Limit conversion that adds a global actor to a function type to subtyping.

Without this, we'll end up accepting invalid conversions when there
is a global actor-qualified function type in a non-top-level structural
position.

Author: DougGregor

lib/AST/ASTDumper.cpp

@@ -3837,6 +3837,11 @@ namespace {
         T->getClangTypeInfo().dump(os, ctx);
         printField("clang_type", os.str());
       }
+
+      if (Type globalActor = T->getGlobalActor()) {
+        printField("global_actor", globalActor.getString());
+      }
+
       printAnyFunctionParams(T->getParams(), "input");
       Indent -=2;
       printRec("output", T->getResult());

lib/Sema/CSSimplify.cpp

@@ -2025,11 +2025,16 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
         return getTypeMatchFailure(locator);
     } else if (func1->getGlobalActor()) {
       // Cannot remove a global actor.
+      if (!shouldAttemptFixes())
+        return getTypeMatchFailure(locator);
+
       auto *fix = MarkGlobalActorFunction::create(
           *this, func1, func2, getConstraintLocator(locator));
 
       if (recordFix(fix))
         return getTypeMatchFailure(locator);
+    } else if (kind < ConstraintKind::Subtype) {
+      return getTypeMatchFailure(locator);
     }
   }
 

test/Concurrency/global_actor_function_types.swift

@@ -93,3 +93,19 @@ func testClosuresOld() {
     return i
   }
 }
+
+// Test conversions that happen in various structural positions.
+struct X<T> { } // expected-note{{arguments to generic parameter 'T' ('() -> Void' and '@SomeGlobalActor () -> Void') are expected to be equal}}
+
+func f(_: (@SomeGlobalActor () -> Void)?) { }
+
+func g(fn: (() -> Void)?) {
+  f(fn)
+}
+
+func f2(_ x: X<@SomeGlobalActor () -> Void>) {
+  g2(x) // expected-error{{converting function value of type '@SomeGlobalActor () -> Void' to '() -> Void' loses global actor 'SomeGlobalActor'}}
+}
+func g2(_ x: X<() -> Void>) {
+  f2(x) // expected-error{{cannot convert value of type 'X<() -> Void>' to expected argument type 'X<@SomeGlobalActor () -> Void>'}}
+}