[Concurrency] Don't allow erasing global actor isolation when the function

value crosses an isolation boundary.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -1917,7 +1917,7 @@ bool swift::isAsyncDecl(ConcreteDeclRef declRef) {
 /// \param ty a function type where \c globalActor was removed from it.
 /// \return true if it is safe to drop the global-actor qualifier.
 static bool safeToDropGlobalActor(
-    DeclContext *dc, Type globalActor, Type ty) {
+    DeclContext *dc, Type globalActor, Type ty, ApplyExpr *call) {
   auto funcTy = ty->getAs<AnyFunctionType>();
   if (!funcTy)
     return false;
@@ -1942,6 +1942,11 @@ static bool safeToDropGlobalActor(
   if (funcTy->isAsync())
     return true;
 
+  // If the argument is passed over an isolation boundary, it's not
+  // safe to erase actor isolation, because the caller
+  if (call && call->getIsolationCrossing())
+    return false;
+
   // fundamentally cannot be sendable if we want to drop isolation info
   if (funcTy->isSendable())
     return false;
@@ -2364,7 +2369,8 @@ namespace {
               return;
 
             auto dc = const_cast<DeclContext*>(getDeclContext());
-            if (!safeToDropGlobalActor(dc, fromActor, toType)) {
+            if (!safeToDropGlobalActor(dc, fromActor, toType,
+                                       getImmediateApply())) {
               // otherwise, it's not a safe cast.
               dc->getASTContext()
                   .Diags

test/Concurrency/actor_isolation.swift

@@ -468,6 +468,8 @@ actor Crystal {
   await asyncGlobalActorFunc()
 }
 
+func crossIsolationBoundary(_ closure: () -> Void) async {}
+
 @available(SwiftStdlib 5.1, *)
 func testGlobalActorClosures() {
   let _: Int = acceptAsyncClosure { @SomeGlobalActor in
@@ -480,6 +482,15 @@ func testGlobalActorClosures() {
   }
 
   acceptConcurrentClosure { @SomeGlobalActor in 5 } // expected-warning {{converting function value of type '@SomeGlobalActor @Sendable () -> Int' to '@Sendable () -> Int' loses global actor 'SomeGlobalActor'}}
+
+  @MainActor func test() async {
+    let closure = { @MainActor @Sendable in
+      MainActor.assertIsolated()
+    }
+
+    await crossIsolationBoundary(closure)
+    // expected-warning@-1 {{converting function value of type '@MainActor @Sendable () -> ()' to '() -> Void' loses global actor 'MainActor'; this is an error in the Swift 6 language mode}}
+  }
 }
 
 @available(SwiftStdlib 5.1, *)