostics(getFunction());
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCalleeIsolation());
+    auto callerIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCallerIsolation());
+    bool isDisconnected = namedValuesIsolationInfo.isDisconnected();
+    diagnoseNote(loc,
+                 diag::regionbasedisolation_named_isolated_closure_yields_race,
+                 isDisconnected, descriptiveKindStr, name, calleeIsolationStr,
+                 callerIsolationStr)
         .highlight(loc.getSourceRange());
     emitRequireInstDiagnostics();
   }
@@ -837,10 +839,17 @@ class UseAfterSendDiagnosticEmitter {
                   inferredType)
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
+
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCalleeIsolation());
+    auto callerIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCallerIsolation());
+
     diagnoseNote(loc,
                  diag::regionbasedisolation_type_isolated_capture_yields_race,
-                 inferredType, isolationCrossing.getCalleeIsolation(),
-                 isolationCrossing.getCallerIsolation());
+                 inferredType, calleeIsolationStr, callerIsolationStr);
     emitRequireInstDiagnostics();
   }
 
@@ -1106,8 +1115,7 @@ struct UseAfterSendDiagnosticInferrer::AutoClosureWalker : ASTWalker {
           if (valueDecl->hasName()) {
             foundTypeInfo.diagnosticEmitter.emitNamedIsolationCrossingError(
                 foundTypeInfo.baseLoc, targetDecl->getBaseIdentifier(),
-                targetDeclIsolationInfo, *isolationCrossing,
-                valueDecl->getName(), valueDecl->getDescriptiveKind());
+                targetDeclIsolationInfo, *isolationCrossing, valueDecl);
             continue;
           }
 
@@ -1341,10 +1349,9 @@ class SendNeverSentDiagnosticEmitter {
         getOperand()->getFunction());
   }
 
-  /// If we can find a callee decl name, return that. None otherwise.
-  std::optional<std::pair<DescriptiveDeclKind, DeclName>>
-  getSendingCalleeInfo() const {
-    return getSendingApplyCalleeInfo(sendingOperand->getUser());
+  /// Attempts to retrieve and return the callee declaration.
+  std::optional<const ValueDecl *> getSendingCallee() const {
+    return getSendingApplyCallee(sendingOperand->getUser());
   }
 
   SILLocation getLoc() const { return sendingOperand->getUser()->getLoc(); }
@@ -1378,40 +1385,41 @@ class SendNeverSentDiagnosticEmitter {
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
 
-    SmallString<64> descriptiveKindStr;
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().printForDiagnostics(os);
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), crossing.getCalleeIsolation());
 
-    if (auto calleeInfo = getSendingCalleeInfo()) {
+    if (auto callee = getSendingCallee()) {
       diagnoseNote(
           loc,
           diag::regionbasedisolation_typed_sendneversendable_via_arg_callee,
-          descriptiveKindStr, inferredType, crossing.getCalleeIsolation(),
-          calleeInfo->first, calleeInfo->second);
+          descriptiveKindStr, inferredType, calleeIsolationStr, callee.value());
     } else {
-      diagnoseNote(
-          loc, diag::regionbasedisolation_typed_sendneversendable_via_arg,
-          descriptiveKindStr, inferredType, crossing.getCalleeIsolation());
+      diagnoseNote(loc,
+                   diag::regionbasedisolation_typed_sendneversendable_via_arg,
+                   descriptiveKindStr, inferredType, calleeIsolationStr);
     }
   }
 
   void emitNamedFunctionArgumentClosure(SILLocation loc, Identifier name,
                                         ApplyIsolationCrossing crossing) {
     emitNamedOnlyError(loc, name);
-    SmallString<64> descriptiveKindStr;
-    {
-      if (!getIsolationRegionInfo().isDisconnected()) {
-        llvm::raw_svector_ostream os(descriptiveKindStr);
-        getIsolationRegionInfo().printForDiagnostics(os);
-        os << ' ';
-      }
-    }
+
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), crossing.getCalleeIsolation());
+    auto callerIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), crossing.getCallerIsolation());
+    bool isDisconnected = getIsolationRegionInfo().isDisconnected();
     diagnoseNote(loc,
                  diag::regionbasedisolation_named_isolated_closure_yields_race,
-                 descriptiveKindStr, name, crossing.getCalleeIsolation(),
-                 crossing.getCallerIsolation())
+                 isDisconnected, descriptiveKindStr, name, calleeIsolationStr,
+                 callerIsolationStr)
         .highlight(loc.getSourceRange());
   }
 
@@ -1422,17 +1430,13 @@ class SendNeverSentDiagnosticEmitter {
         .highlight(loc.getSourceRange())
         .limitBehaviorIf(getBehaviorLimit());
 
-    SmallString<64> descriptiveKindStr;
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().printForDiagnostics(os);
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
 
-    if (auto calleeInfo = getSendingCalleeInfo()) {
+    if (auto callee = getSendingCallee()) {
       diagnoseNote(
           loc, diag::regionbasedisolation_typed_tns_passed_to_sending_callee,
-          descriptiveKindStr, inferredType, calleeInfo->first,
-          calleeInfo->second);
+          descriptiveKindStr, inferredType, callee.value());
     } else {
       diagnoseNote(loc, diag::regionbasedisolation_typed_tns_passed_to_sending,
                    descriptiveKindStr, inferredType);
@@ -1445,11 +1449,8 @@ class SendNeverSentDiagnosticEmitter {
   void emitClosureErrorWithCapturedActor(Operand *partialApplyOp,
                                          Operand *actualUse,
                                          SILArgument *fArg) {
-    SmallString<64> descriptiveKindStr;
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().getIsolationInfo().printForCodeDiagnostic(os);
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForCodeDiagnostic(getFunction());
 
     diagnoseError(partialApplyOp,
                   diag::regionbasedisolation_typed_tns_passed_sending_closure,
@@ -1457,11 +1458,8 @@ class SendNeverSentDiagnosticEmitter {
         .limitBehaviorIf(getDiagnosticBehaviorLimitForOperands(
             actualUse->getFunction(), {actualUse}));
 
-    descriptiveKindStr.clear();
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().getIsolationInfo().printForDiagnostics(os);
-    }
+    descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
     diagnoseNote(actualUse, diag::regionbasedisolation_closure_captures_actor,
                  fArg->getDecl()->getName(), descriptiveKindStr);
   }
@@ -1476,11 +1474,8 @@ class SendNeverSentDiagnosticEmitter {
   void emitSendingClosureParamDirectlyIsolated(Operand *partialApplyOp,
                                                Operand *actualUse,
                                                SILArgument *fArg) {
-    SmallString<64> descriptiveKindStr;
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().getIsolationInfo().printForCodeDiagnostic(os);
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForCodeDiagnostic(getFunction());
 
     diagnoseError(partialApplyOp,
                   diag::regionbasedisolation_typed_tns_passed_sending_closure,
@@ -1494,8 +1489,7 @@ class SendNeverSentDiagnosticEmitter {
           fArg->getType().is<SILBoxType>()) {
         auto diag = diag::
             regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_boxed_value_task_isolated;
-        diagnoseNote(actualUse, diag, fArg->getDecl()->getName(),
-                     fArg->getDecl()->getDescriptiveKind());
+        diagnoseNote(actualUse, diag, fArg->getDecl());
         return;
       }
 
@@ -1505,11 +1499,8 @@ class SendNeverSentDiagnosticEmitter {
       return;
     }
 
-    descriptiveKindStr.clear();
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().printForDiagnostics(os);
-    }
+    descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
 
     auto diag = diag::
         regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_value;
@@ -1526,11 +1517,8 @@ class SendNeverSentDiagnosticEmitter {
       return;
     }
 
-    SmallString<64> descriptiveKindStr;
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo()->printForCodeDiagnostic(os);
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo()->printForCodeDiagnostic(getFunction());
 
     auto emitMainError = [&] {
       auto behaviorLimit = getDiagnosticBehaviorLimitForOperands(
@@ -1562,11 +1550,8 @@ class SendNeverSentDiagnosticEmitter {
       }
 
       emitMainError();
-      descriptiveKindStr.clear();
-      {
-        llvm::raw_svector_ostream os(descriptiveKindStr);
-        getIsolationRegionInfo().printForDiagnostics(os);
-      }
+      descriptiveKindStr =
+          getIsolationRegionInfo().printForDiagnostics(getFunction());
       auto diag = diag::
           regionbasedisolation_typed_tns_passed_to_sending_closure_helper_have_value_region;
       diagnoseNote(actualUseInfo->first, diag, descriptiveKindStr,
@@ -1607,11 +1592,8 @@ class SendNeverSentDiagnosticEmitter {
            "Should never be disconnected?!");
     emitNamedOnlyError(loc, name);
 
-    SmallString<64> descriptiveKindStr;
-    {
-      llvm::raw_svector_ostream os(descriptiveKindStr);
-      getIsolationRegionInfo().printForDiagnostics(os);
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
 
     diagnoseNote(loc, diag::regionbasedisolation_named_send_nt_asynclet_capture,
                  name, descriptiveKindStr);
@@ -1620,62 +1602,44 @@ class SendNeverSentDiagnosticEmitter {
   void emitNamedIsolation(SILLocation loc, Identifier name,
                           ApplyIsolationCrossing isolationCrossing) {
     emitNamedOnlyError(loc, name);
-    SmallString<64> descriptiveKindStr;
-    SmallString<64> descriptiveKindStrWithSpace;
-    {
-      if (!getIsolationRegionInfo().isDisconnected()) {
-        {
-          llvm::raw_svector_ostream os(descriptiveKindStr);
-          getIsolationRegionInfo().printForDiagnostics(os);
-        }
-        descriptiveKindStrWithSpace = descriptiveKindStr;
-        descriptiveKindStrWithSpace.push_back(' ');
-      }
-    }
-    if (auto calleeInfo = getSendingCalleeInfo()) {
+
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
+    auto calleeIsolationStr =
+        SILIsolationInfo::printActorIsolationForDiagnostics(
+            getFunction(), isolationCrossing.getCalleeIsolation());
+    bool isDisconnected = getIsolationRegionInfo().isDisconnected();
+
+    if (auto callee = getSendingCallee()) {
       diagnoseNote(loc,
                    diag::regionbasedisolation_named_send_never_sendable_callee,
-                   name, descriptiveKindStrWithSpace,
-                   isolationCrossing.getCalleeIsolation(), calleeInfo->first,
-                   calleeInfo->second, descriptiveKindStr);
+                   isDisconnected, name, descriptiveKindStr, calleeIsolationStr,
+                   callee.value(), descriptiveKindStr);
     } else {
       diagnoseNote(loc, diag::regionbasedisolation_named_send_never_sendable,
-                   name, descriptiveKindStrWithSpace,
-                   isolationCrossing.getCalleeIsolation(), descriptiveKindStr);
+                   isDisconnected, name, descriptiveKindStr, calleeIsolationStr,
+                   descriptiveKindStr);
     }
   }
 
   void emitNamedSendingNeverSendableToSendingParam(SILLocation loc,
                                                    Identifier varName) {
     emitNamedOnlyError(loc, varName);
-    SmallString<64> descriptiveKindStr;
-    {
-      if (!getIsolationRegionInfo().isDisconnected()) {
-        llvm::raw_svector_ostream os(descriptiveKindStr);
-        getIsolationRegionInfo().printForDiagnostics(os);
-        os << ' ';
-      }
-    }
+
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
+    bool isDisconnected = getIsolationRegionInfo().isDisconnected();
     auto diag = diag::regionbasedisolation_named_send_into_sending_param;
-    diagnoseNote(loc, diag, descriptiveKindStr, varName);
+    diagnoseNote(loc, diag, isDisconnected, descriptiveKindStr, varName);
   }
 
   void emitNamedSendingReturn(SILLocation loc, Identifier varName) {
     emitNamedOnlyError(loc, varName);
-    SmallString<64> descriptiveKindStr;
-    SmallString<64> descriptiveKindStrWithSpace;
-    {
-      if (!getIsolationRegionInfo().isDisconnected()) {
-        {
-          llvm::raw_svector_ostream os(descriptiveKindStr);
-          getIsolationRegionInfo().printForDiagnostics(os);
-        }
-        descriptiveKindStrWithSpace = descriptiveKindStr;
-        descriptiveKindStrWithSpace.push_back(' ');
-      }
-    }
+    auto descriptiveKindStr =
+        getIsolationRegionInfo().printForDiagnostics(getFunction());
+    bool isDisconnected = getIsolationRegionInfo().isDisconnected();
     auto diag = diag::regionbasedisolation_named_nosend_send_into_result;
-    diagnoseNote(loc, diag, descriptiveKindStrWithSpace, varName,
+    diagnoseNote(loc, diag, isDisconnected, descriptiveKindStr, varName,
                  descriptiveKindStr);
   }
 
@@ -1990,7 +1954,6 @@ bool SentNeverSendableDiagnosticInferrer::run() {
           return true;
 
         // See if we can infer a name from the value.
-        SmallString<64> resultingName;
         if (auto varName = inferNameHelper(op->get())) {
           diagnosticEmitter.emitNamedSendingNeverSendableToSendingParam(
               loc, *varName);
@@ -2031,7 +1994,6 @@ bool SentNeverSendableDiagnosticInferrer::run() {
     }
 
     // See if we can infer a name from the value.
-    SmallString<64> resultingName;
     if (auto name = inferNameHelper(op->get())) {
       diagnosticEmitter.emitNamedIsolation(loc, *name, *isolation);
       return true;
@@ -2079,7 +2041,6 @@ bool SentNeverSendableDiagnosticInferrer::run() {
                           }) &&
              "All result info must be the same... if that changes... update "
              "this code!");
-      SmallString<64> resultingName;
       if (auto name = inferNameHelper(op->get())) {
         diagnosticEmitter.emitNamedSendingReturn(loc, *name);
         return true;
@@ -2228,12 +2189,8 @@ void InOutSendingNotDisconnectedDiagnosticEmitter::emit() {
   }
 
   // Then emit the note with greater context.
-  SmallString<64> descriptiveKindStr;
-  {
-    llvm::raw_svector_ostream os(descriptiveKindStr);
-    actorIsolatedRegionInfo.printForDiagnostics(os);
-    os << ' ';
-  }
+  auto descriptiveKindStr =
+      actorIsolatedRegionInfo.printForDiagnostics(getFunction());
 
   diagnoseError(
       functionExitingInst,
@@ -2388,11 +2345,8 @@ static SILValue findOutParameter(SILValue v) {
 
 void AssignIsolatedIntoSendingResultDiagnosticEmitter::emit() {
   // Then emit the note with greater context.
-  SmallString<64> descriptiveKindStr;
-  {
-    llvm::raw_svector_ostream os(descriptiveKindStr);
-    isolatedValueIsolationRegionInfo.printForDiagnostics(os);
-  }
+  auto descriptiveKindStr =
+      isolatedValueIsolationRegionInfo.printForDiagnostics(getFunction());
 
   // Grab the var name if we can find it.
   if (auto varName = VariableNameInferrer::inferName(srcOperand->get())) {
@@ -2516,6 +2470,10 @@ struct NonSendableIsolationCrossingResultDiagnosticEmitter {
       : valueMap(valueMap), error(error),
         representative(valueMap.getRepresentative(error.returnValueElement)) {}
 
+  SILFunction *getFunction() const {
+    return error.op->getSourceInst()->getFunction();
+  }
+
   void emit();
 
   ASTContext &getASTContext() const {
@@ -2632,17 +2590,22 @@ void NonSendableIsolationCrossingResultDiagnosticEmitter::emit() {
   if (!isolationCrossing)
     return emitUnknownPatternError();
 
