[Concurrency] Always infer actor isolation from overridden declaration.

Infer actor isolation from the overridden declaration unless some other
isolation attribute was explicitly specified directly on that
declaration. This allows type- and extension-level annotations with
a global actor to not break overrides.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -1807,16 +1807,15 @@ ActorIsolation ActorIsolationRequest::evaluate(
   // If the declaration overrides another declaration, it must have the same
   // actor isolation.
   if (auto overriddenValue = value->getOverriddenDecl()) {
-    if (auto isolation = getActorIsolation(overriddenValue)) {
-      SubstitutionMap subs;
-      if (auto env = value->getInnermostDeclContext()
-              ->getGenericEnvironmentOfContext()) {
-        subs = SubstitutionMap::getOverrideSubstitutions(
-            overriddenValue, value, subs);
-      }
-
-      return inferredIsolation(isolation.subst(subs));
+    auto isolation = getActorIsolation(overriddenValue);
+    SubstitutionMap subs;
+    if (auto env = value->getInnermostDeclContext()
+            ->getGenericEnvironmentOfContext()) {
+      subs = SubstitutionMap::getOverrideSubstitutions(
+        overriddenValue, value, subs);
     }
+
+    return inferredIsolation(isolation.subst(subs));
   }
 
   // If this is an accessor, use the actor isolation of its storage

test/Concurrency/global_actor_inference.swift

@@ -177,6 +177,21 @@ struct OtherContainer<U> {
   }
 }
 
+class SuperclassWithGlobalActors {
+  @GenericGlobalActor<Int> func f() { }
+  @GenericGlobalActor<Int> func g() { } // expected-note{{overridden declaration is here}}
+  func h() { }
+}
+
+@GenericGlobalActor<String>
+class SubclassWithGlobalActors : SuperclassWithGlobalActors {
+  override func f() { } // okay: inferred to @GenericGlobalActor<Int>
+
+  @GenericGlobalActor<String> override func g() { } // expected-error{{global actor 'GenericGlobalActor<String>'-isolated instance method 'g()' has different actor isolation from global actor 'GenericGlobalActor<Int>'-isolated overridden declaration}}
+
+  override func h() { } // okay: inferred to unspecified
+}
+
 // ----------------------------------------------------------------------
 // Global actor inference for unspecified contexts
 // ----------------------------------------------------------------------