[TypeChecker] Warn if witness mismatches only on sendability until Sw…

lib/Sema/CSSimplify.cpp

@@ -5616,16 +5616,17 @@ bool ConstraintSystem::repairFailures(
   }
 
   if (auto *VD = getAsDecl<ValueDecl>(anchor)) {
-    // Matching a witness to an ObjC protocol requirement.
-    if (VD->isObjC() &&
-        isa<ProtocolDecl>(VD->getDeclContext()) &&
+    // 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
-        // the concurrency annotations from the inner most type.
+        // the concurrency annotations from the innermost type.
         conversionsOrFixes.empty()) {
-      // Allow requirements to introduce `swift_attr` annotations
+      // Allow requirements to introduce `swift_attr` and other
+      // concurrency related annotations (e.g. `& Sendable` or `@Sendable`)
       // (note that `swift_attr` in type contexts weren't supported
       // before) and for witnesses to adopt them gradually by matching
       // with a warning in non-strict concurrency mode.
@@ -5637,11 +5638,15 @@ bool ConstraintSystem::repairFailures(
         auto strippedRHS = rhs->stripConcurrency(/*recursive=*/true,
                                                  /*dropGlobalActor=*/true);
 
-        auto result = matchTypes(strippedLHS, strippedRHS, matchKind,
-                                 flags | TMF_ApplyingFix, locator);
-        if (!result.isFailure()) {
-          increaseScore(SK_MissingSynthesizableConformance, locator);
-          return true;
+        // If nothing got stripped there is no reason to re-match
+        // the types.
+        if (!strippedLHS->isEqual(lhs) || !strippedRHS->isEqual(rhs)) {
+          auto result = matchTypes(strippedLHS, strippedRHS, matchKind,
+                                   flags | TMF_ApplyingFix, locator);
+          if (!result.isFailure()) {
+            increaseScore(SK_MissingSynthesizableConformance, locator);
+            return true;
+          }
         }
       }
     }

lib/Sema/TypeCheckProtocol.cpp

@@ -1261,9 +1261,11 @@ 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 req->isObjC() &&
+      // 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

@@ -0,0 +1,55 @@
+// RUN: %target-typecheck-verify-swift -swift-version 5 -verify-additional-prefix swift5-
+// RUN: %target-typecheck-verify-swift -swift-version 5 -strict-concurrency=complete -verify-additional-prefix swift6-
+// RUN: %target-typecheck-verify-swift -swift-version 6 -verify-additional-prefix swift6-
+
+protocol P {
+  @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]'}}
+  @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) -> ()'}}
+}
+
+struct S1 : P { // expected-swift6-error {{type 'S1' does not conform to protocol 'P'}} expected-swift6-note {{add stubs for conformance}}
+  var prop: [String: Any] = [:]
+  // expected-swift5-warning@-1 {{sendability of function types in property 'prop' does not match requirement in protocol 'P'; this is an error in the Swift 6 language mode}}
+  // expected-swift6-note@-2 {{candidate has non-matching type '[String : Any]'}}
+  func reqFn(_: [String: Any], _: (Any?) -> Void) {}
+  // expected-swift5-warning@-1 {{sendability of function types in instance method 'reqFn' does not match requirement in protocol 'P'; this is an error in the Swift 6 language mode}}
+  // expected-swift6-note@-2 {{candidate has non-matching type '([String : Any], (Any?) -> Void) -> ()'}}
+}
+
+struct S2 : P { // expected-swift6-error {{type 'S2' does not conform to protocol 'P'}} expected-swift6-note {{add stubs for conformance}}
+  var prop: [String: Any] = [:]
+  // expected-swift5-warning@-1 {{sendability of function types in property 'prop' does not match requirement in protocol 'P'; this is an error in the Swift 6 language mode}}
+  // expected-swift6-note@-2 {{candidate has non-matching type '[String : Any]'}}
+
+  func reqFn(_: [String: Any], _: ((any Sendable)?) -> Void) {}
+  // expected-swift5-warning@-1 {{sendability of function types in instance method 'reqFn' does not match requirement in protocol 'P'; this is an error in the Swift 6 language mode}}
+  // expected-swift6-note@-2 {{candidate has non-matching type '([String : Any], ((any Sendable)?) -> Void) -> ()'}}
+}
+
+struct S3 : P { // expected-swift6-error {{type 'S3' does not conform to protocol 'P'}} expected-swift6-note {{add stubs for conformance}}
+  var prop: [String: Any] = [:]
+  // expected-swift5-warning@-1 {{sendability of function types in property 'prop' does not match requirement in protocol 'P'; this is an error in the Swift 6 language mode}}
+  // expected-swift6-note@-2 {{candidate has non-matching type '[String : Any]'}}
+
+  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?]) -> ()'}}
+}