[Runtime] Handle dynamic casting to NSObject via error bridging.

stdlib/public/runtime/Casting.cpp

@@ -37,6 +37,7 @@
 #include "llvm/Support/Compiler.h"
 #if SWIFT_OBJC_INTEROP
 #include "swift/Runtime/ObjCBridge.h"
+#include "SwiftObject.h"
 #include "SwiftValue.h"
 #endif
 
@@ -2330,9 +2331,10 @@ static bool swift_dynamicCastImpl(OpaqueValue *dest, OpaqueValue *src,
   case MetadataKind::Class:
   case MetadataKind::ObjCClassWrapper:
 #if SWIFT_OBJC_INTEROP
-    // If the destination type is an NSError, and the source type is an
-    // Error, then the cast can succeed by NSError bridging.
-    if (targetType == getNSErrorMetadata()) {
+    // If the destination type is an NSError or NSObject, and the source type
+    // is an Error, then the cast can succeed by NSError bridging.
+    if (targetType == getNSErrorMetadata() ||
+        targetType == getNSObjectMetadata()) {
       // Don't rebridge if the source is already some kind of NSError.
       if (srcType->isAnyClass()
           && swift_dynamicCastObjCClass(*reinterpret_cast<id*>(src),

stdlib/public/runtime/SwiftObject.h

@@ -29,6 +29,7 @@
 
 
 #if SWIFT_OBJC_INTEROP
+#if __OBJC__
 
 // Source code: "SwiftObject"
 // Real class name: mangled "Swift._SwiftObject"
@@ -83,5 +84,13 @@ id getDescription(OpaqueValue *value, const Metadata *type);
 }
 
 #endif
+#endif
+
+namespace swift {
+
+/// Get the NSObject metadata.
+const Metadata *getNSObjectMetadata();
+
+}
 
 #endif

stdlib/public/runtime/SwiftObject.mm

@@ -1552,6 +1552,11 @@ void swift_objc_swift3ImplicitObjCEntrypoint(id self, SEL selector,
   free(nullTerminatedFilename);
 }
 
+const Metadata *swift::getNSObjectMetadata() {
+  return SWIFT_LAZY_CONSTANT(
+      swift_getObjCClassMetadata((const ClassMetadata *)[NSObject class]));
+}
+
 #endif
 
 const ClassMetadata *swift::getRootSuperclass() {

test/stdlib/ErrorBridged.swift

@@ -767,4 +767,29 @@ ErrorBridgingTests.test("@objc error domains for nested types") {
               String(reflecting: NonPrintAsObjCError.self))
 }
 
+@inline(never)
+@_optimize(none)
+func anyToAny<T, U>(_ a: T, _ : U.Type) -> U {
+  return a as! U
+}
+
+ErrorBridgingTests.test("error-to-NSObject casts") {
+  let error = MyCustomizedError(code: 12345)
+
+  // Unconditional cast
+  let nsErrorAsObject1 = unconditionalCast(error, to: NSObject.self)
+  let nsError1 = unconditionalCast(nsErrorAsObject1, to: NSError.self)
+  expectEqual("custom", nsError1.domain)
+  expectEqual(12345, nsError1.code)
+
+  // Conditional cast
+  let nsErrorAsObject2 = conditionalCast(error, to: NSObject.self)!
+  let nsError2 = unconditionalCast(nsErrorAsObject2, to: NSError.self)
+  expectEqual("custom", nsError2.domain)
+  expectEqual(12345, nsError2.code)
+
+  // "is" check
+  expectTrue(error is NSObject)
+}
+
 runAllTests()