Merge pull request swiftlang#75252 from hborla/6.0-isolation-inference-inherited-conformance

[6.0][Concurrency] Don't infer actor isolation from inherited conformances.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -4878,12 +4878,30 @@ getIsolationFromWitnessedRequirements(ValueDecl *value) {
 /// are directly specified on the type.
 static std::optional<ActorIsolation>
 getIsolationFromConformances(NominalTypeDecl *nominal) {
+  auto &ctx = nominal->getASTContext();
+
   if (isa<ProtocolDecl>(nominal))
     return std::nullopt;
 
   std::optional<ActorIsolation> foundIsolation;
-  for (auto proto :
-       nominal->getLocalProtocols(ConformanceLookupKind::NonStructural)) {
+  for (auto conformance :
+       nominal->getLocalConformances(ConformanceLookupKind::NonStructural)) {
+
+    // Don't include inherited conformances. If a conformance is inherited
+    // from a superclass, the isolation of the subclass should be inferred
+    // from the superclass, which is done directly in ActorIsolationRequest.
+    // If the superclass has opted out of global actor inference, such as
+    // by conforming to the protocol in an extension, then the subclass should
+    // not infer isolation from the protocol.
+    //
+    // Gate this change behind an upcoming feature flag; isolation inference
+    // changes can break source in language modes < 6.
+    if (conformance->getKind() == ProtocolConformanceKind::Inherited &&
+        ctx.LangOpts.hasFeature(Feature::GlobalActorIsolatedTypesUsability)) {
+      continue;
+    }
+
+    auto *proto = conformance->getProtocol();
     switch (auto protoIsolation = getActorIsolation(proto)) {
     case ActorIsolation::ActorInstance:
     case ActorIsolation::Unspecified:

test/Concurrency/global_actor_inference_swift6.swift

@@ -213,3 +213,13 @@ class C2: MainActorSuperclass, InferenceConflictWithSuperclass {
 }
 
 
+class ConformInExtension {}
+extension ConformInExtension: InferMainActor {}
+
+class InheritConformance: ConformInExtension {
+  func f() {}
+}
+
+func testInheritedMainActorConformance() {
+  InheritConformance().f() // okay; this is not main actor isolated
+}