[CSApply] Don't inject global actor into closure type if it's marked as @concurrent

When the attribute is specified explicitly passing a `@concurrent`
closure to a global actor-isolated parameter or contextual type
should result in a conversion and closure itself should be nonisolated.

Resolves: rdar://151797372

Author: xedin

lib/Sema/CSApply.cpp

@@ -7731,11 +7731,13 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
     }
 
     // If we have a ClosureExpr, then we can safely propagate a global actor
-    // to the closure without invalidating prior analysis.
+    // to the closure if it's not explicitly marked as `@concurrent` without
+    // invalidating prior analysis.
     fromEI = fromFunc->getExtInfo();
-    if (toEI.getGlobalActor() && !fromEI.getGlobalActor()) {
-      auto newFromFuncType = fromFunc->withExtInfo(
-          fromEI.withGlobalActor(toEI.getGlobalActor()));
+    if (toEI.getGlobalActor() && !fromEI.getGlobalActor() &&
+        !isClosureMarkedAsConcurrent(expr)) {
+      auto newFromFuncType =
+          fromFunc->withExtInfo(fromEI.withGlobalActor(toEI.getGlobalActor()));
       if (applyTypeToClosureExpr(cs, expr, newFromFuncType)) {
         fromFunc = newFromFuncType->castTo<FunctionType>();
 

test/Concurrency/attr_execution/conversions_silgen.swift

@@ -462,3 +462,13 @@ func testNoIsolationTransfer() {
   // CHECK: hop_to_executor [[GENERIC_EXECUTOR]]
   testErasure { @concurrent in }
 }
+
+func testClosuresDontAssumeGlobalActorWithMarkedAsConcurrent() {
+  func test(_ fn: @MainActor () async -> Void) {}
+
+  // CHECK-LABEL: sil private [ossa] @$s21attr_execution_silgen55testClosuresDontAssumeGlobalActorWithMarkedAsConcurrentyyFyyYaYbXEfU_
+  // CHECK: [[GENERIC_EXECUTOR:%.*]] = enum $Optional<Builtin.Executor>, #Optional.none!enumelt
+  // CHECK-NEXT: hop_to_executor [[GENERIC_EXECUTOR]]
+  test { @Sendable @concurrent in
+  }
+}