[TypeChecker] Limit sendability downgrade to preconcurrency requirements only

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -5619,6 +5619,7 @@ bool ConstraintSystem::repairFailures(
     // Matching a witness to an protocol requirement.
     if (isa<ProtocolDecl>(VD->getDeclContext()) &&
         VD->isProtocolRequirement() &&
+        VD->preconcurrency() &&
         path[0].is<LocatorPathElt::Witness>() &&
         // Note that the condition below is very important,
         // we need to wait until the very last moment to strip

lib/Sema/TypeCheckProtocol.cpp

@@ -1261,9 +1261,12 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
 
       // If there are no other issues, let's check whether this are
       // missing Sendable conformances when matching ObjC requirements.
-      // This is not an error until Swift 6 because `swift_attr` wasn't
-      // allowed in type contexts initially.
-      return solution->getFixedScore()
+      // This is not an error until Swift 6 because i.e. `swift_attr` wasn't
+      // allowed in type contexts initially and introducing new concurrency
+      // attributes shouldn't break witnesses without strict concurrency
+      // enabled.
+      return req->preconcurrency() &&
+             solution->getFixedScore()
                      .Data[SK_MissingSynthesizableConformance] > 0;
     }();
 

test/Concurrency/witness_matching_with_sendable.swift

@@ -3,10 +3,10 @@
 // RUN: %target-typecheck-verify-swift -swift-version 6 -verify-additional-prefix swift6-
 
 protocol P {
-  var prop: [String: any Sendable] { get set }
+  @preconcurrency var prop: [String: any Sendable] { get set }
   // expected-swift5-note@-1 3 {{expected sendability to match requirement here}}
   // expected-swift6-note@-2 3 {{protocol requires property 'prop' with type '[String : any Sendable]'}}
-  func reqFn(_: [String: any Sendable], _: @Sendable ((any Sendable)?) -> Void)
+  @preconcurrency func reqFn(_: [String: any Sendable], _: @Sendable ((any Sendable)?) -> Void)
   // expected-swift5-note@-1 2 {{expected sendability to match requirement here}}
   // expected-swift6-note@-2 2 {{protocol requires function 'reqFn' with type '([String : any Sendable], @Sendable ((any Sendable)?) -> Void) -> ()'}}
 }
@@ -37,3 +37,19 @@ struct S3 : P { // expected-swift6-error {{type 'S3' does not conform to protoco
 
   func reqFn(_: [String: any Sendable], _: ((any Sendable)?) -> Void) {} // Ok
 }
+
+protocol Q {
+  var prop: [String: [String: any Sendable]] { get set }
+  // expected-note@-1 {{protocol requires property 'prop' with type '[String : [String : any Sendable]]'}}
+
+  func test(_: [() -> (any Sendable)?])
+  // expected-note@-1 {{protocol requires function 'test' with type '([() -> (any Sendable)?]) -> ()'}}
+}
+
+struct S4 : Q { // expected-error {{type 'S4' does not conform to protocol 'Q'}} expected-note {{add stubs for conformance}}
+  var prop: [String: [String: Any]] = [:]
+  // expected-note@-1 {{candidate has non-matching type '[String : [String : Any]]'}}
+
+  func test(_: [() -> Any?]) {}
+  // expected-note@-1 {{candidate has non-matching type '([() -> Any?]) -> ()'}}
+}

test/decl/protocol/conforms/isolated_any.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -typecheck -verify -swift-version 6 %s
+// RUN: %target-swift-frontend -typecheck -verify %s
 
 struct A<T> {
   // expected-note @+1 {{candidate has non-matching type}}