+  auto calleeIsolationStr = SILIsolationInfo::printActorIsolationForDiagnostics(
+      getFunction(), isolationCrossing->getCalleeIsolation());
+  auto callerIsolationStr = SILIsolationInfo::printActorIsolationForDiagnostics(
+      getFunction(), isolationCrossing->getCallerIsolation());
+
   auto type = getType();
   if (getCalledDecl()) {
-    diagnoseError(error.op->getSourceInst(), diag::rbi_isolation_crossing_result,
-                  type, isolationCrossing->getCalleeIsolation(), getCalledDecl(),
-                  isolationCrossing->getCallerIsolation())
-      .limitBehaviorIf(getBehaviorLimit());
+    diagnoseError(error.op->getSourceInst(),
+                  diag::rbi_isolation_crossing_result, type, calleeIsolationStr,
+                  getCalledDecl(), callerIsolationStr)
+        .limitBehaviorIf(getBehaviorLimit());
   } else {
-    diagnoseError(error.op->getSourceInst(), diag::rbi_isolation_crossing_result_no_decl,
-                  type, isolationCrossing->getCalleeIsolation(),
-                  isolationCrossing->getCallerIsolation())
-      .limitBehaviorIf(getBehaviorLimit());
+    diagnoseError(error.op->getSourceInst(),
+                  diag::rbi_isolation_crossing_result_no_decl, type,
+                  calleeIsolationStr, callerIsolationStr)
+        .limitBehaviorIf(getBehaviorLimit());
   }
   if (type->is<FunctionType>()) {
     diagnoseNote(error.op->getSourceInst(),
diff --git a/lib/SILOptimizer/Utils/PartitionUtils.cpp b/lib/SILOptimizer/Utils/PartitionUtils.cpp
index 4980743c20278..55a75e42f548c 100644
--- a/lib/SILOptimizer/Utils/PartitionUtils.cpp
+++ b/lib/SILOptimizer/Utils/PartitionUtils.cpp
@@ -46,7 +46,7 @@ void PartitionOpError::SentNeverSendableError::print(
      << "        ID:  %%" << sentElement << "\n"
      << "        Rep: " << *info.getRepresentative(sentElement)
      << "        Dynamic Isolation Region: ";
-  isolationRegionInfo.printForOneLineLogging(os);
+  isolationRegionInfo.printForOneLineLogging(info.getFunction(), os);
   os << '\n';
   if (auto isolatedValue = isolationRegionInfo->maybeGetIsolatedValue()) {
     os << "        Isolated Value: " << isolatedValue;
@@ -90,7 +90,7 @@ void PartitionOpError::InOutSendingNotDisconnectedAtExitError::print(
      << "        ID:  %%" << inoutSendingElement << "\n"
      << "        Rep: " << valueMap.getRepresentativeValue(inoutSendingElement)
      << "        Dynamic Isolation Region: ";
-  isolationInfo.printForOneLineLogging(os);
+  isolationInfo.printForOneLineLogging(valueMap.getFunction(), os);
   os << '\n';
 }
 
diff --git a/lib/SILOptimizer/Utils/SILIsolationInfo.cpp b/lib/SILOptimizer/Utils/SILIsolationInfo.cpp
index cfe3b918682ef..b4af1a75e28fc 100644
--- a/lib/SILOptimizer/Utils/SILIsolationInfo.cpp
+++ b/lib/SILOptimizer/Utils/SILIsolationInfo.cpp
@@ -447,6 +447,21 @@ SILIsolationInfo SILIsolationInfo::get(SILInstruction *inst) {
         return info;
     }
 
+    // See if our function apply site has an implicit isolated parameter. In
+    // such a case, we know that we have a caller inheriting isolated
+    // function. Return that this has disconnected isolation.
+    //
+    // DISCUSSION: The reason why we are doing this is that we already know that
+    // the AST is not going to label this as an isolation crossing point so we
+    // will only perform a merge. We want to just perform an isolation merge
+    // without adding additional isolation info. Otherwise, a nonisolated
+    // function that
+    if (auto paramInfo = fas.getSubstCalleeType()->maybeGetIsolatedParameter();
+        paramInfo && paramInfo->hasOption(SILParameterInfo::ImplicitLeading) &&
+        paramInfo->hasOption(SILParameterInfo::Isolated)) {
+      return SILIsolationInfo::getDisconnected(false /*unsafe nonisolated*/);
+    }
+
     if (auto *isolatedOp = fas.getIsolatedArgumentOperandOrNullPtr()) {
       // First look through ActorInstance agnostic values so we can find the
       // type of the actual underlying actor (e.x.: copy_value,
@@ -539,9 +554,8 @@ SILIsolationInfo SILIsolationInfo::get(SILInstruction *inst) {
         if (actorInstance) {
           if (auto actualIsolatedValue =
                   ActorInstance::getForValue(actorInstance)) {
-            // See if we have a function parameter. In that case, we want to see
-            // if we have a function argument. In such a case, we need to use
-            // the right parameter and the var decl.
+            // See if we have a function parameter. In such a case, we need to
+            // use the right parameter and the var decl.
             if (auto *fArg = dyn_cast<SILFunctionArgument>(
                     actualIsolatedValue.getValue())) {
               if (auto info =
@@ -924,6 +938,12 @@ SILIsolationInfo SILIsolationInfo::get(SILInstruction *inst) {
   /// Consider non-Sendable metatypes to be task-isolated, so they cannot cross
   /// into another isolation domain.
   if (auto *mi = dyn_cast<MetatypeInst>(inst)) {
+    if (auto funcIsolation = mi->getFunction()->getActorIsolation();
+        funcIsolation && funcIsolation->isCallerIsolationInheriting()) {
+      return SILIsolationInfo::getTaskIsolated(mi)
+          .withNonisolatedNonsendingTaskIsolated(true);
+    }
+
     return SILIsolationInfo::getTaskIsolated(mi);
   }
 
@@ -985,6 +1005,14 @@ SILIsolationInfo SILIsolationInfo::get(SILArgument *arg) {
   // handles isolated self and specifically marked isolated.
   if (auto *isolatedArg = llvm::cast_or_null<SILFunctionArgument>(
           fArg->getFunction()->maybeGetIsolatedArgument())) {
+    // See if the function is nonisolated(nonsending). In such a case, return
+    // task isolated.
+    if (auto funcIsolation = fArg->getFunction()->getActorIsolation();
+        funcIsolation && funcIsolation->isCallerIsolationInheriting()) {
+      return SILIsolationInfo::getTaskIsolated(fArg)
+          .withNonisolatedNonsendingTaskIsolated(true);
+    }
+
     auto astType = isolatedArg->getType().getASTType();
     if (astType->lookThroughAllOptionalTypes()->getAnyActor()) {
       return SILIsolationInfo::getActorInstanceIsolated(fArg, isolatedArg);
@@ -1216,7 +1244,41 @@ void SILIsolationInfo::printOptions(llvm::raw_ostream &os) const {
   llvm::interleave(data, os, ", ");
 }
 
-void SILIsolationInfo::print(llvm::raw_ostream &os) const {
+StringRef SILIsolationInfo::printActorIsolationForDiagnostics(
+    SILFunction *fn, ActorIsolation iso, StringRef openingQuotationMark,
+    bool asNoun) {
+  SmallString<64> string;
+  {
+    llvm::raw_svector_ostream os(string);
+    printActorIsolationForDiagnostics(fn, iso, os, openingQuotationMark,
+                                      asNoun);
+  }
+
+  return fn->getASTContext().getIdentifier(string).str();
+}
+
+void SILIsolationInfo::printActorIsolationForDiagnostics(
+    SILFunction *fn, ActorIsolation iso, llvm::raw_ostream &os,
+    StringRef openingQuotationMark, bool asNoun) {
+  // If we have NonisolatedNonsendingByDefault enabled, we need to return
+  // @concurrent for nonisolated and nonisolated for caller isolation inherited.
+  if (fn->isAsync() && fn->getASTContext().LangOpts.hasFeature(
+                           Feature::NonisolatedNonsendingByDefault)) {
+    if (iso.isCallerIsolationInheriting()) {
+      os << "nonisolated";
+      return;
+    }
+
+    if (iso.isNonisolated()) {
+      os << "@concurrent";
+      return;
+    }
+  }
+
+  return iso.printForDiagnostics(os, openingQuotationMark, asNoun);
+}
+
+void SILIsolationInfo::print(SILFunction *fn, llvm::raw_ostream &os) const {
   switch (Kind(*this)) {
   case Unknown:
     os << "unknown";
@@ -1263,7 +1325,7 @@ void SILIsolationInfo::print(llvm::raw_ostream &os) const {
       }
     }
 
-    getActorIsolation().printForDiagnostics(os);
+    printActorIsolationForDiagnostics(fn, getActorIsolation(), os);
     printOptions(os);
     return;
   case Task:
@@ -1334,6 +1396,7 @@ bool SILIsolationInfo::isEqual(const SILIsolationInfo &other) const {
 
 void SILIsolationInfo::Profile(llvm::FoldingSetNodeID &id) const {
   id.AddInteger(getKind());
+  id.AddInteger(getOptions().toRaw());
   switch (getKind()) {
   case Unknown:
   case Disconnected:
@@ -1350,7 +1413,18 @@ void SILIsolationInfo::Profile(llvm::FoldingSetNodeID &id) const {
   }
 }
 
-void SILIsolationInfo::printForDiagnostics(llvm::raw_ostream &os) const {
+StringRef SILIsolationInfo::printForDiagnostics(SILFunction *fn) const {
+  SmallString<64> string;
+  {
+    llvm::raw_svector_ostream os(string);
+    printForDiagnostics(fn, os);
+  }
+
+  return fn->getASTContext().getIdentifier(string).str();
+}
+
+void SILIsolationInfo::printForDiagnostics(SILFunction *fn,
+                                           llvm::raw_ostream &os) const {
   switch (Kind(*this)) {
   case Unknown:
     llvm::report_fatal_error("Printing unknown for diagnostics?!");
@@ -1385,15 +1459,41 @@ void SILIsolationInfo::printForDiagnostics(llvm::raw_ostream &os) const {
       }
     }
 
-    getActorIsolation().printForDiagnostics(os);
+    printActorIsolationForDiagnostics(fn, getActorIsolation(), os);
     return;
   case Task:
+    if (fn->isAsync() && fn->getASTContext().LangOpts.hasFeature(
+                             Feature::NonisolatedNonsendingByDefault)) {
+      if (isNonisolatedNonsendingTaskIsolated()) {
+        os << "task-isolated";
+        return;
+      }
+      os << "@concurrent task-isolated";
+      return;
+    }
+
+    if (isNonisolatedNonsendingTaskIsolated()) {
+      os << "nonisolated(nonsending) task-isolated";
+      return;
+    }
+
     os << "task-isolated";
     return;
   }
 }
 
-void SILIsolationInfo::printForCodeDiagnostic(llvm::raw_ostream &os) const {
+StringRef SILIsolationInfo::printForCodeDiagnostic(SILFunction *fn) const {
+  SmallString<64> string;
+  {
+    llvm::raw_svector_ostream os(string);
+    printForCodeDiagnostic(fn, os);
+  }
+
+  return fn->getASTContext().getIdentifier(string).str();
+}
+
+void SILIsolationInfo::printForCodeDiagnostic(SILFunction *fn,
+                                              llvm::raw_ostream &os) const {
   switch (Kind(*this)) {
   case Unknown:
     llvm::report_fatal_error("Printing unknown for code diagnostic?!");
@@ -1428,7 +1528,7 @@ void SILIsolationInfo::printForCodeDiagnostic(llvm::raw_ostream &os) const {
       }
     }
 
-    getActorIsolation().printForDiagnostics(os);
+    printActorIsolationForDiagnostics(fn, getActorIsolation(), os);
     os << " code";
     return;
   case Task:
@@ -1437,7 +1537,8 @@ void SILIsolationInfo::printForCodeDiagnostic(llvm::raw_ostream &os) const {
   }
 }
 
-void SILIsolationInfo::printForOneLineLogging(llvm::raw_ostream &os) const {
+void SILIsolationInfo::printForOneLineLogging(SILFunction *fn,
+                                              llvm::raw_ostream &os) const {
   switch (Kind(*this)) {
   case Unknown:
     os << "unknown";
@@ -1477,7 +1578,7 @@ void SILIsolationInfo::printForOneLineLogging(llvm::raw_ostream &os) const {
       }
     }
 
-    getActorIsolation().printForDiagnostics(os);
+    printActorIsolationForDiagnostics(fn, getActorIsolation(), os);
     printOptions(os);
     return;
   case Task:
@@ -1606,8 +1707,13 @@ std::optional<SILDynamicMergedIsolationInfo>
 SILDynamicMergedIsolationInfo::merge(SILIsolationInfo other) const {
   // If we are greater than the other kind, then we are further along the
   // lattice. We ignore the change.
-  if (unsigned(innerInfo.getKind() > unsigned(other.getKind())))
+  //
+  // NOTE: If we are further along, then we both cannot be task isolated. In
+  // such a case, we are the only potential thing that can be
+  // nonisolated(unsafe)... so we do not need to try to propagate.
+  if (unsigned(innerInfo.getKind() > unsigned(other.getKind()))) {
     return {*this};
+  }
 
   // If we are both actor isolated...
   if (innerInfo.isActorIsolated() && other.isActorIsolated()) {
@@ -1642,6 +1748,21 @@ SILDynamicMergedIsolationInfo::merge(SILIsolationInfo other) const {
     return {other.withUnsafeNonIsolated(false)};
   }
 
+  // We know that we are either the same as other or other is further along. If
+  // other is further along, it is the only thing that can propagate the task
+  // isolated bit. So we do not need to do anything. If we are equal though, we
+  // may need to propagate the bit. This ensures that when we emit a diagnostic
+  // we appropriately say potentially actor isolated code instead of code in the
+  // current task.
+  //
+  // TODO: We should really represent this as a separate isolation info
+  // kind... but that would be a larger change than we want for 6.2.
+  if (innerInfo.isTaskIsolated() && other.isTaskIsolated()) {
+    if (innerInfo.isNonisolatedNonsendingTaskIsolated() ||
+        other.isNonisolatedNonsendingTaskIsolated())
+      return other.withNonisolatedNonsendingTaskIsolated(true);
+  }
+
   // Otherwise, just return other.
   return {other};
 }
@@ -1684,7 +1805,8 @@ static FunctionTest
                                   SILIsolationInfo::get(value);
                               llvm::outs() << "Input Value: " << *value;
                               llvm::outs() << "Isolation: ";
-                              info.printForOneLineLogging(llvm::outs());
+                              info.printForOneLineLogging(&function,
+                                                          llvm::outs());
                               llvm::outs() << "\n";
                             });
 
@@ -1704,13 +1826,13 @@ static FunctionTest IsolationMergeTest(
       mergedInfo = mergedInfo->merge(secondValueInfo);
       llvm::outs() << "First Value: " << *firstValue;
       llvm::outs() << "First Isolation: ";
-      firstValueInfo.printForOneLineLogging(llvm::outs());
+      firstValueInfo.printForOneLineLogging(&function, llvm::outs());
       llvm::outs() << "\nSecond Value: " << *secondValue;
       llvm::outs() << "Second Isolation: ";
-      secondValueInfo.printForOneLineLogging(llvm::outs());
+      secondValueInfo.printForOneLineLogging(&function, llvm::outs());
       llvm::outs() << "\nMerged Isolation: ";
       if (mergedInfo) {
-        mergedInfo->printForOneLineLogging(llvm::outs());
+        mergedInfo->printForOneLineLogging(&function, llvm::outs());
       } else {
         llvm::outs() << "Merge failure!";
       }
diff --git a/lib/Sema/CSApply.cpp b/lib/Sema/CSApply.cpp
index a214f7377bea9..5fcbf38a3a07d 100644
--- a/lib/Sema/CSApply.cpp
+++ b/lib/Sema/CSApply.cpp
@@ -7861,23 +7861,6 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       }
     }
 
-    // If we have a ClosureExpr, then we can safely propagate the
-    // 'nonisolated(nonsending)' isolation if it's not explicitly
-    // marked as `@concurrent`.
-    if (toEI.getIsolation().isNonIsolatedCaller() &&
-        (fromEI.getIsolation().isNonIsolated() &&
-         !isClosureMarkedAsConcurrent(expr))) {
-      auto newFromFuncType = fromFunc->withIsolation(
-          FunctionTypeIsolation::forNonIsolatedCaller());
-      if (applyTypeToClosureExpr(cs, expr, newFromFuncType)) {
-        fromFunc = newFromFuncType->castTo<FunctionType>();
-        // Propagating 'nonisolated(nonsending)' might have satisfied the entire
-        // conversion. If so, we're done, otherwise keep converting.
-        if (fromFunc->isEqual(toType))
-          return expr;
-      }
-    }
-
     if (ctx.LangOpts.isDynamicActorIsolationCheckingEnabled()) {
       // Passing a synchronous global actor-isolated function value and
       // parameter that expects a synchronous non-isolated function type could
diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index 97a0927f7c843..2ae325f28b30e 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -4067,6 +4067,10 @@ namespace {
       Expr *argForIsolatedParam = nullptr;
       auto calleeDecl = apply->getCalledValue(/*skipFunctionConversions=*/true);
 
+      if (calleeDecl &&
+          calleeDecl->getAttrs().hasAttribute<UnsafeInheritExecutorAttr>())
+        return false;
+
       auto fnTypeIsolation = fnType->getIsolation();
       if (fnTypeIsolation.isGlobalActor()) {
         // If the function type is global-actor-qualified, determine whether
@@ -4112,10 +4116,6 @@ namespace {
           calleeDecl = memberRef->first.getDecl();
           argForIsolatedParam = selfApplyFn->getBase();
         }
-      } else if (calleeDecl &&
-                 calleeDecl->getAttrs()
-                     .hasAttribute<UnsafeInheritExecutorAttr>()) {
-        return false;
       }
 
       // Check for isolated parameters.
@@ -4166,11 +4166,25 @@ namespace {
         break;
       }
 
