Improve misleading diagnostics regarding implicit Sendable conformances

(cherry picked from commit 7c0bdff)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -1916,6 +1916,12 @@ ERROR(type_cannot_conform, none,
 NOTE(only_concrete_types_conform_to_protocols,none,
      "only concrete types such as structs, enums and classes can conform to protocols",
      ())
+NOTE(nonsendable_function_type,none,
+     "a function type must be marked '@Sendable' to conform to 'Sendable'", ())
+NOTE(nonsendable_tuple_type,none,
+     "a tuple type must be composed of 'Sendable' elements to conform to "
+     "'Sendable'", ())
+
 NOTE(required_by_opaque_return,none,
     "required by opaque return type of %0 %1", (DescriptiveDeclKind, DeclName))
 NOTE(required_by_decl,none,

lib/Sema/CSDiagnostics.cpp

@@ -533,7 +533,21 @@ bool MissingConformanceFailure::diagnoseTypeCannotConform(
                  nonConformingType->isEqual(protocolType),
                  protocolType);
 
-  emitDiagnostic(diag::only_concrete_types_conform_to_protocols);
+  bool emittedSpecializedNote = false;
+  if (auto protoType = protocolType->getAs<ProtocolType>()) {
+    if (protoType->getDecl()->isSpecificProtocol(KnownProtocolKind::Sendable)) {
+      if (nonConformingType->is<FunctionType>()) {
+        emitDiagnostic(diag::nonsendable_function_type);
+        emittedSpecializedNote = true;
+      } else if (nonConformingType->is<TupleType>()) {
+        emitDiagnostic(diag::nonsendable_tuple_type);
+        emittedSpecializedNote = true;
+      }
+    }
+  }
+
+  if (!emittedSpecializedNote)
+    emitDiagnostic(diag::only_concrete_types_conform_to_protocols);
 
   if (auto *OTD = dyn_cast<OpaqueTypeDecl>(AffectedDecl)) {
     auto *namingDecl = OTD->getNamingDecl();

test/decl/protocol/special/Sendable.swift

@@ -22,11 +22,12 @@ func testSendableBuiltinConformances(
   // Errors
   acceptSendable((i, ns)) // expected-error{{global function 'acceptSendable' requires that 'NotSendable' conform to 'Sendable'}}
   acceptSendable(nsf) // expected-error{{type '() -> Void' cannot conform to 'Sendable'}}
-  // FIXME: expected-note@-1{{only concrete types such as structs, enums and classes can conform to protocols}}
+  // expected-note@-1{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   acceptSendable((nsf, i)) // expected-error{{type '() -> Void' cannot conform to 'Sendable'}}
-  // FIXME: expected-note@-1{{only concrete types such as structs, enums and classes can conform to protocols}}
+  // expected-note@-1{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   // expected-note@-2{{requirement from conditional conformance of '(() -> Void, Int)' to 'Sendable'}}
   acceptSendable(funNotSendable) // expected-error{{type '() -> Void' cannot conform to 'Sendable'}}
-  // FIXME: expected-note@-1{{only concrete types such as structs, enums and classes can conform to protocols}}
+  // expected-note@-1{{a function type must be marked '@Sendable' to conform to 'Sendable'}}
   // expected-note@-2{{requirement from conditional conformance of '(Int, () -> Void, NotSendable.Type)' to 'Sendable'}}
+  acceptSendable((i, ns)) // expected-error{{global function 'acceptSendable' requires that 'NotSendable' conform to 'Sendable'}}
 }