Correct scope change of objc id var and explain.

Swift SVN r24821

stdlib/runtime/Casting.cpp

@@ -765,6 +765,11 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
                                       const Metadata *srcType,
                                       const ExistentialTypeMetadata *targetType,
                                       DynamicCastFlags flags) {
+#if SWIFT_OBJC_INTEROP
+  // This variable's lifetime needs to be for the whole function, but is
+  // only valid with Objective-C interop enabled.
+  id tmp;
+#endif
   // Find the actual type of the source.
   OpaqueValue *srcDynamicValue;
   const Metadata *srcDynamicType;
@@ -785,9 +790,10 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
       // Class metadata can be used as an object when ObjC interop is available.
       auto metatypePtr = reinterpret_cast<const Metadata **>(src);
       auto metatype = *metatypePtr;
+      tmp = swift_dynamicCastMetatypeToObjectConditional(metatype);
       // If the cast succeeded, use the result value as the class instance
       // below.
-      if (id tmp = swift_dynamicCastMetatypeToObjectConditional(metatype)) {
+      if (tmp) {
         srcDynamicValue = reinterpret_cast<OpaqueValue*>(&tmp);
         srcDynamicType = reinterpret_cast<const Metadata*>(tmp);
         break;