[Isolation] Propagate global actors from witnesses.

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -2669,7 +2669,8 @@ ActorIsolation ActorIsolationRequest::evaluate(
     // If the declaration witnesses a protocol requirement that is isolated,
     // use that.
     if (auto witnessedIsolation = getIsolationFromWitnessedRequirements(value)) {
-      if (auto inferred = inferredIsolation(*witnessedIsolation))
+      if (auto inferred = inferredIsolation(
+              *witnessedIsolation, /*propagateUnsafe=*/defaultIsolation))
         return inferred;
     }
 

test/ClangImporter/objc_async.swift

@@ -77,7 +77,7 @@ func testSlowServerOldSchool(slowServer: SlowServer) {
 func globalAsync() async { }
 
 actor MySubclassCheckingSwiftAttributes : ProtocolWithSwiftAttributes {
-  func syncMethod() { } // expected-note {{calls to instance method 'syncMethod()' from outside of its actor context are implicitly asynchronous}}
+  func syncMethod() { } // expected-note 2{{calls to instance method 'syncMethod()' from outside of its actor context are implicitly asynchronous}}
 
   func independentMethod() {
     syncMethod() // expected-error{{ctor-isolated instance method 'syncMethod()' can not be referenced from a non-isolated context}}
@@ -88,7 +88,7 @@ actor MySubclassCheckingSwiftAttributes : ProtocolWithSwiftAttributes {
   }
 
   func mainActorMethod() {
-    syncMethod()
+    syncMethod() // expected-error{{actor-isolated instance method 'syncMethod()' can not be referenced from synchronous context of global actor 'MainActor'}}
   }
 
   func uiActorMethod() { }