Provide a Bincompat override for the newly tightened casting semantics

Author: tbkka

include/swift/Runtime/Bincompat.h

@@ -38,6 +38,9 @@ bool useLegacyOptionalNilInjectionInCasting();
 /// Obj-C interop
 bool useLegacyObjCBoxingInCasting();
 
+/// Whether to use legacy semantics when unboxing __SwiftValue
+bool useLegacySwiftValueUnboxingInCasting();
+
 } // namespace bincompat
 
 } // namespace runtime

stdlib/public/runtime/Bincompat.cpp

@@ -195,6 +195,26 @@ bool useLegacyObjCBoxingInCasting() {
 #endif
 }
 
+// Should casting be strict about protocol conformance when
+// unboxing values that were boxed for Obj-C use?
+
+// Similar to `useLegacyObjCBoxingInCasting()`, but
+// this applies to the case where you have already boxed
+// some Swift non-reference-type into a `__SwiftValue`
+// and are now casting to a protocol.
+
+// For example, this cast
+// `x as! AnyObject as? NSCopying`
+// always succeeded with the legacy semantics.
+
+bool useLegacySwiftValueUnboxingInCasting() {
+#if BINARY_COMPATIBILITY_APPLE
+  return true; // For now, continue using the legacy behavior on Apple OSes
+#else
+  return false; // Always use the new behavior on non-Apple OSes
+#endif
+}
+
 } // namespace bincompat
 
 } // namespace runtime

stdlib/public/runtime/DynamicCast.cpp

@@ -1494,10 +1494,12 @@ tryCastToClassExistential(
 #if SWIFT_OBJC_INTEROP
     id srcObject;
     memcpy(&srcObject, srcValue, sizeof(id));
-    if (getAsSwiftValue(srcObject) != nullptr) {
-      // Do not directly cast a `__SwiftValue` box
-      // Return failure so our caller will unwrap and try again
-      return DynamicCastResult::Failure;
+    if (!runtime::bincompat::useLegacySwiftValueUnboxingInCasting()) {
+      if (getAsSwiftValue(srcObject) != nullptr) {
+	// Do not directly cast a `__SwiftValue` box
+	// Return failure so our caller will unwrap and try again
+	return DynamicCastResult::Failure;
+      }
     }
 #endif
     SWIFT_FALLTHROUGH;

stdlib/public/runtime/SwiftObject.mm

@@ -25,6 +25,7 @@
 #endif
 #include "llvm/ADT/StringRef.h"
 #include "swift/Basic/Lazy.h"
+#include "swift/Runtime/Bincompat.h"
 #include "swift/Runtime/Casting.h"
 #include "swift/Runtime/CustomRRABI.h"
 #include "swift/Runtime/Debug.h"
@@ -1270,9 +1271,11 @@ id swift_dynamicCastObjCProtocolUnconditional(id object,
 id swift_dynamicCastObjCProtocolConditional(id object,
                                             size_t numProtocols,
                                             Protocol * const *protocols) {
-  if (getAsSwiftValue(object) != nil) {
-    // SwiftValue wrapper never holds a class object
-    return nil;
+  if (!runtime::bincompat::useLegacySwiftValueUnboxingInCasting()) {
+    if (getAsSwiftValue(object) != nil) {
+      // SwiftValue wrapper never holds a class object
+      return nil;
+    }
   }
   for (size_t i = 0; i < numProtocols; ++i) {
     if (![object conformsToProtocol:protocols[i]]) {