[Runtime] Allow weak references to ErrorObjects.

When ObjC interop is not available, Error values are represented in ErrorObject boxes. These are full HeapObjects, but unowned refcounting ops asserted that the metadata was class metadata. This assert would be hit when destroying an ErrorObject that was weakly referenced. Expand the asserts to accept ErrorObject metadata as well.

rdar://150214921

Author: mikeash

stdlib/public/runtime/HeapObject.cpp

@@ -547,6 +547,16 @@ HeapObject *swift::swift_unownedRetain(HeapObject *object) {
 #endif
 }
 
+// Assert that the metadata is a class or ErrorObject, for unowned operations.
+// Other types of metadata are not supposed to be used with unowned.
+static void checkMetadataForUnownedRR(HeapObject *object) {
+  assert(object->metadata->isClassObject() ||
+         object->metadata->getKind() == MetadataKind::ErrorObject);
+  if (object->metadata->isClassObject())
+    assert(
+        static_cast<const ClassMetadata *>(object->metadata)->isTypeMetadata());
+}
+
 void swift::swift_unownedRelease(HeapObject *object) {
 #ifdef SWIFT_THREADING_NONE
   swift_nonatomic_unownedRelease(object);
@@ -555,9 +565,7 @@ void swift::swift_unownedRelease(HeapObject *object) {
   if (!isValidPointerForNativeRetain(object))
     return;
 
-  // Only class objects can be unowned-retained and unowned-released.
-  assert(object->metadata->isClassObject());
-  assert(static_cast<const ClassMetadata*>(object->metadata)->isTypeMetadata());
+  checkMetadataForUnownedRR(object);
 
   if (object->refCounts.decrementUnownedShouldFree(1)) {
     auto classMetadata = static_cast<const ClassMetadata*>(object->metadata);
@@ -582,9 +590,7 @@ void swift::swift_nonatomic_unownedRelease(HeapObject *object) {
   if (!isValidPointerForNativeRetain(object))
     return;
 
-  // Only class objects can be unowned-retained and unowned-released.
-  assert(object->metadata->isClassObject());
-  assert(static_cast<const ClassMetadata*>(object->metadata)->isTypeMetadata());
+  checkMetadataForUnownedRR(object);
 
   if (object->refCounts.decrementUnownedShouldFreeNonAtomic(1)) {
     auto classMetadata = static_cast<const ClassMetadata*>(object->metadata);
@@ -615,9 +621,7 @@ void swift::swift_unownedRelease_n(HeapObject *object, int n) {
   if (!isValidPointerForNativeRetain(object))
     return;
 
-  // Only class objects can be unowned-retained and unowned-released.
-  assert(object->metadata->isClassObject());
-  assert(static_cast<const ClassMetadata*>(object->metadata)->isTypeMetadata());
+  checkMetadataForUnownedRR(object);
 
   if (object->refCounts.decrementUnownedShouldFree(n)) {
     auto classMetadata = static_cast<const ClassMetadata*>(object->metadata);
@@ -641,9 +645,7 @@ void swift::swift_nonatomic_unownedRelease_n(HeapObject *object, int n) {
   if (!isValidPointerForNativeRetain(object))
     return;
 
-  // Only class objects can be unowned-retained and unowned-released.
-  assert(object->metadata->isClassObject());
-  assert(static_cast<const ClassMetadata*>(object->metadata)->isTypeMetadata());
+  checkMetadataForUnownedRR(object);
 
   if (object->refCounts.decrementUnownedShouldFreeNonAtomic(n)) {
     auto classMetadata = static_cast<const ClassMetadata*>(object->metadata);