-      // If we're calling an async function that's nonisolated, and we're in
-      // an isolated context, then we're exiting the actor context.
-      if (mayExitToNonisolated && fnType->isAsync() &&
-          getContextIsolation().isActorIsolated())
-        unsatisfiedIsolation = ActorIsolation::forNonisolated(/*unsafe=*/false);
+      // If we're calling an async function that's nonisolated, and we're in an
+      // isolated context, then we're exiting the actor context unless we have
+      // nonisolated(nonsending) isolation.
+      //
+      // NOTE: We do not check fnTypeIsolation since that is the AST level
+      // actual isolation which does not have nonisolated(nonsending) added to
+      // it yet. Instead, we want the direct callee including casts since those
+      // casts is what would add the nonisolated(nonsending) bit to the function
+      // type isolation.
+      if (mayExitToNonisolated && fnType->isAsync()) {
+        if (getContextIsolation().isActorIsolated() &&
+            !fnTypeIsolation.isNonIsolatedCaller())
+          unsatisfiedIsolation =
+              ActorIsolation::forNonisolated(/*unsafe=*/false);
+        else if (getContextIsolation().isCallerIsolationInheriting() &&
+                 fnTypeIsolation.isNonIsolated())
+          unsatisfiedIsolation =
+              ActorIsolation::forNonisolated(/*unsafe=*/false);
+      }
 
       // If there was no unsatisfied actor isolation, we're done.
       if (!unsatisfiedIsolation)
@@ -4973,8 +4987,33 @@ ActorIsolation ActorIsolationChecker::determineClosureIsolation(
         closure->getParent(), getClosureActorIsolation);
     preconcurrency |= parentIsolation.preconcurrency();
 
-    return computeClosureIsolationFromParent(closure, parentIsolation,
-                                             checkIsolatedCapture);
+    auto normalIsolation = computeClosureIsolationFromParent(
+        closure, parentIsolation, checkIsolatedCapture);
+
+    // The solver has to be conservative and produce a conversion to
+    // `nonisolated(nonsending)` because at solution application time
+    // we don't yet know whether there are any captures which would
+    // make closure isolated.
+    //
+    // At this point we know that closure is not explicitly annotated with
+    // global actor, nonisolated/@concurrent attributes and doesn't have
+    // isolated parameters. If our closure is nonisolated and we have a
+    // conversion to nonisolated(nonsending), then we should respect that.
+    if (auto *explicitClosure = dyn_cast<ClosureExpr>(closure);
+        !normalIsolation.isGlobalActor()) {
+      if (auto *fce =
+              dyn_cast_or_null<FunctionConversionExpr>(Parent.getAsExpr())) {
+        auto expectedIsolation =
+            fce->getType()->castTo<FunctionType>()->getIsolation();
+        if (expectedIsolation.isNonIsolatedCaller()) {
+          auto captureInfo = explicitClosure->getCaptureInfo();
+          if (!captureInfo.getIsolatedParamCapture())
+            return ActorIsolation::forCallerIsolationInheriting();
+        }
+      }
+    }
+
+    return normalIsolation;
   }();
 
   // Apply computed preconcurrency.
@@ -8129,12 +8168,14 @@ ActorReferenceResult ActorReferenceResult::forReference(
   // When the declaration is not actor-isolated, it can always be accessed
   // directly.
   if (!declIsolation.isActorIsolated()) {
+    if (declRef.getDecl()->getAttrs().hasAttribute<UnsafeInheritExecutorAttr>())
+      return forSameConcurrencyDomain(declIsolation, options);
+
     // If the declaration is asynchronous and we are in an actor-isolated
-    // context (of any kind), then we exit the actor to the nonisolated context.
-    if (decl->isAsync() && contextIsolation.isActorIsolated() &&
-        !declRef.getDecl()
-             ->getAttrs()
-             .hasAttribute<UnsafeInheritExecutorAttr>())
+    // context (of any kind) or it is a caller isolation inheriting, then
+    // we exit the actor to the nonisolated context.
+    if (decl->isAsync() && (contextIsolation.isActorIsolated() ||
+                            contextIsolation.isCallerIsolationInheriting()))
       return forExitsActorToNonisolated(contextIsolation, options);
 
     // Otherwise, we stay in the same concurrency domain, whether on an actor
diff --git a/test/ClangImporter/Inputs/regionbasedisolation.h b/test/ClangImporter/Inputs/regionbasedisolation.h
index 7852358ca5a3e..6385570570d3e 100644
--- a/test/ClangImporter/Inputs/regionbasedisolation.h
+++ b/test/ClangImporter/Inputs/regionbasedisolation.h
@@ -13,6 +13,9 @@ NS_ASSUME_NONNULL_BEGIN
     (void (^)(NSArray<NSObject *> *_Nullable,
                                NSError *_Nullable))completionHandler;
 
+- (void)useValue:(id)object
+    withCompletionHandler:(void (^)(NSObject *_Nullable))completionHandler;
+
 @end
 
 NS_ASSUME_NONNULL_END
diff --git a/test/ClangImporter/regionbasedisolation.swift b/test/ClangImporter/regionbasedisolation.swift
index 63a2364dafbb2..4f86aedd320a0 100644
--- a/test/ClangImporter/regionbasedisolation.swift
+++ b/test/ClangImporter/regionbasedisolation.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend %s -import-objc-header %S/Inputs/regionbasedisolation.h -verify -c -swift-version 6
+// RUN: %target-swift-frontend %s -import-objc-header %S/Inputs/regionbasedisolation.h -verify -verify-additional-prefix ni- -c -swift-version 6
+// RUN: %target-swift-frontend %s -import-objc-header %S/Inputs/regionbasedisolation.h -verify -verify-additional-prefix ni-ns- -c -swift-version 6 -enable-upcoming-feature NonisolatedNonsendingByDefault
 // RUN: %target-swift-frontend %s -import-objc-header %S/Inputs/regionbasedisolation.h -emit-silgen -swift-version 6 | %FileCheck %s
 
 // REQUIRES: objc_interop
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 extension ObjCObject {
   // CHECK-LABEL: sil hidden [ossa] @$sSo10ObjCObjectC20regionbasedisolationE11sendObjectsSaySo8NSObjectCGyYaKF : $@convention(method) @async (@guaranteed ObjCObject) -> (@sil_sending @owned Array<NSObject>, @error any Error) {
@@ -123,3 +125,40 @@ extension ObjCObject {
     try await loadObjects2()
   } // expected-error {{task or actor isolated value cannot be sent}}
 }
+
+@concurrent func useValueConcurrently<T>(_ t: T) async {}
+@MainActor func useValueMainActor<T>(_ t: T) async {}
+nonisolated(nonsending) func useValueNonIsolatedNonSending<T>(_ t: T) async {}
+
+@MainActor func testMainActorMerging(_ y: NSObject) async {
+  let x = ObjCObject()
+  await x.useValue(y)
+  await useValueConcurrently(x)  // expected-error {{sending 'x' risks causing data races}}
+  // expected-ni-note @-1 {{sending main actor-isolated 'x' to nonisolated global function 'useValueConcurrently' risks causing data races between nonisolated and main actor-isolated uses}}
+  // expected-ni-ns-note @-2 {{sending main actor-isolated 'x' to @concurrent global function 'useValueConcurrently' risks causing data races between @concurrent and main actor-isolated uses}}
+}
+
+func testTaskLocal(_ y: NSObject) async {
+  let x = ObjCObject()
+  await x.useValue(y)
+  await useValueConcurrently(x) // expected-ni-ns-error {{sending 'x' risks causing data races}}
+  // expected-ni-ns-note @-1 {{sending task-isolated 'x' to @concurrent global function 'useValueConcurrently' risks causing data races between @concurrent and task-isolated uses}}
+
+  // This is not safe since we merge x into y's region making x task
+  // isolated. We then try to send it to a main actor function.
+  await useValueMainActor(x) // expected-error {{sending 'x' risks causing data races}}
+  // expected-note @-1 {{sending task-isolated 'x' to main actor-isolated global function 'useValueMainActor' risks causing data races between main actor-isolated and task-isolated uses}}
+}
+
+actor MyActor {
+  var field = NSObject()
+
+  // This is safe since x.useValue is going to be nonisolated(nonsending) so we
+  // are going to merge x into the actor. And useValueNonIsolatedNonSending
+  // inherits from the context.
+  func test() async {
+    let x = ObjCObject()
+    await x.useValue(field)
+    await useValueNonIsolatedNonSending(x)
+  }  
+}
diff --git a/test/Concurrency/attr_execution/attr_execution.swift b/test/Concurrency/attr_execution/attr_execution.swift
index 93702c1c8aacb..c9f855cb770c9 100644
--- a/test/Concurrency/attr_execution/attr_execution.swift
+++ b/test/Concurrency/attr_execution/attr_execution.swift
@@ -18,7 +18,7 @@ func callerTest() async {}
 struct Test {
   // CHECK-LABEL: // closure #1 in variable initialization expression of Test.x
   // CHECK: // Isolation: caller_isolation_inheriting
-  // CHECK: sil private [ossa] @$s14attr_execution4TestV1xyyYaYCcvpfiyyYaYCcfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
+  // CHECK: sil private [ossa] @$s14attr_execution4TestV1xyyYaYCcvpfiyyYacfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
   var x: () async -> Void = {}
 
   // CHECK-LABEL: // Test.test()
@@ -67,7 +67,7 @@ func takesClosure(fn: () async -> Void) {
 }
 
 // CHECK-LABEL: sil hidden [ossa] @$s14attr_execution11testClosureyyF : $@convention(thin) () -> ()
-// CHECK:  [[CLOSURE:%.*]] = function_ref @$s14attr_execution11testClosureyyFyyYaYCXEfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
+// CHECK:  [[CLOSURE:%.*]] = function_ref @$s14attr_execution11testClosureyyFyyYaXEfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
 // CHECK:  [[THUNKED_CLOSURE:%.*]] = thin_to_thick_function %0 to $@noescape @async @callee_guaranteed (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
 // CHECK:  [[TAKES_CLOSURE:%.*]] = function_ref @$s14attr_execution12takesClosure2fnyyyYaYCXE_tF : $@convention(thin) (@guaranteed @noescape @async @callee_guaranteed (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()) -> ()
 // CHECK:  apply [[TAKES_CLOSURE]]([[THUNKED_CLOSURE]])
@@ -75,7 +75,7 @@ func takesClosure(fn: () async -> Void) {
 
 // CHECK-LABEL: // closure #1 in testClosure()
 // CHECK: // Isolation: caller_isolation_inheriting
-// CHECK: sil private [ossa] @$s14attr_execution11testClosureyyFyyYaYCXEfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
+// CHECK: sil private [ossa] @$s14attr_execution11testClosureyyFyyYaXEfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
 func testClosure() {
   takesClosure {
   }
diff --git a/test/Concurrency/attr_execution/conversions_silgen.swift b/test/Concurrency/attr_execution/conversions_silgen.swift
index 7b97a59fc57e5..2271c686ed172 100644
--- a/test/Concurrency/attr_execution/conversions_silgen.swift
+++ b/test/Concurrency/attr_execution/conversions_silgen.swift
@@ -421,7 +421,7 @@ func conversionsFromSyncToAsync(_ x: @escaping @Sendable (NonSendableKlass) -> V
 }
 
 func testThatClosuresAssumeIsolation(fn: inout nonisolated(nonsending) (Int) async -> Void) {
-  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYaYCcfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYacfU_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> () {
   // CHECK: bb0([[EXECUTOR:%.*]] : @guaranteed $Optional<any Actor>):
   // CHECK: hop_to_executor [[EXECUTOR]]
   let _: nonisolated(nonsending) () async -> Void = {
@@ -430,7 +430,7 @@ func testThatClosuresAssumeIsolation(fn: inout nonisolated(nonsending) (Int) asy
 
   func testParam(_: nonisolated(nonsending) () async throws -> Void) {}
 
-  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYaYCXEfU0_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> @error any Error
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFyyYaXEfU0_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> () {
   // CHECK: bb0([[EXECUTOR:%.*]] : @guaranteed $Optional<any Actor>):
   // CHECK: hop_to_executor [[EXECUTOR]]
   testParam { 42 }
@@ -440,7 +440,7 @@ func testThatClosuresAssumeIsolation(fn: inout nonisolated(nonsending) (Int) asy
   // CHECK: hop_to_executor [[GENERIC_EXECUTOR]]
   testParam { @concurrent in 42 }
 
-  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFySiYaYCcfU2_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, Int) -> ()
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen31testThatClosuresAssumeIsolation2fnyySiYaYCcz_tFySiYacfU2_ : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, Int) -> () {
   // CHECK: bb0([[EXECUTOR:%.*]] : @guaranteed $Optional<any Actor>, %1 : $Int):
   // CHECK: hop_to_executor [[EXECUTOR]]
   fn = { _ in }
diff --git a/test/Concurrency/concurrent_value_checking.swift b/test/Concurrency/concurrent_value_checking.swift
index 0738d4ea0dd0a..7dc503268a0d3 100644
--- a/test/Concurrency/concurrent_value_checking.swift
+++ b/test/Concurrency/concurrent_value_checking.swift
@@ -1,9 +1,11 @@
 // RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete -parse-as-library %s -emit-sil -o /dev/null -verify -DALLOW_TYPECHECKER_ERRORS -verify-additional-prefix typechecker- -verify-additional-prefix tns-allow-typechecker-
 
-// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete -parse-as-library %s -emit-sil -o /dev/null -verify -verify-additional-prefix tns-
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete -parse-as-library %s -emit-sil -o /dev/null -verify -verify-additional-prefix tns-ni- -verify-additional-prefix tns-
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete -parse-as-library %s -emit-sil -o /dev/null -verify -verify-additional-prefix tns-ni-ns- -verify-additional-prefix tns- -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 class NotConcurrent { } // expected-note 12{{class 'NotConcurrent' does not conform to the 'Sendable' protocol}}
 // expected-tns-allow-typechecker-note @-1 {{class 'NotConcurrent' does not conform to the 'Sendable' protocol}}
diff --git a/test/Concurrency/concurrent_value_checking_objc.swift b/test/Concurrency/concurrent_value_checking_objc.swift
index a08c6a3ebb2bc..5b16ce46233de 100644
--- a/test/Concurrency/concurrent_value_checking_objc.swift
+++ b/test/Concurrency/concurrent_value_checking_objc.swift
@@ -1,8 +1,10 @@
-// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete %s -emit-sil -o /dev/null -verify
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete %s -emit-sil -o /dev/null -verify -verify-additional-prefix ni-
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -strict-concurrency=complete %s -emit-sil -o /dev/null -verify -verify-additional-prefix ni-ns- -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: objc_interop
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 import Foundation
 
diff --git a/test/Concurrency/nonisolated_inherits_isolation.swift b/test/Concurrency/nonisolated_inherits_isolation.swift
index ec1e6d8211d84..82dc816633761 100644
--- a/test/Concurrency/nonisolated_inherits_isolation.swift
+++ b/test/Concurrency/nonisolated_inherits_isolation.swift
@@ -13,7 +13,12 @@
 // Declarations //
 //////////////////
 
-class NonSendableKlass {}
+class NonSendableKlass {
+  func unspecifiedCaller() async {}
+  nonisolated func nonisolatedCaller() async {}
+  nonisolated(nonsending) func nonisolatedNonSendingCaller() async {}
+  @concurrent func concurrentCaller() async {}
+}
 
 nonisolated class NonIsolatedNonSendableKlass {
   func unspecifiedMethod() async {}
@@ -24,6 +29,8 @@ func unspecifiedSyncUse<T>(_ t: T) {}
 func unspecifiedAsyncUse<T>(_ t: T) async {}
 nonisolated func nonisolatedSyncUse<T>(_ t: T) {}
 nonisolated func nonisolatedAsyncUse<T>(_ t: T) async {}
+nonisolated(nonsending) func nonisolatedNonSendingAsyncUse<T>(_ t: T) async {}
+@concurrent func concurrentAsyncUse<T>(_ t: T) async {}
 
 func unspecifiedSyncUseWithResult<T>(_ t: T) -> T { t }
 func unspecifiedAsyncUseWithResult<T>(_ t: T) async -> T { t }
@@ -34,6 +41,8 @@ func unspecifiedSyncResult() -> NonSendableKlass { fatalError() }
 func unspecifiedAsyncResult() async -> NonSendableKlass { fatalError() }
 nonisolated func nonisolatedSyncResult() -> NonSendableKlass { fatalError() }
 nonisolated func nonisolatedAsyncResult() async -> NonSendableKlass { fatalError() }
+func sendingParameter<T>(_ t: sending T) async {}
+func useValue<T>(_ t: T) {}
 
