[Concurrency] Make sure that conversion that switches from one actor to another is not accepted

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -2795,15 +2795,6 @@ namespace {
                   toFnType, /*downgradeToWarning=*/true);
               break;
 
-            case FunctionTypeIsolation::Kind::GlobalActor:
-              // Actor isolation change.
-              if (fromIsolation.getGlobalActorType()->isEqual(
-                      toIsolation.getGlobalActorType())) {
-                diagnoseNonSendableParametersAndResult(
-                    toFnType, /*downgradeToWarning=*/true);
-              }
-              break;
-
             case FunctionTypeIsolation::Kind::NonIsolated: {
               // Since @execution(concurrent) as an asynchronous
               // function it would mean that without Sendable
@@ -2821,6 +2812,9 @@ namespace {
             // Runs on the actor.
             case FunctionTypeIsolation::Kind::NonIsolatedCaller:
               break;
+
+            case FunctionTypeIsolation::Kind::GlobalActor:
+              llvm_unreachable("invalid conversion");
             }
             break;
           }

test/Concurrency/attr_execution_conversions.swift

@@ -4,6 +4,11 @@
 // REQUIRES: concurrency
 // REQUIRES: swift_feature_ExecutionAttribute
 
+@globalActor
+actor MyActor {
+  static let shared = MyActor()
+}
+
 @execution(concurrent)
 func concurrentTest() async {
 }
@@ -132,4 +137,9 @@ func testNonSendableDiagnostics(
 
   let _: @MainActor (NonSendable) async -> Void = caller1 // Ok
   let _: @MainActor () async -> NonSendable = caller2 // Ok
+
+  let _: @MyActor (NonSendable) async -> Void = globalActor1
+  // expected-error@-1 {{cannot convert value actor-isolated to 'MainActor' to specified type actor-isolated to 'MyActor'}}
+  let _: @MyActor () async -> NonSendable = globalActor2
+  // expected-error@-1 {{cannot convert value actor-isolated to 'MainActor' to specified type actor-isolated to 'MyActor'}}
 }