c++ - Can I restrict a test for a nested enum using SFINAE to a class without base classes? -

when use sfinae detect @ compile time if class has type, seems include derived types. ideally test existence of enum, shows same effect: both tests work in general, return true if class derives type or enum.

live example here!

#include <iostream>  struct { enum { istagged }; struct istaggedstruct{}; }; struct b :a {}; struct c {};  template< int > struct hastag;  template< class tst > struct check {     template< class t2 > static char (&test( ... ))[1];     template< class t2 > static char (&test( hastag<t2::istagged>* ))[2];     //template< class t2 > static char (&test( typename t2::istaggedstruct* ))[2];     static const bool value = sizeof(test<tst>(0))==sizeof(char[2]); };  int main(int argc, char* argv[]) {     std::cout << "value a=" << check< >::value << std::endl;     std::cout << "value b=" << check< b >::value << std::endl;     std::cout << "value c=" << check< c >::value << std::endl;     return 0; } 

output:

+ g++ -std=c++03 main.cpp + ./a.out value a=1 value b=1 value c=0 

wanted output a=1, rest 0. 1 test enum or type enabled @ time, of course. gcc c++03.

tl;dr: sfinae test if type exists in class true derived classes, how restrict class?