Infer protocol isolation from its inherited protocols.

We somehow missed this inference rule. Put it behind SE-0434's
`GlobalActorIsolatedTypesUsability` flag. Fixes rdar://127843050.

(cherry picked from commit 90341b2)

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -4543,6 +4543,37 @@ getIsolationFromConformances(NominalTypeDecl *nominal) {
   return foundIsolation;
 }
 
+/// Compute the isolation of a protocol
+static std::optional<ActorIsolation>
+getIsolationFromInheritedProtocols(ProtocolDecl *protocol) {
+  std::optional<ActorIsolation> foundIsolation;
+  for (auto inherited : protocol->getInheritedProtocols()) {
+    switch (auto protoIsolation = getActorIsolation(inherited)) {
+    case ActorIsolation::ActorInstance:
+    case ActorIsolation::Unspecified:
+    case ActorIsolation::Nonisolated:
+    case ActorIsolation::NonisolatedUnsafe:
+      break;
+
+    case ActorIsolation::Erased:
+      llvm_unreachable("protocol cannot have erased isolation");
+
+    case ActorIsolation::GlobalActor:
+      if (!foundIsolation) {
+        foundIsolation = protoIsolation;
+        continue;
+      }
+
+      if (*foundIsolation != protoIsolation)
+        return std::nullopt;
+
+      break;
+    }
+  }
+
+  return foundIsolation;
+}
+
 /// Compute the isolation of a nominal type from the property wrappers on
 /// any stored properties.
 static std::optional<ActorIsolation>
@@ -5231,6 +5262,17 @@ ActorIsolation ActorIsolationRequest::evaluate(
         if (auto inferred = inferredIsolation(*conformanceIsolation))
           return inferred;
 
+      // For a protocol, inherit isolation from the directly-inherited
+      // protocols.
+      if (ctx.LangOpts.hasFeature(Feature::GlobalActorIsolatedTypesUsability)) {
+        if (auto proto = dyn_cast<ProtocolDecl>(nominal)) {
+          if (auto protoIsolation = getIsolationFromInheritedProtocols(proto)) {
+            if (auto inferred = inferredIsolation(*protoIsolation))
+              return inferred;
+          }
+        }
+      }
+
       // Before Swift 6: If the declaration is a nominal type and any property
       // wrappers on its stored properties require isolation, use that.
       if (auto wrapperIsolation = getIsolationFromWrappers(nominal)) {

test/Concurrency/global_actor_inference_swift6.swift

@@ -154,3 +154,29 @@ class C {
 struct S: InferMainActor {
   @Wrapper var value: C // okay, 'S' is isolated to 'MainActor'
 }
+
+protocol InferMainActorInherited: InferMainActor {
+  func f() // expected-note{{mark the protocol requirement 'f()' 'async' to allow actor-isolated conformances}}
+  func g()
+}
+
+@SomeGlobalActor
+protocol InferSomeGlobalActor { }
+
+protocol InferenceConflict: InferMainActorInherited, InferSomeGlobalActor { }
+
+struct S2: InferMainActorInherited {
+  func f() { } // okay, 'f' is MainActor isolated, as is the requirement
+  @MainActor func g() { } // okay for the same reasons, but more explicitly
+}
+
+@SomeGlobalActor
+struct S3: InferenceConflict {
+  nonisolated func g() { }
+}
+
+extension S3 {
+  func f() { }
+  // expected-error@-1{{global actor 'SomeGlobalActor'-isolated instance method 'f()' cannot be used to satisfy main actor-isolated protocol requirement}}
+  //expected-note@-2{{add 'nonisolated' to 'f()' to make this instance method not isolated to the actor}}
+}