 @MainActor func sendToMain<T>(_ t: T) async {}
 
@@ -49,6 +58,13 @@ struct CustomActor {
 
 @CustomActor func sendToCustom<T>(_ t: T) async {}
 
+@MainActor
+final class MainActorKlass {
+  var ns = NonSendableKlass()
+
+  func useValueAsync(_ x: NonSendableKlass) async {}
+}
+
 ///////////
 // Tests //
 ///////////
@@ -90,22 +106,17 @@ actor ActorTest {
     await sendToMain(x1)
 
     let x2 = await unspecifiedAsyncResult()
-    await sendToMain(x2) // expected-enabled-error {{sending 'x2' risks causing data races}}
-    // expected-enabled-note @-1 {{sending 'self'-isolated 'x2' to main actor-isolated global function 'sendToMain' risks causing data races between main actor-isolated and 'self'-isolated uses}}
+    await sendToMain(x2)
 
     let x3 = nonisolatedSyncResult()
     await sendToMain(x3)
 
     let x4 = await nonisolatedAsyncResult()
-    await sendToMain(x4) // expected-enabled-error {{sending 'x4' risks causing data races}}
-    // expected-enabled-note @-1 {{sending 'self'-isolated 'x4' to main actor-isolated global function 'sendToMain' risks causing data races between main actor-isolated and 'self'-isolated uses}}
+    await sendToMain(x4)
   }
 }
 
-@MainActor
-class MainActorKlass {
-  var ns = NonSendableKlass()
-
+extension MainActorKlass {
   func callNonIsolatedWithParam() async {
     unspecifiedSyncUse(ns)
     await unspecifiedAsyncUse(ns) // expected-disabled-error {{sending 'self.ns' risks causing data races}}
@@ -141,15 +152,13 @@ class MainActorKlass {
     await sendToCustom(x1)
 
     let x2 = await unspecifiedAsyncResult()
-    await sendToCustom(x2) // expected-enabled-error {{sending 'x2' risks causing data races}}
-    // expected-enabled-note @-1 {{sending main actor-isolated 'x2' to global actor 'CustomActor'-isolated global function 'sendToCustom' risks causing data races between global actor 'CustomActor'-isolated and main actor-isolated uses}}
+    await sendToCustom(x2)
 
     let x3 = nonisolatedSyncResult()
     await sendToCustom(x3)
 
     let x4 = await nonisolatedAsyncResult()
-    await sendToCustom(x4) // expected-enabled-error {{sending 'x4' risks causing data races}}
-    // expected-enabled-note @-1 {{sending main actor-isolated 'x4' to global actor 'CustomActor'-isolated global function 'sendToCustom' risks causing data races between global actor 'CustomActor'-isolated and main actor-isolated uses}}
+    await sendToCustom(x4)
   }
 }
 
@@ -166,3 +175,171 @@ func validateNonisolatedOnClassMeansCallerIsolationInheritingOnFuncDecl(
   await c.nonisolatedMethod() // expected-disabled-error {{sending 'c' risks causing data races}}
   // expected-disabled-note @-1 {{sending main actor-isolated 'c' to nonisolated instance method 'nonisolatedMethod()' risks causing data races between nonisolated and main actor-isolated uses}}
 }
+
+// Shouldn't get an error here since we are not using after we send to main.
+func nonisolatedCallingNonIsolated() async {
+  let c = NonSendableKlass()
+
+  await unspecifiedAsyncUse(c)
+  await unspecifiedAsyncUse(c)
+
+  await sendToMain(c)
+}
+
+func nonisolatedCallingNonIsolated2() async {
+  let c = NonSendableKlass()
+
+  await unspecifiedAsyncUse(c)
+  await unspecifiedAsyncUse(c)
+
+  await sendToMain(c) // expected-error {{sending 'c' risks causing data races}}
+  // expected-note @-1 {{sending 'c' to main actor-isolated global function 'sendToMain' risks causing data races between main actor-isolated and local nonisolated uses}}
+
+  await unspecifiedAsyncUse(c) // expected-note {{access can happen concurrently}}
+}
+
+// We should emit an error here despite the function context being @MainActor.
+@MainActor
+func sendingWithMainActor() async {
+  let c = NonSendableKlass()
+  await sendingParameter(c) // expected-error {{sending 'c' risks causing data races}}
+  // expected-note @-1 {{'c' used after being passed as a 'sending' parameter}}
+  useValue(c) // expected-note {{access can happen concurrently}}
+}
+
+// We should emit an error here despite the function context have an explicit
+// isolated parameter.
+func sendingWithIsolatedParam(_ a: isolated Optional<Actor>) async {
+  let c = NonSendableKlass()
+  await sendingParameter(c) // expected-error {{sending 'c' risks causing data races}}
+  // expected-note @-1 {{'c' used after being passed as a 'sending' parameter}}
+  useValue(c) // expected-note {{access can happen concurrently}}
+}
+
+// This errors only when disabled since the first time we call
+// unspecifiedAsyncUse we have a disconnected value and the second time we have
+// a value in a main actor isolated region and the unspecifiedAsyncUse runs on
+// the main actor.
+//
+// TODO: This doesn't error if we have a non-final class because of a known bug
+// where we infer isolation wrong for non-final class setters. That is why
+// MainActorKlass is made final so we can test this appropriately.
+@MainActor
+func testUnrolledLoop(_ a: MainActorKlass) async {
+  let k = NonSendableKlass()
+  await unspecifiedAsyncUse(k)
+  a.ns = k
+  await unspecifiedAsyncUse(k) // expected-disabled-error {{sending 'k' risks causing data races}}
+  // expected-disabled-note @-1 {{sending main actor-isolated 'k' to nonisolated global function 'unspecifiedAsyncUse' risks causing data races between nonisolated and main actor-isolated uses}}
+}
+
+// We emit an error in both modes since we are now in an @MainActor isolated
+// function.
+func testUnrolledLoop2(_ a: MainActorKlass) async {
+  let k = NonSendableKlass()
+  await unspecifiedAsyncUse(k)
+  await a.useValueAsync(k) // expected-error {{sending 'k' risks causing data races}}
+  // expected-note @-1 {{sending 'k' to main actor-isolated instance method 'useValueAsync' risks causing data races between main actor-isolated and local nonisolated uses}}
+  await unspecifiedAsyncUse(k) // expected-note {{access can happen concurrently}}
+}
+
+func testUnrolledLoopWithAsyncLet(_ a: MainActorKlass) async {
+  let k = NonSendableKlass()
+  await unspecifiedAsyncUse(k)
+  // This is valid since our valid is disconnected here.
+  async let value: () = await unspecifiedAsyncUse(k)
+  _ = await value
+
+  await a.useValueAsync(k) // expected-error {{sending 'k' risks causing data races}}
+  // expected-note @-1 {{sending 'k' to main actor-isolated instance method 'useValueAsync' risks causing data races between main actor-isolated and local nonisolated uses}}
+  async let value2: () = await unspecifiedAsyncUse(k) // expected-note {{access can happen concurrently}}
+  _ = await value2
+}
+
+@MainActor
+func testUnrolledLoopWithAsyncLet2(_ a: MainActorKlass) async {
+  let k = NonSendableKlass()
+  await unspecifiedAsyncUse(k)
+  // This is valid since our valid is disconnected here.
+  async let value: () = await unspecifiedAsyncUse(k)
+  _ = await value
+
+  await a.useValueAsync(k)
+  async let value2: () = await unspecifiedAsyncUse(k) // expected-error {{sending 'k' risks causing data races}}
+  // expected-note @-1 {{sending main actor-isolated 'k' into async let risks causing data races between nonisolated and main actor-isolated uses}}
+  _ = await value2
+}
+
+@MainActor
+func testUnrolledLoopWithAsyncLet3(_ a: MainActorKlass) async {
+  let k = NonSendableKlass()
+  await unspecifiedAsyncUse(k)
+  // This is valid since our valid is disconnected here.
+  async let value: () = await unspecifiedAsyncUse(k)
+  _ = await value
+
+  await a.useValueAsync(k)
+  async let value2: () = await sendToMain(k) // expected-error {{sending 'k' risks causing data races}}
+  // expected-note @-1 {{sending main actor-isolated 'k' into async let risks causing data races between nonisolated and main actor-isolated uses}}
+  _ = await value2
+}
+
+func unspecifiedCallingVariousNonisolated(_ x: NonSendableKlass) async {
+  await x.unspecifiedCaller()
+  await x.nonisolatedCaller()
+  await x.nonisolatedNonSendingCaller()
+  await x.concurrentCaller() // expected-enabled-error {{sending 'x' risks causing data races}}
+  // expected-enabled-note @-1 {{sending task-isolated 'x' to @concurrent instance method 'concurrentCaller()' risks causing data races between @concurrent and task-isolated uses}}
+
+  await unspecifiedAsyncUse(x)
+  await nonisolatedAsyncUse(x)
+  await nonisolatedNonSendingAsyncUse(x)
+  await concurrentAsyncUse(x) // expected-enabled-error {{sending 'x' risks causing data races}}
+  // expected-enabled-note @-1 {{sending task-isolated 'x' to @concurrent global function 'concurrentAsyncUse' risks causing data races between @concurrent and task-isolated uses}}
+}
+
+nonisolated func nonisolatedCallingVariousNonisolated(_ x: NonSendableKlass) async {
+  await x.unspecifiedCaller()
+  await x.nonisolatedCaller()
+  await x.nonisolatedNonSendingCaller()
+  await x.concurrentCaller() // expected-enabled-error {{sending 'x' risks causing data races}}
+  // expected-enabled-note @-1 {{sending task-isolated 'x' to @concurrent instance method 'concurrentCaller()' risks causing data races between @concurrent and task-isolated uses}}
+
+  await unspecifiedAsyncUse(x)
+  await nonisolatedAsyncUse(x)
+  await nonisolatedNonSendingAsyncUse(x)
+  await concurrentAsyncUse(x) // expected-enabled-error {{sending 'x' risks causing data races}}
+  // expected-enabled-note @-1 {{sending task-isolated 'x' to @concurrent global function 'concurrentAsyncUse' risks causing data races between @concurrent and task-isolated uses}}
+}
+
+nonisolated(nonsending) func nonisolatedNonSendingCallingVariousNonisolated(_ x: NonSendableKlass) async {
+  await x.unspecifiedCaller() // expected-disabled-error {{sending 'x' risks causing data races}}
+  // expected-disabled-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated instance method 'unspecifiedCaller()' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  await x.nonisolatedCaller() // expected-disabled-error {{sending 'x' risks causing data races}}
+  // expected-disabled-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated instance method 'nonisolatedCaller()' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  await x.nonisolatedNonSendingCaller()
+  await x.concurrentCaller() // expected-error {{sending 'x' risks causing data races}}
+  // expected-disabled-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated instance method 'concurrentCaller()' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  // expected-enabled-note @-2 {{sending task-isolated 'x' to @concurrent instance method 'concurrentCaller()' risks causing data races between @concurrent and task-isolated uses}}
+
+  await unspecifiedAsyncUse(x) // expected-disabled-error {{sending 'x' risks causing data races}}
+  // expected-disabled-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated global function 'unspecifiedAsyncUse' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  await nonisolatedAsyncUse(x) // expected-disabled-error {{sending 'x' risks causing data races}}
+  // expected-disabled-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated global function 'nonisolatedAsyncUse' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  await nonisolatedNonSendingAsyncUse(x)
+  await concurrentAsyncUse(x) // expected-error {{sending 'x' risks causing data races}}
+  // expected-disabled-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated global function 'concurrentAsyncUse' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  // expected-enabled-note @-2 {{sending task-isolated 'x' to @concurrent global function 'concurrentAsyncUse' risks causing data races between @concurrent and task-isolated uses}}
+}
+
+@concurrent func concurrentCallingVariousNonisolated(_ x: NonSendableKlass) async {
+  await x.unspecifiedCaller()
+  await x.nonisolatedCaller()
+  await x.nonisolatedNonSendingCaller()
+  await x.concurrentCaller()
+
+  await unspecifiedAsyncUse(x)
+  await nonisolatedAsyncUse(x)
+  await nonisolatedNonSendingAsyncUse(x)
+  await concurrentAsyncUse(x)
+}
diff --git a/test/Concurrency/sendable_checking.swift b/test/Concurrency/sendable_checking.swift
index 3ffdc6b7a206a..9b9a96103cc43 100644
--- a/test/Concurrency/sendable_checking.swift
+++ b/test/Concurrency/sendable_checking.swift
@@ -1,9 +1,11 @@
-// RUN: %target-swift-frontend -target %target-swift-5.0-abi-triple -verify -strict-concurrency=targeted -verify-additional-prefix targeted- -emit-sil -o /dev/null %s
-// RUN: %target-swift-frontend -target %target-swift-5.0-abi-triple -verify -strict-concurrency=complete -verify-additional-prefix tns- -verify-additional-prefix complete-and-tns- -emit-sil -o /dev/null %s
+// RUN: %target-swift-frontend -target %target-swift-5.0-abi-triple -verify -verify-additional-prefix targeted-and-ni- -strict-concurrency=targeted -verify-additional-prefix targeted- -emit-sil -o /dev/null %s
+// RUN: %target-swift-frontend -target %target-swift-5.0-abi-triple -verify -strict-concurrency=complete -verify-additional-prefix tns-ni- -verify-additional-prefix tns- -emit-sil -o /dev/null %s -swift-version 5 -verify-additional-prefix targeted-and-ni-
+// RUN: %target-swift-frontend -target %target-swift-5.0-abi-triple -verify -strict-concurrency=complete -verify-additional-prefix tns-ni-ns- -verify-additional-prefix tns- -emit-sil -o /dev/null %s -enable-upcoming-feature NonisolatedNonsendingByDefault -swift-version 5 -DNONISOLATEDNONSENDING
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
 // REQUIRES: OS=macosx
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 @available(SwiftStdlib 5.1, *)
 struct NS1 { }
@@ -15,7 +17,7 @@ extension NS1: Sendable { }
 
 @available(SwiftStdlib 5.1, *)
 struct NS2 { // expected-note {{consider making struct 'NS2' conform to the 'Sendable' protocol}}
-  // expected-complete-and-tns-note @-1 {{consider making struct 'NS2' conform to the 'Sendable' protocol}}
+  // expected-tns-note @-1 {{consider making struct 'NS2' conform to the 'Sendable' protocol}}
   var ns1: NS1
 }
 
@@ -27,7 +29,7 @@ extension NS3: Sendable { }
 
 @available(SwiftStdlib 5.1, *)
 class NS4 { } // expected-note {{class 'NS4' does not conform to the 'Sendable' protocol}}
-// expected-complete-and-tns-note @-1 {{class 'NS4' does not conform to the 'Sendable' protocol}}
+// expected-tns-note @-1 {{class 'NS4' does not conform to the 'Sendable' protocol}}
 
 @available(SwiftStdlib 5.1, *)
 func acceptCV<T: Sendable>(_: T) { }
@@ -44,15 +46,15 @@ func testCV(
 
   acceptCV(ns1array) // expected-warning {{conformance of 'NS1' to 'Sendable' is unavailable}}
 
-  acceptCV(ns2) // expected-complete-and-tns-warning {{type 'NS2' does not conform to the 'Sendable' protocol}}
+  acceptCV(ns2) // expected-tns-warning {{type 'NS2' does not conform to the 'Sendable' protocol}}
 
   acceptCV(ns3) // expected-warning {{conformance of 'NS3' to 'Sendable' is only available in macOS 11.0 or newer}}
   // expected-note @-1 {{add 'if #available' version check}}
 
-  acceptCV(ns4) // expected-complete-and-tns-warning {{type 'NS4' does not conform to the 'Sendable' protocol}}
+  acceptCV(ns4) // expected-tns-warning {{type 'NS4' does not conform to the 'Sendable' protocol}}
 
-  acceptCV(fn) // expected-complete-and-tns-warning {{type '() -> Void' does not conform to the 'Sendable' protocol}}
-  // expected-complete-and-tns-note @-1 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+  acceptCV(fn) // expected-tns-warning {{type '() -> Void' does not conform to the 'Sendable' protocol}}
+  // expected-tns-note @-1 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 
   acceptSendableFn(fn) // expected-warning{{passing non-Sendable parameter 'fn' to function expecting a '@Sendable' closure}}
 }
@@ -93,8 +95,8 @@ public actor MyActor: MyProto {
 
   func g(ns1: NS1) async {
     await nonisolatedAsyncFunc1(ns1)
-    // expected-tns-warning @-1 {{sending 'ns1' risks causing data races}}
-    // expected-tns-note @-2 {{sending 'self'-isolated 'ns1' to nonisolated global function 'nonisolatedAsyncFunc1' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-tns-ni-warning @-1 {{sending 'ns1' risks causing data races}}
+    // expected-tns-ni-note @-2 {{sending 'self'-isolated 'ns1' to nonisolated global function 'nonisolatedAsyncFunc1' risks causing data races between nonisolated and 'self'-isolated uses}}
     _ = await nonisolatedAsyncFunc2()
   }
 }
@@ -162,18 +164,18 @@ class Super {
 
