[Concurrency] Don't crash if a #isolation default argument has a type

mismatch.

The actor isolation checker was setting the actor value for
`#isolation` when type checking the expression failed. Later on, the
isolation checker would crash when querying the type of the actor, e.g.
to check if it's a distributed actor. Instead, keep the actor value as
null if there's a type mismatch.

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -3986,7 +3986,7 @@ namespace {
       // do not cause implicit 'self' capture diagnostics, etc.
 
       Expr *actorExpr = nullptr;
-      Type optionalAnyActorType = isolationExpr->getType();
+      Type isolationType = isolationExpr->getType();
       switch (isolation) {
       case ActorIsolation::ActorInstance: {
         if (auto *instance = isolation.getActorInstanceExpr()) {
@@ -4040,10 +4040,16 @@ namespace {
 
 
       // Convert the actor argument to the appropriate type.
-      (void)TypeChecker::typeCheckExpression(
+      auto result = TypeChecker::typeCheckExpression(
           actorExpr, dc,
           constraints::ContextualTypeInfo(
-            optionalAnyActorType, CTP_CallArgument));
+            isolationType, CTP_CallArgument));
+
+      // Don't set the actor if there's a type mismatch. The isolation
+      // checker will treat calls using this #isolation value for an
+      // isolated argument as crossing an isolation boundary.
+      if (!result)
+        return;
 
       isolationExpr->setActor(actorExpr);
     }

test/Concurrency/isolation_macro.swift

@@ -100,6 +100,14 @@ extension A {
 }
 
 #if TEST_DIAGNOSTICS
+@available(SwiftStdlib 5.1, *)
+actor ConcreteActor {}
+
+@available(SwiftStdlib 5.1, *)
+func concreteActorIsolation(
+  actor: isolated ConcreteActor = #isolation
+) async {}
+
 @available(SwiftStdlib 5.1, *)
 @MainActor
 func testContextualType() {
@@ -111,5 +119,10 @@ func testContextualType() {
   // CHECK-DIAGS: note: in expansion of macro 'isolation' here
   // CHECK-DIAGS: let _: Int = #isolation
   let _: Int = #isolation
+
+  // CHECK-DIAGS: error: cannot convert value of type 'MainActor' to expected argument type 'ConcreteActor'
+  // CHECK-DIAGS: note: in expansion of macro 'isolation' here
+  // CHECK-DIAGS: await concreteActorIsolation()
+  await concreteActorIsolation()
 }
 #endif