[Runtime] Extract an embedded NSError when dynamic casting to an Error existential.

SILGen already attempts to extract an embedded NSError when
type-erasing to an Error existential; make the runtime do the same
thing dynamically.

Huge thanks to Joe Groff who noticed that I missed this path.

(cherry picked from commit e83fb64)

Author: DougGregor

stdlib/public/runtime/Casting.cpp

@@ -872,6 +872,16 @@ swift_dynamicCastMetatypeToObjectUnconditional(const Metadata *metatype) {
   }
   }
 }
+
+// @_silgen_name("swift_stdlib_getErrorEmbeddedNSErrorIndirect")
+// public func _stdlib_getErrorEmbeddedNSErrorIndirect<T : Error>(
+///    _ x: UnsafePointer<T>) -> AnyObject?
+SWIFT_CC(swift)
+extern "C" id swift_stdlib_getErrorEmbeddedNSErrorIndirect(
+                const OpaqueValue *error,
+                const Metadata *T,
+                const WitnessTable *Error);
+
 #endif
 
 /// Perform a dynamic cast to an existential type.
@@ -1040,7 +1050,20 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
                               targetType->Protocols,
                               &errorWitness))
       return _fail(src, srcType, targetType, flags, srcDynamicType);
-    
+
+#if SWIFT_OBJC_INTEROP
+    // Check whether there is an embedded NSError. If so, use that for our Error
+    // representation.
+    if (auto embedded =
+          swift_stdlib_getErrorEmbeddedNSErrorIndirect(srcDynamicValue,
+                                                       srcDynamicType,
+                                                       errorWitness)) {
+      *destBoxAddr = reinterpret_cast<SwiftError*>(embedded);
+      maybeDeallocateSourceAfterSuccess();
+      return true;
+    }
+#endif
+
     BoxPair destBox = swift_allocError(srcDynamicType, errorWitness,
                                        srcDynamicValue,
                /*isTake*/ canTake && (flags & DynamicCastFlags::TakeOnSuccess));
@@ -2006,20 +2029,11 @@ static bool _dynamicCastToFunction(OpaqueValue *dest,
 }
 
 #if SWIFT_OBJC_INTEROP
-// @_silgen_name("swift_stdlib_getErrorEmbeddedNSErrorIndirect")
-// public func _stdlib_getErrorEmbeddedNSErrorIndirect<T : Error>(
-///    _ x: UnsafePointer<T>) -> AnyObject?
-SWIFT_CC(swift)
-extern "C" id swift_stdlib_getErrorEmbeddedNSErrorIndirect(
-                const OpaqueValue *error,
-                const Metadata *T,
-                const WitnessTable *Error);
-
 static id dynamicCastValueToNSError(OpaqueValue *src,
                                     const Metadata *srcType,
                                     const WitnessTable *srcErrorWitness,
                                     DynamicCastFlags flags) {
-  // Check whether there is an embedded error.
+  // Check whether there is an embedded NSError.
   if (auto embedded =
           swift_stdlib_getErrorEmbeddedNSErrorIndirect(src, srcType,
                                                        srcErrorWitness)) {

test/1_stdlib/ErrorBridged.swift

@@ -633,11 +633,24 @@ extension Error {
 	}
 }
 
+func unconditionalCast<T>(_ x: Any, to: T.Type) -> T {
+  return x as! T
+}
+
+func conditionalCast<T>(_ x: Any, to: T.Type) -> T? {
+  return x as? T
+}
+
 // SR-1562
 ErrorBridgingTests.test("Error archetype identity") {
   let myError = NSError(domain: "myErrorDomain", code: 0,
                         userInfo: [ AnyHashable("one") : 1 ])
   expectTrue(myError === myError.asNSError())
+
+  expectTrue(unconditionalCast(myError, to: Error.self) as NSError
+     === myError)
+  expectTrue(conditionalCast(myError, to: Error.self)! as NSError
+     === myError)
 }
 
 runAllTests()