Avoid type confusion in exceptions.

When creating these typedefs, if we use in-place
declared types, we wind up with surprising aliasing
of various types, via namespace exposure.

Thread.h

@@ -15,7 +15,8 @@ namespace Alepha::Hydrogen  ::detail::  Thread_m
 {
 	inline namespace exports
 	{
-		using CrossThreadNotificationRethrowError= synthetic_exception< struct cross_thread_notification_failure, Error >;
+		enum CrossThread_tag {};
+		using CrossThreadNotificationRethrowError= synthetic_exception< CrossThread_tag, Error >;
 		namespace this_thread
 		{
 			//void sleep( std::chrono::duration );