   @MainActor
   func bar (x : () -> ()) async {}
-  // expected-note@-1 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
+  // expected-targeted-and-ni-note @-1 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 
   @MainActor
   func foo2<T : Sendable>(x: T) async {}
 
   @MainActor
   func bar2<T>(x: T) async {}
-  // expected-note@-1 {{consider making generic parameter 'T' conform to the 'Sendable' protocol}} {{14-14=: Sendable}}
+  // expected-targeted-and-ni-note @-1 {{consider making generic parameter 'T' conform to the 'Sendable' protocol}} {{14-14=: Sendable}}
 
   @MainActor
   func bar3<T: Equatable>(x: T) async {}
-  // expected-note@-1 {{consider making generic parameter 'T' conform to the 'Sendable' protocol}} {{25-25= & Sendable}}
+  // expected-targeted-and-ni-note @-1 {{consider making generic parameter 'T' conform to the 'Sendable' protocol}} {{25-25= & Sendable}}
 }
 
 // Make sure isolation crossing overrides check sendability
@@ -182,16 +184,25 @@ class Super {
 class Sub : Super {
   override nonisolated func foo(x : () -> ()) async {}
 
+  // We do not emit an error for the override with
+  // NonisolatedNonsendingByDefault since the vtable thunk passes in MainActor
+  // so we are still in the main actor even though we are nonisolated. In the
+  // case of that being disabled, we cannot do this since we will hop off the
+  // actor.
   override nonisolated func bar(x : () -> ()) async {}
-  // expected-warning@-1 {{non-Sendable parameter type '() -> ()' cannot be sent from caller of superclass instance method 'bar(x:)' into nonisolated override}}
+  // expected-targeted-and-ni-warning@-1 {{non-Sendable parameter type '() -> ()' cannot be sent from caller of superclass instance method 'bar(x:)' into nonisolated override}}
 
   override nonisolated func foo2<T>(x: T) async {}
 
+  // See comment above about why nonisolated overrides of superclass are allowed
+  // when is enabled NonisolatedNonsendingByDefault.
   override nonisolated func bar2<T>(x: T) async {}
-  // expected-warning@-1 {{non-Sendable parameter type 'T' cannot be sent from caller of superclass instance method 'bar2(x:)' into nonisolated override}}
+  // expected-targeted-and-ni-warning @-1 {{non-Sendable parameter type 'T' cannot be sent from caller of superclass instance method 'bar2(x:)' into nonisolated override}}
 
+  // See comment above about why nonisolated overrides of superclass are allowed
+  // when is enabled NonisolatedNonsendingByDefault.
   override nonisolated func bar3<T>(x: T) async {}
-  // expected-warning@-1 {{non-Sendable parameter type 'T' cannot be sent from caller of superclass instance method 'bar3(x:)' into nonisolated override}}
+  // expected-targeted-and-ni-warning @-1 {{non-Sendable parameter type 'T' cannot be sent from caller of superclass instance method 'bar3(x:)' into nonisolated override}}
 }
 
 @available(SwiftStdlib 5.1, *)
@@ -217,7 +228,8 @@ class SubWSafeSubscript : SuperWSafeSubscript {
 class SuperWUnsafeSubscript {
   @MainActor
   subscript<T>(x : T) -> Int {
-    // expected-note@-1 2{{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+    // expected-targeted-and-ni-note @-1 {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+    // expected-targeted-and-ni-note @-2 {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
     get async {
       return 0
     }
@@ -226,10 +238,15 @@ class SuperWUnsafeSubscript {
 
 @available(SwiftStdlib 5.1, *)
 class SubWUnsafeSubscript : SuperWUnsafeSubscript {
+  // We do not emit an error for the override with
+  // NonisolatedNonsendingByDefault since the vtable thunk passes in MainActor
+  // so we are still in the main actor even though we are nonisolated. In the
+  // case of that being disabled, we cannot do this since we will hop off the
+  // actor.
   override nonisolated subscript<T>(x : T) -> Int {
     get async {
-      // expected-warning@-2{{non-Sendable parameter type 'T' cannot be sent from caller of superclass subscript 'subscript(_:)' into nonisolated override}}
-      // expected-warning@-2{{non-Sendable parameter type 'T' cannot be sent from caller of superclass getter for subscript 'subscript(_:)' into nonisolated override}}
+      // expected-targeted-and-ni-warning@-2{{non-Sendable parameter type 'T' cannot be sent from caller of superclass subscript 'subscript(_:)' into nonisolated override}}
+      // expected-targeted-and-ni-warning@-2{{non-Sendable parameter type 'T' cannot be sent from caller of superclass getter for subscript 'subscript(_:)' into nonisolated override}}
       // there really shouldn't be two warnings produced here, see rdar://110846040 (Sendable diagnostics reported twice for subscript getters)
       return 0
     }
@@ -263,7 +280,7 @@ struct CustomActor {
 final class NonSendable {
   // expected-note @-1 5 {{class 'NonSendable' does not conform to the 'Sendable' protocol}}
   // TransferNonSendable emits 3 fewer errors here.
-  // expected-complete-and-tns-note @-3 {{class 'NonSendable' does not conform to the 'Sendable' protocol}}
+  // expected-tns-note @-3 {{class 'NonSendable' does not conform to the 'Sendable' protocol}}
   var value = ""
 
   @MainActor
@@ -341,6 +358,7 @@ func testNonSendableBaseArg3() async {
 @Sendable
 func globalSendable(_ ns: NonSendable) async {}
 
+#if !NONISOLATEDNONSENDING
 @available(SwiftStdlib 5.1, *)
 @MainActor
 func callNonisolatedAsyncClosure(
@@ -348,14 +366,15 @@ func callNonisolatedAsyncClosure(
   g: (NonSendable) async -> Void
 ) async {
   await g(ns)
-  // expected-tns-warning @-1 {{sending 'ns' risks causing data races}}
-  // expected-tns-note @-2 {{sending main actor-isolated 'ns' to nonisolated callee risks causing data races between nonisolated and main actor-isolated uses}}
+  // expected-tns-ni-warning @-1 {{sending 'ns' risks causing data races}}
+  // expected-tns-ni-note @-2 {{sending main actor-isolated 'ns' to nonisolated callee risks causing data races between nonisolated and main actor-isolated uses}}
 
   let f: (NonSendable) async -> () = globalSendable // okay
   await f(ns)
-  // expected-tns-warning @-1 {{sending 'ns' risks causing data races}}
-  // expected-tns-note @-2 {{sending main actor-isolated 'ns' to nonisolated callee risks causing data races between nonisolated and main actor-isolated uses}}
+  // expected-tns-ni-warning @-1 {{sending 'ns' risks causing data races}}
+  // expected-tns-ni-note @-2 {{sending main actor-isolated 'ns' to nonisolated callee risks causing data races between nonisolated and main actor-isolated uses}}
 }
+#endif
 
 @available(SwiftStdlib 5.1, *)
 func testLocalCaptures() {
@@ -364,7 +383,7 @@ func testLocalCaptures() {
   @Sendable
   func a2() -> NonSendable {
     return ns
-    // expected-complete-and-tns-warning @-1 {{capture of 'ns' with non-Sendable type 'NonSendable' in a '@Sendable' local function}}
+    // expected-tns-warning @-1 {{capture of 'ns' with non-Sendable type 'NonSendable' in a '@Sendable' local function}}
   }
 }
 
diff --git a/test/Concurrency/transfernonsendable.swift b/test/Concurrency/transfernonsendable.swift
index ae26109da4a91..755a37371d8f1 100644
--- a/test/Concurrency/transfernonsendable.swift
+++ b/test/Concurrency/transfernonsendable.swift
@@ -1,4 +1,5 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -disable-availability-checking -verify -verify-additional-prefix tns-  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -disable-availability-checking -verify -verify-additional-prefix tns-ni- -verify-additional-prefix tns- %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -disable-availability-checking -verify -verify-additional-prefix tns-ni-ns- -verify-additional-prefix tns-  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // This run validates that for specific test cases around closures, we properly
 // emit errors in the type checker before we run sns. This ensures that we know that
@@ -8,6 +9,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
@@ -69,6 +71,7 @@ final class FinalMainActorIsolatedKlass {
 func useInOut<T>(_ x: inout T) {}
 func useValue<T>(_ x: T) {}
 func useValueAsync<T>(_ x: T) async {}
+@concurrent func useValueAsyncConcurrent<T>(_ x: T) async {}
 
 @MainActor func transferToMain<T>(_ t: T) async {}
 
@@ -106,15 +109,15 @@ enum MyEnum<T> {
 extension MyActor {
   func warningIfCallingGetter() async {
     await self.klass.asyncCall() // expected-complete-warning {{passing argument of non-Sendable type 'NonSendableKlass' outside of actor-isolated context may introduce data races}}
-    // expected-tns-warning @-1 {{sending 'self.klass' risks causing data races}}
-    // expected-tns-note @-2 {{sending 'self'-isolated 'self.klass' to nonisolated instance method 'asyncCall()' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-tns-ni-warning @-1 {{sending 'self.klass' risks causing data races}}
+    // expected-tns-ni-note @-2 {{sending 'self'-isolated 'self.klass' to nonisolated instance method 'asyncCall()' risks causing data races between nonisolated and 'self'-isolated uses}}
   }
 
   func warningIfCallingAsyncOnFinalField() async {
     // Since we are calling finalKlass directly, we emit a warning here.
     await self.finalKlass.asyncCall() // expected-complete-warning {{passing argument of non-Sendable type 'NonSendableKlass' outside of actor-isolated context may introduce data races}}
-    // expected-tns-warning @-1 {{sending 'self.finalKlass' risks causing data races}}
-    // expected-tns-note @-2 {{sending 'self'-isolated 'self.finalKlass' to nonisolated instance method 'asyncCall()' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-tns-ni-warning @-1 {{sending 'self.finalKlass' risks causing data races}}
+    // expected-tns-ni-note @-2 {{sending 'self'-isolated 'self.finalKlass' to nonisolated instance method 'asyncCall()' risks causing data races between nonisolated and 'self'-isolated uses}}
   }
 
   // We do not warn on this since we warn in the caller of our getter instead.
@@ -127,8 +130,8 @@ extension FinalActor {
   func warningIfCallingAsyncOnFinalField() async {
     // Since our whole class is final, we emit the error directly here.
     await self.klass.asyncCall() // expected-complete-warning {{passing argument of non-Sendable type 'NonSendableKlass' outside of actor-isolated context may introduce data races}}
-    // expected-tns-warning @-1 {{sending 'self.klass' risks causing data races}}
-    // expected-tns-note @-2 {{sending 'self'-isolated 'self.klass' to nonisolated instance method 'asyncCall()' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-tns-ni-warning @-1 {{sending 'self.klass' risks causing data races}}
+    // expected-tns-ni-note @-2 {{sending 'self'-isolated 'self.klass' to nonisolated instance method 'asyncCall()' risks causing data races between nonisolated and 'self'-isolated uses}}
   }
 }
 
@@ -1572,8 +1575,12 @@ func functionArgumentIntoClosure(_ x: @escaping () -> ()) async {
   let a = MainActorIsolatedKlass()
   var c = NonSendableKlass()
   for _ in 0..<1024 {
-    await useValueAsync(c) // expected-tns-warning {{sending 'c' risks causing data races}}
-    // expected-tns-note @-1 {{sending main actor-isolated 'c' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+    // This works with nonisolated(nonsending) since the first time through the
+    // loop we are disconnected... meaning that we are ok. The second time
+    // through the loop, we are now main actor isolated, but again b/c we
+    // inherit isolation, we are ok.
+    await useValueAsync(c) // expected-tns-ni-warning {{sending 'c' risks causing data races}}
+    // expected-tns-ni-note @-1 {{sending main actor-isolated 'c' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
     // expected-complete-warning @-2 {{passing argument of non-Sendable type 'NonSendableKlass' outside of main actor-isolated context may introduce data races}}
     c = a.klass
   }
@@ -1583,8 +1590,8 @@ func functionArgumentIntoClosure(_ x: @escaping () -> ()) async {
   let a = MainActorIsolatedKlass()
   var c = NonSendableKlass()
   for _ in 0..<1024 {
-    await useValueAsync(c) // expected-tns-warning {{sending 'c' risks causing data races}}
-    // expected-tns-note @-1 {{sending main actor-isolated 'c' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+    await useValueAsync(c) // expected-tns-ni-warning {{sending 'c' risks causing data races}}
+    // expected-tns-ni-note @-1 {{sending main actor-isolated 'c' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
     // expected-complete-warning @-2 {{passing argument of non-Sendable type 'NonSendableKlass' outside of main actor-isolated context may introduce data races}}
     c = a.klassLet
   }
@@ -1832,8 +1839,8 @@ actor FunctionWithSendableResultAndIsolationActor {
 @MainActor
 func previouslyBrokenTestCase(ns: NonSendableKlass) async -> SendableGenericStruct? {
   return await { () -> SendableGenericStruct? in
-    return await ns.getSendableGenericStructAsync() // expected-tns-warning {{sending 'ns' risks causing data races}}
-    // expected-tns-note @-1 {{sending main actor-isolated 'ns' to nonisolated instance method 'getSendableGenericStructAsync()' risks causing data races between nonisolated and main actor-isolated uses}}
+    return await ns.getSendableGenericStructAsync() // expected-tns-ni-warning {{sending 'ns' risks causing data races}}
+    // expected-tns-ni-note @-1 {{sending main actor-isolated 'ns' to nonisolated instance method 'getSendableGenericStructAsync()' risks causing data races between nonisolated and main actor-isolated uses}}
   }()
 }
 
@@ -2068,3 +2075,10 @@ func inferLocationOfCapturedTaskIsolatedSelfCorrectly() {
   }
 }
 
+nonisolated(nonsending) func testCallNonisolatedNonsending(_ x: NonSendableKlass) async {
+  await useValueAsync(x) // expected-tns-ni-warning {{sending 'x' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  await useValueAsyncConcurrent(x) // expected-tns-warning {{sending 'x' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending nonisolated(nonsending) task-isolated 'x' to nonisolated global function 'useValueAsyncConcurrent' risks causing data races between nonisolated and nonisolated(nonsending) task-isolated uses}}
+  // expected-tns-ni-ns-note @-2 {{sending task-isolated 'x' to @concurrent global function 'useValueAsyncConcurrent' risks causing data races between @concurrent and task-isolated uses}}
+}
diff --git a/test/Concurrency/transfernonsendable_asynclet.swift b/test/Concurrency/transfernonsendable_asynclet.swift
index 1408f828d39b9..f00bb44a5876b 100644
--- a/test/Concurrency/transfernonsendable_asynclet.swift
+++ b/test/Concurrency/transfernonsendable_asynclet.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -verify-additional-prefix ni-
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault -verify-additional-prefix ni-ns-
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
@@ -84,7 +86,8 @@ struct TwoFieldKlassBox {
 func asyncLet_Let_ActorIsolated_Simple1() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
   useValue(x) // expected-note {{access can happen concurrently}}
   let _ = await y
 }
@@ -92,7 +95,8 @@ func asyncLet_Let_ActorIsolated_Simple1() async {
 func asyncLet_Let_ActorIsolated_Simple2() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
 
   let _ = await y
@@ -102,7 +106,8 @@ func asyncLet_Let_ActorIsolated_Simple2() async {
 func asyncLet_Let_ActorIsolated_Simple3() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}  
 
   // TODO: We shouldn't emit the 2nd error here given the current implementation
   // since it is only accessible along the else path but we already hit
@@ -120,7 +125,8 @@ func asyncLet_Let_ActorIsolated_Simple3() async {
 func asyncLet_Let_ActorIsolated_Simple4() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   if await booleanFlag {
     useValue(x) // expected-note {{access can happen concurrently}}
@@ -133,7 +139,8 @@ func asyncLet_Let_ActorIsolated_Simple4() async {
 func asyncLet_Let_ActorIsolated_Simple5() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   if await booleanFlag {
     let _ = await y
@@ -148,7 +155,8 @@ func asyncLet_Let_ActorIsolated_Simple5() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsClass1() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x.field) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -157,7 +165,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsClass1() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsClass2() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x.field) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.field) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -166,7 +175,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsClass2() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsClass3() async {
   let x = NonSendableKlass()
   async let y = transferToMainInt(x.field) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.field2) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -177,7 +187,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsClass3() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsStruct1() async {
   let x = TwoFieldKlassBox()
   async let y = transferToMainInt(x.k1) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -186,7 +197,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsStruct1() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsStruct2() async {
   let x = TwoFieldKlassBox()
   async let y = transferToMainInt(x.k1) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.k1) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -195,7 +207,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsStruct2() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsStruct3() async {
   let x = TwoFieldKlassBox()
   async let y = transferToMainInt(x.k1) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.k2) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -204,7 +217,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsStruct3() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsStruct4() async {
   let x = TwoFieldKlassBox()
   async let y = transferToMainInt(x.k2.field2) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.k1.field) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -215,7 +229,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsStruct4() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsTuple1() async {
   let x = (TwoFieldKlassBox(), TwoFieldKlassBox())
   async let y = transferToMainInt(x.0.k1) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -224,7 +239,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsTuple1() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsTuple2() async {
   let x = (TwoFieldKlassBox(), TwoFieldKlassBox())
   async let y = transferToMainInt(x.0.k1) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.1) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -233,7 +249,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsTuple2() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsTuple3() async {
   let x = (TwoFieldKlassBox(), TwoFieldKlassBox())
   async let y = transferToMainInt(x.0.k1) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.1.k2) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -242,7 +259,8 @@ func asyncLet_Let_ActorIsolated_AccessFieldsTuple3() async {
 func asyncLet_Let_ActorIsolated_AccessFieldsTuple4() async {
   let x = (TwoFieldKlassBox(), TwoFieldKlassBox())
   async let y = transferToMainInt(x.1.k1.field2) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x.0.k2.field) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -253,9 +271,11 @@ func asyncLet_Let_ActorIsolated_CallBuriedInOtherExpr1() async {
   let x2 = NonSendableKlass()
 
   async let y = useValue(transferToMainInt(x) + transferToMainInt(x2)) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-warning @-2 {{sending 'x2' risks causing data races}}
-  // expected-note @-3 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-warning @-3 {{sending 'x2' risks causing data races}}
+  // expected-ni-note @-4 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-5 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -267,9 +287,11 @@ func asyncLet_Let_ActorIsolated_CallBuriedInOtherExpr2() async {
   let x2 = NonSendableKlass()
 
   async let y = useValue(transferToMainInt(x) + transferToMainInt(x2)) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-warning @-2 {{sending 'x2' risks causing data races}}
-  // expected-note @-3 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-warning @-3 {{sending 'x2' risks causing data races}}
+  // expected-ni-note @-4 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-5 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x2) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -282,9 +304,11 @@ func asyncLet_Let_ActorIsolated_CallBuriedInOtherExpr3() async {
   let x2 = NonSendableKlass()
 
   async let y = useValue(transferToMainInt(x) + transferToMainInt(x2)) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-warning @-2 {{sending 'x2' risks causing data races}}
-  // expected-note @-3 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-warning @-3 {{sending 'x2' risks causing data races}}
+  // expected-ni-note @-4 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-5 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   // We only error on the first value captured if multiple values are captured
   // since we track a single partial_apply as a transfer instruction.
@@ -298,8 +322,9 @@ func asyncLet_Let_ActorIsolated_CallBuriedInOtherExpr4() async {
 
   async let y = useValue(transferToMainInt(x) + transferToMainInt(x))
   // expected-warning @-1:26 {{sending 'x' risks causing data races}}
-  // expected-note @-2:26 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-note @-3:49 {{access can happen concurrently}}
+  // expected-ni-note @-2:26 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-3:26 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-note @-4:49 {{access can happen concurrently}}
 
   let _ = await y
 }
@@ -311,8 +336,9 @@ func asyncLet_Let_ActorIsolated_CallBuriedInOtherExpr5() async {
 
   async let y = useValue(transferToMainInt(x) + transferToCustomInt(x))
   // expected-warning @-1 {{sending 'x' risks causing data races}}
-  // expected-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-note @-3:49 {{access can happen concurrently}}
+  // expected-ni-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-3 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-note @-4:49 {{access can happen concurrently}}
 
   let _ = await y
 }
@@ -323,8 +349,9 @@ func asyncLet_Let_ActorIsolated_MultipleAsyncLet1() async {
   let x = NonSendableKlass()
 
   async let y = useValue(transferToMainInt(x)), z = useValue(transferToMainInt(x)) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-note @-2:53 {{access can happen concurrently}}
+  // expected-ni-note @-1 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-note @-3:53 {{access can happen concurrently}}
 
   let _ = await y
   let _ = await z
@@ -347,7 +374,8 @@ func asyncLet_Let_ActorIsolated_MultipleAsyncLet3() async {
 
   async let y = useValue(transferToMainInt(x)), z = useValue(transferToMainInt(x2))
   // expected-warning @-1 {{sending 'x2' risks causing data races}}
-  // expected-note @-2 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-2 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-3 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   useValue(x2) // expected-note {{access can happen concurrently}}
   let _ = await y
@@ -360,9 +388,11 @@ func asyncLet_Let_ActorIsolated_MultipleAsyncLet4() async {
 
   async let y = useValue(transferToMainInt(x)), z = useValue(transferToMainInt(x2))
   // expected-warning @-1 {{sending 'x' risks causing data races}}
-  // expected-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-warning @-3 {{sending 'x2' risks causing data races}}
-  // expected-note @-4 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-3 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-warning @-4 {{sending 'x2' risks causing data races}}
+  // expected-ni-note @-5 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-6 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   let _ = await y
   let _ = await z
@@ -376,9 +406,11 @@ func asyncLet_Let_ActorIsolated_MultipleAsyncLet5() async {
 
   async let y = useValue(transferToMainInt(x)), z = useValue(transferToMainInt(x2))
   // expected-warning @-1 {{sending 'x' risks causing data races}}
-  // expected-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-warning @-3 {{sending 'x2' risks causing data races}}
-  // expected-note @-4 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-3 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-warning @-4 {{sending 'x2' risks causing data races}}
+  // expected-ni-note @-5 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-6 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   let _ = await y
   useValue(x) // expected-note {{access can happen concurrently}}
@@ -392,9 +424,11 @@ func asyncLet_Let_ActorIsolated_MultipleAsyncLet6() async {
 
   async let y = useValue(transferToMainInt(x)), z = useValue(transferToMainInt(x2))
   // expected-warning @-1 {{sending 'x' risks causing data races}}
-  // expected-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
-  // expected-warning @-3 {{sending 'x2' risks causing data races}}
-  // expected-note @-4 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-2 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-3 {{sending 'x' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
+  // expected-warning @-4 {{sending 'x2' risks causing data races}}
+  // expected-ni-note @-5 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-6 {{sending 'x2' to main actor-isolated global function 'transferToMainInt' risks causing data races between main actor-isolated and local @concurrent uses}}
 
   let _ = await y
   useValue(x) // expected-note {{access can happen concurrently}}
@@ -722,7 +756,8 @@ func asyncLetWithoutCapture() async {
   //
   // NOTE: Error below will go away in next commit.
   async let x: NonSendableKlass = await returnValueFromMain()
-  // expected-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'returnValueFromMain()' to nonisolated context}}
+  // expected-ni-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'returnValueFromMain()' to nonisolated context}}
+  // expected-ni-ns-warning @-2 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'returnValueFromMain()' to @concurrent context}}
   let y = await x
   await transferToMain(y) // expected-warning {{sending 'y' risks causing data races}}
   // expected-note @-1 {{sending 'y' to main actor-isolated global function 'transferToMain' risks causing data races between main actor-isolated and local nonisolated uses}}
@@ -734,7 +769,8 @@ func asyncLet_Let_ActorIsolated_Method() async {
   let a = MyActor()
   let x = NonSendableKlass()
   async let y = a.useKlass(x) // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{sending 'x' to actor-isolated instance method 'useKlass' risks causing data races between actor-isolated and local nonisolated uses}}
+  // expected-ni-note @-1 {{sending 'x' to actor-isolated instance method 'useKlass' risks causing data races between actor-isolated and local nonisolated uses}}
+  // expected-ni-ns-note @-2 {{sending 'x' to actor-isolated instance method 'useKlass' risks causing data races between actor-isolated and local @concurrent uses}}
 
   useValue(x) // expected-note {{access can happen concurrently}}
   let _ = await y
diff --git a/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift b/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift
index 0c52bb1d73cbd..fb3669656dfd4 100644
--- a/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift
+++ b/test/Concurrency/transfernonsendable_closureliterals_isolationinference.swift
@@ -1,8 +1,11 @@
-// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -swift-version 5 -strict-concurrency=complete %s -o - | %FileCheck %s
-// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -swift-version 6 -verify %s -o /dev/null
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -swift-version 5 -strict-concurrency=complete %s -o - 2>/dev/null | %FileCheck %s
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -swift-version 6 -verify %s -o /dev/null -verify-additional-prefix ni-
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -swift-version 5 -strict-concurrency=complete %s -o - -enable-upcoming-feature NonisolatedNonsendingByDefault 2>/dev/null | %FileCheck %s
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -swift-version 6 -verify %s -o /dev/null -enable-upcoming-feature NonisolatedNonsendingByDefault -verify-additional-prefix ni-ns-
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // This test validates the behavior of transfernonsendable around
 // closure literals
@@ -204,8 +207,8 @@ func test_CallerAsyncNormal_CalleeSyncNonIsolated() async {
 
     // CHECK-LABEL: closure #1 in test_CallerAsyncNormal_CalleeSyncNonIsolated()
     // CHECK-NEXT: Isolation: global_actor. type: CustomActor
-    await asyncNormalAcceptsClosure { } // expected-error {{sending value of non-Sendable type '() -> ()' risks causing data races}}
-    // expected-note @-1 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() -> ()' to nonisolated global function 'asyncNormalAcceptsClosure' risks causing races in between global actor 'CustomActor'-isolated and nonisolated uses}}
+    await asyncNormalAcceptsClosure { } // expected-ni-error {{sending value of non-Sendable type '() -> ()' risks causing data races}}
+    // expected-ni-note @-1 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() -> ()' to nonisolated global function 'asyncNormalAcceptsClosure' risks causing races in between global actor 'CustomActor'-isolated and nonisolated uses}}
 
     // CHECK-LABEL: closure #2 in test_CallerAsyncNormal_CalleeSyncNonIsolated()
     // CHECK-NEXT: Isolation: nonisolated
@@ -244,8 +247,8 @@ func test_CallerAsyncNormal_CalleeAsyncNonIsolated() async {
 
     // CHECK-LABEL: closure #1 in test_CallerAsyncNormal_CalleeAsyncNonIsolated()
     // CHECK-NEXT: Isolation: global_actor. type: CustomActor
-    await asyncNormalAcceptsAsyncClosure { } // expected-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-    // expected-note @-1 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() async -> ()' to nonisolated global function 'asyncNormalAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and nonisolated uses}}
+    await asyncNormalAcceptsAsyncClosure { } // expected-ni-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+    // expected-ni-note @-1 {{sending global actor 'CustomActor'-isolated value of non-Sendable type '() async -> ()' to nonisolated global function 'asyncNormalAcceptsAsyncClosure' risks causing races in between global actor 'CustomActor'-isolated and nonisolated uses}}
 
     // CHECK-LABEL: closure #2 in test_CallerAsyncNormal_CalleeAsyncNonIsolated()
     // CHECK-NEXT: Isolation: nonisolated
@@ -432,8 +435,8 @@ extension MyActor {
 
         // CHECK-LABEL: closure #1 in MyActor.test_CallerAsyncNormal_CalleeSyncNonIsolated()
         // CHECK-NEXT: Isolation: actor_instance. name: 'self'
-        await asyncNormalAcceptsClosure { print(self) } // expected-error {{sending value of non-Sendable type '() -> ()' risks causing data races}}
-        // expected-note @-1 {{sending 'self'-isolated value of non-Sendable type '() -> ()' to nonisolated global function 'asyncNormalAcceptsClosure' risks causing races in between 'self'-isolated and nonisolated uses}}
+        await asyncNormalAcceptsClosure { print(self) } // expected-ni-error {{sending value of non-Sendable type '() -> ()' risks causing data races}}
+        // expected-ni-note @-1 {{sending 'self'-isolated value of non-Sendable type '() -> ()' to nonisolated global function 'asyncNormalAcceptsClosure' risks causing races in between 'self'-isolated and nonisolated uses}}
 
         // CHECK-LABEL: closure #2 in MyActor.test_CallerAsyncNormal_CalleeSyncNonIsolated()
         // CHECK-NEXT: Isolation: nonisolated
@@ -473,8 +476,8 @@ extension MyActor {
 
         // CHECK-LABEL: closure #1 in MyActor.test_CallerAsyncNormal_CalleeAsyncNonIsolated()
         // CHECK-NEXT: Isolation: actor_instance. name: 'self'
-        await asyncNormalAcceptsAsyncClosure { print(self) } // expected-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
-        // expected-note @-1 {{sending 'self'-isolated value of non-Sendable type '() async -> ()' to nonisolated global function 'asyncNormalAcceptsAsyncClosure' risks causing races in between 'self'-isolated and nonisolated uses}}
+        await asyncNormalAcceptsAsyncClosure { print(self) } // expected-ni-error {{sending value of non-Sendable type '() async -> ()' risks causing data races}}
+        // expected-ni-note @-1 {{sending 'self'-isolated value of non-Sendable type '() async -> ()' to nonisolated global function 'asyncNormalAcceptsAsyncClosure' risks causing races in between 'self'-isolated and nonisolated uses}}
 
         // CHECK-LABEL: closure #2 in MyActor.test_CallerAsyncNormal_CalleeAsyncNonIsolated()
         // CHECK-NEXT: Isolation: nonisolated
diff --git a/test/Concurrency/transfernonsendable_functionsubtyping.swift b/test/Concurrency/transfernonsendable_functionsubtyping.swift
index de7b2fc8bb893..22739fcc20053 100644
--- a/test/Concurrency/transfernonsendable_functionsubtyping.swift
+++ b/test/Concurrency/transfernonsendable_functionsubtyping.swift
@@ -1,4 +1,5 @@
 // RUN: %target-typecheck-verify-swift -swift-version 6
+// RUN: %target-typecheck-verify-swift -swift-version 6 -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // READ THIS! This file only contains tests that validate that the relevant
 // function subtyping rules for sending work. Please do not put other tests in
@@ -6,6 +7,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
diff --git a/test/Concurrency/transfernonsendable_global_actor.swift b/test/Concurrency/transfernonsendable_global_actor.swift
index fa440e65ec268..b991837f2fd01 100644
--- a/test/Concurrency/transfernonsendable_global_actor.swift
+++ b/test/Concurrency/transfernonsendable_global_actor.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix tns-  %s -o /dev/null -parse-as-library -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix tns-ni- -verify-additional-prefix tns-  %s -o /dev/null -parse-as-library -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix tns-ni-ns- -verify-additional-prefix tns-  %s -o /dev/null -parse-as-library -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
@@ -150,8 +152,8 @@ private struct StructContainingValue { // expected-complete-note 2{{}}
   var x = StructContainingValue()
   x.x = firstList
 
-  await transferToNonIsolated(x) // expected-tns-warning {{sending 'x' risks causing data races}}
-  // expected-tns-note @-1 {{sending global actor 'CustomActor'-isolated 'x' to nonisolated global function 'transferToNonIsolated' risks causing data races between nonisolated and global actor 'CustomActor'-isolated uses}}
+  await transferToNonIsolated(x) // expected-tns-ni-warning {{sending 'x' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending global actor 'CustomActor'-isolated 'x' to nonisolated global function 'transferToNonIsolated' risks causing data races between nonisolated and global actor 'CustomActor'-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type 'StructContainingValue' outside of global actor 'CustomActor'-isolated context may introduce data races}}
 
   useValue(x)
@@ -174,8 +176,8 @@ private struct StructContainingValue { // expected-complete-note 2{{}}
 
   x.1 = firstList
 
-  await transferToNonIsolated(x) // expected-tns-warning {{sending 'x' risks causing data races}}
-  // expected-tns-note @-1 {{sending global actor 'CustomActor'-isolated 'x' to nonisolated global function 'transferToNonIsolated' risks causing data races between nonisolated and global actor 'CustomActor'-isolated uses}}
+  await transferToNonIsolated(x) // expected-tns-ni-warning {{sending 'x' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending global actor 'CustomActor'-isolated 'x' to nonisolated global function 'transferToNonIsolated' risks causing data races between nonisolated and global actor 'CustomActor'-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type '(NonSendableLinkedList<Int>, NonSendableLinkedList<Int>)' outside of global actor 'CustomActor'-isolated context may introduce data races}}
   // expected-complete-warning @-3 {{passing argument of non-Sendable type '(NonSendableLinkedList<Int>, NonSendableLinkedList<Int>)' outside of global actor 'CustomActor'-isolated context may introduce data races}}
 
@@ -195,8 +197,8 @@ struct Clock {
 // We used to crash when inferring the type for the diagnostic below.
 @MainActor func testIndirectParametersHandledCorrectly() async {
   let c = Clock()
-  let _: Int = await c.measure { // expected-tns-warning {{sending value of non-Sendable type '() async -> Int' risks causing data races}}
-    // expected-tns-note @-1 {{sending main actor-isolated value of non-Sendable type '() async -> Int' to nonisolated instance method 'measure' risks causing races in between main actor-isolated and nonisolated uses}}
+  let _: Int = await c.measure { // expected-tns-ni-warning {{sending value of non-Sendable type '() async -> Int' risks causing data races}}
+    // expected-tns-ni-note @-1 {{sending main actor-isolated value of non-Sendable type '() async -> Int' to nonisolated instance method 'measure' risks causing races in between main actor-isolated and nonisolated uses}}
     try! await c.sleep()
   }
 }
@@ -255,8 +257,8 @@ struct Clock {
 
   let erased: () -> Void = closure
 
-  await useValueAsync(erased) // expected-tns-warning {{sending 'erased' risks causing data races}}
-  // expected-tns-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-tns-ni-warning {{sending 'erased' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type '() -> Void' outside of main actor-isolated context may introduce data races}}
   // expected-complete-note @-3 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 }
@@ -264,8 +266,8 @@ struct Clock {
 @MainActor func synchronousActorIsolatedFunctionError() async {
   let erased: () -> Void = mainActorFunction
 
-  await useValueAsync(erased) // expected-tns-warning {{sending 'erased' risks causing data races}}
-  // expected-tns-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-tns-ni-warning {{sending 'erased' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type '() -> Void' outside of main actor-isolated context may introduce data races}}
   // expected-complete-note @-3 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 }
@@ -273,8 +275,8 @@ struct Clock {
 @MainActor func synchronousActorIsolatedGenericFunctionError<T>(_ t: T) async {
   let erased: (T) -> Void = useValueMainActor
 
-  await useValueAsync(erased) // expected-tns-warning {{sending 'erased' risks causing data races}}
-  // expected-tns-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-tns-ni-warning {{sending 'erased' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type '(T) -> Void' outside of main actor-isolated context may introduce data races}}
   // expected-complete-note @-3 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 }
@@ -287,8 +289,8 @@ struct Clock {
   let t = Test()
   let erased: () -> Void = t.foo
 
-  await useValueAsync(erased) // expected-tns-warning {{sending 'erased' risks causing data races}}
-  // expected-tns-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-tns-ni-warning {{sending 'erased' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type '() -> Void' outside of main actor-isolated context may introduce data races}}
   // expected-complete-note @-3 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 }
@@ -301,8 +303,8 @@ struct Clock {
   let t = Test()
   let erased: () -> Void = t.foo
 
-  await useValueAsync(erased) // expected-tns-warning {{sending 'erased' risks causing data races}}
-  // expected-tns-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-tns-ni-warning {{sending 'erased' risks causing data races}}
+  // expected-tns-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
   // expected-complete-warning @-2 {{passing argument of non-Sendable type '() -> Void' outside of main actor-isolated context may introduce data races}}
   // expected-complete-note @-3 {{a function type must be marked '@Sendable' to conform to 'Sendable'}}
 }
diff --git a/test/Concurrency/transfernonsendable_global_actor_nonsendable.swift b/test/Concurrency/transfernonsendable_global_actor_nonsendable.swift
index f4fe9669ef6af..b6553a9cc6807 100644
--- a/test/Concurrency/transfernonsendable_global_actor_nonsendable.swift
+++ b/test/Concurrency/transfernonsendable_global_actor_nonsendable.swift
@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -parse-as-library
+// RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -parse-as-library -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // READ THIS: This test is testing specifically behavior around global actor
 // isolated types that are nonsendable. This is a bit of a corner case so we use
@@ -6,6 +7,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
diff --git a/test/Concurrency/transfernonsendable_global_actor_sending.swift b/test/Concurrency/transfernonsendable_global_actor_sending.swift
index a0fe45df8881c..37c446a1eee62 100644
--- a/test/Concurrency/transfernonsendable_global_actor_sending.swift
+++ b/test/Concurrency/transfernonsendable_global_actor_sending.swift
@@ -1,9 +1,13 @@
 // RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -parse-as-library
+// RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -parse-as-library -enable-upcoming-feature NonisolatedNonsendingByDefault
+
 
 // README: Once we loosen the parser so that sending is rejected in Sema
 // instead of the parser, move into the normal
 // transfernonsendable_global_actor.swift
 
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
+
 ////////////////////////
 // MARK: Declarations //
 ////////////////////////
@@ -30,9 +34,8 @@ func useValue<T>(_ t: T) {}
 @MainActor func testGlobalFakeInit() {
   let ns = NonSendableKlass()
 
-  // Will be resolved once @MainActor is @Sendable.
-  Task.fakeInit { @MainActor in // expected-error {{passing closure as a 'sending' parameter risks causing data races between main actor-isolated code and concurrent execution of the closure}}
-    print(ns) // expected-note {{closure captures 'ns' which is accessible to main actor-isolated code}}
+  Task.fakeInit { @MainActor in
+    print(ns)
   }
 
   useValue(ns)
diff --git a/test/Concurrency/transfernonsendable_global_actor_swift6.swift b/test/Concurrency/transfernonsendable_global_actor_swift6.swift
index a8bf80094f830..9c18e13977c77 100644
--- a/test/Concurrency/transfernonsendable_global_actor_swift6.swift
+++ b/test/Concurrency/transfernonsendable_global_actor_swift6.swift
@@ -1,4 +1,5 @@
-// RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -parse-as-library
+// RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix ni- %s -o /dev/null -parse-as-library
+// RUN: %target-swift-frontend -emit-sil -swift-version 6 -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix ni-ns- %s -o /dev/null -parse-as-library -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // README: This is testing specific patterns around global actors that are
 // slightly different in between swift 5 and swift 6. The normal global actor
@@ -6,6 +7,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
@@ -44,22 +46,22 @@ var booleanFlag: Bool { false }
 
   let erased: () -> Void = closure
 
-  await useValueAsync(erased) // expected-error {{sending 'erased' risks causing data races}}
-  // expected-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-ni-error {{sending 'erased' risks causing data races}}
+  // expected-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
 }
 
 @MainActor func synchronousActorIsolatedFunctionError() async {
   let erased: () -> Void = mainActorFunction
 
-  await useValueAsync(erased) // expected-error {{sending 'erased' risks causing data races}}
-  // expected-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-ni-error {{sending 'erased' risks causing data races}}
+  // expected-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
 }
 
 @MainActor func synchronousActorIsolatedGenericFunctionError<T>(_ t: T) async {
   let erased: (T) -> Void = useValueMainActor
 
-  await useValueAsync(erased) // expected-error {{sending 'erased' risks causing data races}}
-  // expected-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-ni-error {{sending 'erased' risks causing data races}}
+  // expected-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
 }
 
 @MainActor func synchronousActorIsolatedClassMethodError() async {
@@ -70,8 +72,8 @@ var booleanFlag: Bool { false }
   let t = Test()
   let erased: () -> Void = t.foo
 
-  await useValueAsync(erased) // expected-error {{sending 'erased' risks causing data races}}
-  // expected-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-ni-error {{sending 'erased' risks causing data races}}
+  // expected-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
 }
 
 @MainActor func synchronousActorIsolatedFinalClassMethodError() async {
@@ -82,6 +84,6 @@ var booleanFlag: Bool { false }
   let t = Test()
   let erased: () -> Void = t.foo
 
-  await useValueAsync(erased) // expected-error {{sending 'erased' risks causing data races}}
-  // expected-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
+  await useValueAsync(erased) // expected-ni-error {{sending 'erased' risks causing data races}}
+  // expected-ni-note @-1 {{sending main actor-isolated 'erased' to nonisolated global function 'useValueAsync' risks causing data races between nonisolated and main actor-isolated uses}}
 }
diff --git a/test/Concurrency/transfernonsendable_initializers.swift b/test/Concurrency/transfernonsendable_initializers.swift
index 4eb41f9ec759d..b298fadbb3835 100644
--- a/test/Concurrency/transfernonsendable_initializers.swift
+++ b/test/Concurrency/transfernonsendable_initializers.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -swift-version 6 -verify %s -o /dev/null
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -swift-version 6 -verify -verify-additional-prefix ni- %s -o /dev/null
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -swift-version 6 -verify -verify-additional-prefix ni-ns- %s -o /dev/null -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // This test validates the behavior of transfernonsendable around initializers.
 
@@ -34,7 +36,8 @@ actor ActorWithSynchronousNonIsolatedInit {
     // TODO: This should say actor isolated.
     let _ = { @MainActor in
       print(newK) // expected-error {{sending 'newK' risks causing data races}}
-      // expected-note @-1 {{task-isolated 'newK' is captured by a main actor-isolated closure. main actor-isolated uses in closure may race against later nonisolated uses}}
+      // expected-ni-note @-1 {{task-isolated 'newK' is captured by a main actor-isolated closure. main actor-isolated uses in closure may race against later nonisolated uses}}
+      // expected-ni-ns-note @-2 {{task-isolated 'newK' is captured by a main actor-isolated closure. main actor-isolated uses in closure may race against later nonisolated uses}}
     }
   }
 
diff --git a/test/Concurrency/transfernonsendable_isolationcrossing_partialapply.swift b/test/Concurrency/transfernonsendable_isolationcrossing_partialapply.swift
index c24a94e778149..0a293980726a1 100644
--- a/test/Concurrency/transfernonsendable_isolationcrossing_partialapply.swift
+++ b/test/Concurrency/transfernonsendable_isolationcrossing_partialapply.swift
@@ -1,7 +1,9 @@
 // RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -disable-availability-checking -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -disable-availability-checking -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // This test validates how we handle partial applies that are isolated to a
 // specific isolation domain (causing isolation crossings to occur).
diff --git a/test/Concurrency/transfernonsendable_nonisolated.swift b/test/Concurrency/transfernonsendable_nonisolated.swift
index 73aaf5b82225e..6e208f6a2bb54 100644
--- a/test/Concurrency/transfernonsendable_nonisolated.swift
+++ b/test/Concurrency/transfernonsendable_nonisolated.swift
@@ -1,7 +1,9 @@
 // RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // This test validates behavior of transfernonsendable around nonisolated
 // functions and fields.
diff --git a/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift b/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift
index ab9fb1752b5a1..13f1c2e414442 100644
--- a/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift
+++ b/test/Concurrency/transfernonsendable_nonisolatedunsafe.swift
@@ -1,10 +1,12 @@
 // RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix tns-  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -verify-additional-prefix tns-  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // READ THIS: This test is intended to centralize all tests that use
 // nonisolated(unsafe).
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
diff --git a/test/Concurrency/transfernonsendable_objc.swift b/test/Concurrency/transfernonsendable_objc.swift
index b726878b9c7d0..56099d5097b84 100644
--- a/test/Concurrency/transfernonsendable_objc.swift
+++ b/test/Concurrency/transfernonsendable_objc.swift
@@ -1,8 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -enable-upcoming-feature RegionBasedIsolation -target %target-swift-5.1-abi-triple %s -o /dev/null -import-objc-header %S/Inputs/transfernonsendable_objc.h -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple %s -o /dev/null -import-objc-header %S/Inputs/transfernonsendable_objc.h -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple %s -o /dev/null -import-objc-header %S/Inputs/transfernonsendable_objc.h -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: objc_interop
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
-// REQUIRES: swift_feature_RegionBasedIsolation
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 import Foundation
 
diff --git a/test/Concurrency/transfernonsendable_ownership.swift b/test/Concurrency/transfernonsendable_ownership.swift
index fe0fd7102e736..68bed57c74b37 100644
--- a/test/Concurrency/transfernonsendable_ownership.swift
+++ b/test/Concurrency/transfernonsendable_ownership.swift
@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -enable-upcoming-feature GlobalActorIsolatedTypesUsability %s -o /dev/null
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify -enable-upcoming-feature GlobalActorIsolatedTypesUsability %s -o /dev/null -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // This test validates the behavior of transfer non sendable around ownership
 // constructs like non copyable types, consuming/borrowing parameters, and inout
@@ -6,6 +7,7 @@
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
diff --git a/test/Concurrency/transfernonsendable_preconcurrency.swift b/test/Concurrency/transfernonsendable_preconcurrency.swift
index 08f87c3fa89cf..7577948bf8c27 100644
--- a/test/Concurrency/transfernonsendable_preconcurrency.swift
+++ b/test/Concurrency/transfernonsendable_preconcurrency.swift
@@ -11,9 +11,11 @@
 
 // Test swift 6
 // RUN: %target-swift-frontend -swift-version 6 %s -emit-sil -o /dev/null -verify -verify-additional-prefix swift-6- -parse-as-library -I %t -target %target-swift-5.1-abi-triple
+// RUN: %target-swift-frontend -swift-version 6 %s -emit-sil -o /dev/null -verify -verify-additional-prefix swift-6- -parse-as-library -I %t -target %target-swift-5.1-abi-triple -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // README: This test is meant to test the interaction of transfernonsendable and
 // preconcurrency. Please only keep such tests in this file.
diff --git a/test/Concurrency/transfernonsendable_preconcurrency_sending.swift b/test/Concurrency/transfernonsendable_preconcurrency_sending.swift
index dd2d68a356723..a46ee44de1dc2 100644
--- a/test/Concurrency/transfernonsendable_preconcurrency_sending.swift
+++ b/test/Concurrency/transfernonsendable_preconcurrency_sending.swift
@@ -11,9 +11,11 @@
 
 // Test swift 6
 // RUN: %target-swift-frontend -swift-version 6 %s -emit-sil -o /dev/null -verify -verify-additional-prefix swift-6- -parse-as-library -I %t -target %target-swift-5.1-abi-triple
+// RUN: %target-swift-frontend -swift-version 6 %s -emit-sil -o /dev/null -verify -verify-additional-prefix swift-6- -parse-as-library -I %t -target %target-swift-5.1-abi-triple -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // README: This test is meant to test the interaction of transfernonsendable,
 // preconcurrency, and transferring. Please only keep such tests in this file.
diff --git a/test/Concurrency/transfernonsendable_rbi_result.swift b/test/Concurrency/transfernonsendable_rbi_result.swift
index ea730c83b2d92..fc86d0ca4519d 100644
--- a/test/Concurrency/transfernonsendable_rbi_result.swift
+++ b/test/Concurrency/transfernonsendable_rbi_result.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -swift-version 6 -parse-as-library %s -emit-sil -o /dev/null -verify
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -swift-version 6 -disable-availability-checking -parse-as-library %s -emit-sil -o /dev/null -verify -verify-additional-prefix ni-
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -swift-version 6 -disable-availability-checking -parse-as-library %s -emit-sil -o /dev/null -verify -verify-additional-prefix ni-ns- -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: asserts
 // REQUIRES: concurrency
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ///////////////////////
 // MARK: Declaration //
@@ -20,9 +22,13 @@ struct CustomActor {
 class NonSendable {} // expected-note 3{{}}
 
 func passNonSendable(_: NonSendable) async { }
+func passTwoNonSendable(_: NonSendable, _: NonSendable) async { }
 
 func returnsNonSendable() async -> NonSendable { NonSendable() }
 
+@MainActor
+func sendToMain<T>(_ t: T) async {}
+
 @MainActor
 func mainActorPassNonSendable(_: NonSendable) async { }
 
@@ -93,9 +99,22 @@ actor A {
   func callNonisolatedFuncsFromActor(ns: NonSendable) async {
     // Non-sendable value passed from nonisolated to actor isolated
 
+    // We do not error in ni-ns since we just merge pass non sendable.
     await passNonSendable(ns)
-    // expected-error @-1 {{sending 'ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'ns' to nonisolated global function 'passNonSendable' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-ni-error @-1 {{sending 'ns' risks causing data races}}
+    // expected-ni-note @-2 {{sending 'self'-isolated 'ns' to nonisolated global function 'passNonSendable' risks causing data races between nonisolated and 'self'-isolated uses}}
+
+    // We error here in both modes but in different places. In NI mode, we error
+    // on passing ns to a nonisolated function. With ni-ns, we do not error
+    // there since we inherit isolation from the caller.
+    //
+    // In contrast, when ni-ns is enabled, we error on sendToMain since we merge
+    // ns2 into ns's region causing it to be self-isolated.
+    let ns2 = NonSendable()
+    await passTwoNonSendable(ns, ns2) // expected-ni-error {{sending 'ns' risks causing data races}}
+    // expected-ni-note @-1 {{sending 'self'-isolated 'ns' to nonisolated global function 'passTwoNonSendable' risks causing data races between nonisolated and 'self'-isolated uses}}
+    await sendToMain(ns2) // expected-ni-ns-error {{sending 'ns2' risks causing data races}}
+    // expected-ni-ns-note @-1 {{sending 'self'-isolated 'ns2' to main actor-isolated global function 'sendToMain' risks causing data races between main actor-isolated and 'self'-isolated uses}}
 
     _ = await returnsNonSendable()
   }
diff --git a/test/Concurrency/transfernonsendable_region_based_sendability.swift b/test/Concurrency/transfernonsendable_region_based_sendability.swift
index 30932af4c7dbb..1c39c643de782 100644
--- a/test/Concurrency/transfernonsendable_region_based_sendability.swift
+++ b/test/Concurrency/transfernonsendable_region_based_sendability.swift
@@ -1,7 +1,9 @@
 // RUN: %target-swift-frontend  -strict-concurrency=complete -target %target-swift-5.1-abi-triple -parse-as-library -emit-sil -o /dev/null -verify -verify-additional-prefix tns- %s -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend  -strict-concurrency=complete -target %target-swift-5.1-abi-triple -parse-as-library -emit-sil -o /dev/null -verify -verify-additional-prefix tns- %s -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 /*
  This file tests the early features of the flow-sensitive Sendable checking implemented by the TransferNonSendable SIL pass.
diff --git a/test/Concurrency/transfernonsendable_sending_params.swift b/test/Concurrency/transfernonsendable_sending_params.swift
index e71857f4f2a87..55a06267503d2 100644
--- a/test/Concurrency/transfernonsendable_sending_params.swift
+++ b/test/Concurrency/transfernonsendable_sending_params.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -strict-concurrency=complete -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -strict-concurrency=complete -verify -verify-additional-prefix ni- %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -target %target-swift-5.1-abi-triple -strict-concurrency=complete -verify -verify-additional-prefix ni-ns- %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
diff --git a/test/Concurrency/transfernonsendable_sending_results.swift b/test/Concurrency/transfernonsendable_sending_results.swift
index a3615b29546fe..4ffacba9f1a6e 100644
--- a/test/Concurrency/transfernonsendable_sending_results.swift
+++ b/test/Concurrency/transfernonsendable_sending_results.swift
@@ -1,7 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -parse-as-library -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -verify-additional-prefix ni-
+// RUN: %target-swift-frontend -emit-sil -parse-as-library -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault -verify-additional-prefix ni-ns-
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 ////////////////////////
 // MARK: Declarations //
@@ -141,7 +143,8 @@ func transferInAndOut(_ x: sending NonSendableKlass) -> sending NonSendableKlass
 
 func transferReturnArg(_ x: NonSendableKlass) -> sending NonSendableKlass {
   return x // expected-warning {{sending 'x' risks causing data races}}
-  // expected-note @-1 {{task-isolated 'x' cannot be a 'sending' result. task-isolated uses may race with caller uses}}
+  // expected-ni-note @-1 {{task-isolated 'x' cannot be a 'sending' result. task-isolated uses may race with caller uses}}
+  // expected-ni-ns-note @-2 {{task-isolated 'x' cannot be a 'sending' result. task-isolated uses may race with caller uses}}
 }
 
 // TODO: This will be fixed once I represent @MainActor on func types.
@@ -238,7 +241,8 @@ func asyncLetReabstractionThunkTest() async {
 func asyncLetReabstractionThunkTest2() async {
   // We emit the error here since we are returning a main actor isolated value.
   async let newValue: NonSendableKlass = await getMainActorValueAsync()
-  // expected-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to nonisolated context}}
+  // expected-ni-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to nonisolated context}}
+  // expected-ni-ns-warning @-2 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to @concurrent context}}
 
   let _ = await newValue
 
@@ -261,7 +265,8 @@ func asyncLetReabstractionThunkTest2() async {
 @MainActor func asyncLetReabstractionThunkTestGlobalActor2() async {
   // We emit the error here since we are returning a main actor isolated value.
   async let newValue: NonSendableKlass = await getMainActorValueAsync()
-  // expected-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to nonisolated context}}
+  // expected-ni-warning @-1 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to nonisolated context}}
+  // expected-ni-ns-warning @-2 {{non-Sendable 'NonSendableKlass'-typed result can not be returned from main actor-isolated global function 'getMainActorValueAsync()' to @concurrent context}}
 
   let _ = await newValue
 
diff --git a/test/Concurrency/transfernonsendable_typed_errors.swift b/test/Concurrency/transfernonsendable_typed_errors.swift
index cade1abcc2e03..f563ecfdd843a 100644
--- a/test/Concurrency/transfernonsendable_typed_errors.swift
+++ b/test/Concurrency/transfernonsendable_typed_errors.swift
@@ -1,7 +1,9 @@
 // RUN: %target-swift-frontend -swift-version 6 -Xllvm -sil-regionbasedisolation-force-use-of-typed-errors -emit-sil -o /dev/null %s -verify -target %target-swift-5.1-abi-triple
+// RUN: %target-swift-frontend -swift-version 6 -Xllvm -sil-regionbasedisolation-force-use-of-typed-errors -emit-sil -o /dev/null %s -verify -target %target-swift-5.1-abi-triple -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: asserts
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // READ THIS: This test is only intended to test typed errors that are fallback
 // error paths that are only invoked if we can't find a name for the value being
diff --git a/test/Concurrency/transfernonsendable_unavailable_conformance.swift b/test/Concurrency/transfernonsendable_unavailable_conformance.swift
index 883b1447b07fc..aac9d80b2bb87 100644
--- a/test/Concurrency/transfernonsendable_unavailable_conformance.swift
+++ b/test/Concurrency/transfernonsendable_unavailable_conformance.swift
@@ -1,8 +1,9 @@
-// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -enable-upcoming-feature RegionBasedIsolation -target %target-swift-5.1-abi-triple -verify  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify  %s -o /dev/null -enable-upcoming-feature GlobalActorIsolatedTypesUsability -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_GlobalActorIsolatedTypesUsability
-// REQUIRES: swift_feature_RegionBasedIsolation
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // This test makes sure that we treat types with an unavailable Sendable
 // conformance as being non-Sendable.
diff --git a/test/Concurrency/transfernonsendable_warning_until_swift6.swift b/test/Concurrency/transfernonsendable_warning_until_swift6.swift
index 9f194ee040248..3548524be7c9e 100644
--- a/test/Concurrency/transfernonsendable_warning_until_swift6.swift
+++ b/test/Concurrency/transfernonsendable_warning_until_swift6.swift
@@ -1,6 +1,8 @@
 // RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -swift-version 6
+// RUN: %target-swift-frontend -emit-sil -strict-concurrency=complete -target %target-swift-5.1-abi-triple -verify %s -o /dev/null -swift-version 6 -enable-upcoming-feature NonisolatedNonsendingByDefault
 
 // REQUIRES: concurrency
+// REQUIRES: swift_feature_NonisolatedNonsendingByDefault
 
 // This test makes sure that all of our warnings are errors in swift6 mode.
 
diff --git a/test/SILOptimizer/missing_returns.swift b/test/SILOptimizer/missing_returns.swift
index e6bb0299c779c..986a46567115c 100644
--- a/test/SILOptimizer/missing_returns.swift
+++ b/test/SILOptimizer/missing_returns.swift
@@ -10,29 +10,29 @@
 
 struct MissingGetterSubscript1 {
   subscript (i : Int) -> Int {
-  } // expected-error {{missing return in getter expected to return 'Int'}}
+  } // expected-error {{missing return in getter for subscript expected to return 'Int'}}
 }
 
 // MARK: `decl/var/properties`
 
 struct X {}
 
-var x13: X {} // expected-error {{missing return in getter expected to return 'X'}}
+var x13: X {} // expected-error {{missing return in getter for var expected to return 'X'}}
 
 struct X14 {}
 extension X14 {
   var x14: X {
-  } // expected-error {{missing return in getter expected to return 'X'}}
+  } // expected-error {{missing return in getter for property expected to return 'X'}}
 }
 
 // https://github.com/apple/swift/issues/57936
 
 enum E1_57936 {
-  var foo: Int {} // expected-error{{missing return in getter expected to return 'Int'}}
+  var foo: Int {} // expected-error{{missing return in getter for property expected to return 'Int'}}
 }
 
 enum E2_57936<T> {
-  var foo: T {} // expected-error{{missing return in getter expected to return 'T'}}
+  var foo: T {} // expected-error{{missing return in getter for property expected to return 'T'}}
 }
 
 // MARK: `decl/var/result_builders`
@@ -51,7 +51,7 @@ var fv_nop: () {
 }
 
 var fv_missing: String {
-} // expected-error {{missing return in getter expected to return 'String'}}
+} // expected-error {{missing return in getter for var expected to return 'String'}}
 
 enum S_nop {
     subscript() -> () {
@@ -60,30 +60,30 @@ enum S_nop {
 
 enum S_missing {
     subscript() -> String {
-    } // expected-error {{missing return in getter expected to return 'String'}}
+    } // expected-error {{missing return in getter for subscript expected to return 'String'}}
 }
 
 // MARK: `Sema/generic-subscript`
 
 struct S_generic_subscript_missing_return {
   subscript<Value>(x: Int) -> Value {
-  }  // expected-error {{missing return in getter expected to return 'Value'}}
+  }  // expected-error {{missing return in getter for subscript expected to return 'Value'}}
 }
 
 // MARK: New Test Cases
 
 enum MyEmptyType {}
 extension MyEmptyType {
-  var i: Int {} // expected-error{{missing return in getter expected to return 'Int'}}
-  var n: MyEmptyType {} // expected-error{{getter with uninhabited return type 'MyEmptyType' is missing call to another never-returning function on all paths}}
+  var i: Int {} // expected-error{{missing return in getter for property expected to return 'Int'}}
+  var n: MyEmptyType {} // expected-error{{getter for property with uninhabited return type 'MyEmptyType' is missing call to another never-returning function on all paths}}
 
   static subscript<A>(root: MyEmptyType) -> A {}
 
   subscript(_ e: MyEmptyType) -> Int {}
   subscript<T>(_ e: MyEmptyType) -> T {}
-  subscript(_ i: Int) -> Int {} // expected-error{{missing return in getter expected to return 'Int'}}
-  subscript<T>(_ p: Int) -> T {} // expected-error{{missing return in getter expected to return 'T'}}
-  subscript(_ i: Int) -> Self {} // expected-error{{getter with uninhabited return type 'MyEmptyType' is missing call to another never-returning function on all paths}}
+  subscript(_ i: Int) -> Int {} // expected-error{{missing return in getter for subscript expected to return 'Int'}}
+  subscript<T>(_ p: Int) -> T {} // expected-error{{missing return in getter for subscript expected to return 'T'}}
+  subscript(_ i: Int) -> Self {} // expected-error{{getter for subscript with uninhabited return type 'MyEmptyType' is missing call to another never-returning function on all paths}}
   subscript(_ s: Self) -> Self {}
 
   static func unreachable_static_implicit_return(_ e: MyEmptyType) -> Int {}
@@ -97,16 +97,16 @@ extension MyEmptyType {
 }
 
 extension Never {
-  var i: Int {} // expected-error{{missing return in getter expected to return 'Int'}}
-  var n: Never {} // expected-error{{getter with uninhabited return type 'Never' is missing call to another never-returning function on all paths}}
+  var i: Int {} // expected-error{{missing return in getter for property expected to return 'Int'}}
+  var n: Never {} // expected-error{{getter for property with uninhabited return type 'Never' is missing call to another never-returning function on all paths}}
 
   static subscript<A>(root: Never) -> A {}
 
   subscript(_ n: Never) -> Int {}
   subscript<T>(_ e: Never) -> T {}
-  subscript(_ i: Int) -> Int {} // expected-error{{missing return in getter expected to return 'Int'}}
-  subscript<T>(_ p: Int) -> T {} // expected-error{{missing return in getter expected to return 'T'}}
-  subscript(_ i: Int) -> Self {} // expected-error{{getter with uninhabited return type 'Never' is missing call to another never-returning function on all paths}}
+  subscript(_ i: Int) -> Int {} // expected-error{{missing return in getter for subscript expected to return 'Int'}}
+  subscript<T>(_ p: Int) -> T {} // expected-error{{missing return in getter for subscript expected to return 'T'}}
+  subscript(_ i: Int) -> Self {} // expected-error{{getter for subscript with uninhabited return type 'Never' is missing call to another never-returning function on all paths}}
   subscript(_ s: Self) -> Self {}
 
   static func unreachable_static_implicit_return(_ n: Never) -> Int {}
@@ -128,8 +128,8 @@ enum InhabitedType {
   subscript(_ v: MyEmptyType, e: Int) -> Never {}
 
   // Inhabited params
-  subscript(_ i: Int) -> Int {} // expected-error{{missing return in getter expected to return 'Int'}}
+  subscript(_ i: Int) -> Int {} // expected-error{{missing return in getter for subscript expected to return 'Int'}}
   subscript(_ j: Int) -> Void {}
-  subscript(_ k: Int) -> Never {} // expected-error{{getter with uninhabited return type 'Never' is missing call to another never-returning function on all paths}}
+  subscript(_ k: Int) -> Never {} // expected-error{{getter for subscript with uninhabited return type 'Never' is missing call to another never-returning function on all paths}}
   // FIXME: ^ this diagnostic should probably use the word 'subscript' rather than 'getter'
 }
diff --git a/test/SILOptimizer/return.swift b/test/SILOptimizer/return.swift
index 84147017d6506..863b34349110c 100644
--- a/test/SILOptimizer/return.swift
+++ b/test/SILOptimizer/return.swift
@@ -232,14 +232,14 @@ struct S_56857 {
         if b {
             return 0
         }
-    } // expected-error {{missing return in getter expected to return 'Int'}}
+    } // expected-error {{missing return in getter for property expected to return 'Int'}}
 
     var y: Int {
         get {
             if b {
                 return 0
             }
-        } // expected-error {{missing return in getter expected to return 'Int'}}
+        } // expected-error {{missing return in getter for property expected to return 'Int'}}
         set {}
     } 
 }
diff --git a/test/Serialization/caller_isolation_inherit.swift b/test/Serialization/caller_isolation_inherit.swift
index a6d3b23442998..632d746f549ae 100644
--- a/test/Serialization/caller_isolation_inherit.swift
+++ b/test/Serialization/caller_isolation_inherit.swift
@@ -32,7 +32,7 @@ actor A {
   func test1a() async {
     await WithFeature.unspecifiedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated global function 'unspecifiedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent global function 'unspecifiedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // unspecifiedAsyncCaller<A>(_:)
@@ -48,7 +48,7 @@ actor A {
   func test2() async {
     await WithoutFeature.unspecifiedAsync(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated global function 'unspecifiedAsync' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent global function 'unspecifiedAsync' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // unspecifiedAsyncConcurrent<A>(_:)
@@ -59,7 +59,7 @@ actor A {
     // an error.
     await WithoutFeature.unspecifiedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated global function 'unspecifiedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent global function 'unspecifiedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // unspecifiedAsyncCaller<A>(_:)
@@ -82,7 +82,7 @@ actor A {
   func test3a() async {
     await WithFeature.nonisolatedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated global function 'nonisolatedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent global function 'nonisolatedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // nonisolatedAsyncCaller<A>(_:)
@@ -98,7 +98,7 @@ actor A {
   func test4() async {
     await WithoutFeature.nonisolatedAsync(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated global function 'nonisolatedAsync' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent global function 'nonisolatedAsync' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // nonisolatedAsyncConcurrent<A>(_:)
@@ -107,7 +107,7 @@ actor A {
   func test4a() async {
     await WithoutFeature.nonisolatedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated global function 'nonisolatedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent global function 'nonisolatedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // nonisolatedAsyncCaller<A>(_:)
@@ -132,7 +132,7 @@ actor A {
     let s = WithFeature.S()
     await s.unspecifiedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated instance method 'unspecifiedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent instance method 'unspecifiedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // S.unspecifiedAsyncCaller<A>(_:)
@@ -158,7 +158,7 @@ actor A {
     let s = WithFeature.S()
     await s.nonisolatedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated instance method 'nonisolatedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent instance method 'nonisolatedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // S.nonisolatedAsyncCaller<A>(_:)
@@ -176,7 +176,7 @@ actor A {
     let s = WithoutFeature.S()
     await s.unspecifiedAsync(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated instance method 'unspecifiedAsync' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent instance method 'unspecifiedAsync' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // S.unspecifiedAsyncConcurrent<A>(_:)
@@ -186,7 +186,7 @@ actor A {
     let s = WithoutFeature.S()
     await s.unspecifiedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated instance method 'unspecifiedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent instance method 'unspecifiedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // S.unspecifiedAsyncCaller<A>(_:)
@@ -204,7 +204,7 @@ actor A {
     let s = WithoutFeature.S()
     await s.nonisolatedAsync(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated instance method 'nonisolatedAsync' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent instance method 'nonisolatedAsync' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // S.nonisolatedAsyncConcurrent<A>(_:)
@@ -214,7 +214,7 @@ actor A {
     let s = WithoutFeature.S()
     await s.nonisolatedAsyncConcurrent(ns)
     // expected-error @-1 {{sending 'self.ns' risks causing data races}}
-    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to nonisolated instance method 'nonisolatedAsyncConcurrent' risks causing data races between nonisolated and 'self'-isolated uses}}
+    // expected-note @-2 {{sending 'self'-isolated 'self.ns' to @concurrent instance method 'nonisolatedAsyncConcurrent' risks causing data races between @concurrent and 'self'-isolated uses}}
   }
 
   // CHECK-LABEL: // S.nonisolatedAsyncCaller<A>(_:)