What does SFINAE not work correctly with following has_member function?

Question

I\'m trying out examples from Walter Brown\'s TMP talk and I\'m trying to get his has_member implementation working.

However the implementation seems t

Answer 1

The problem is that gcc 4.8.2 (and prior to gcc 5.0) does not regard unused arguments in alias templates as suitable for SFINAE. The workaround is to forward to a voider class template:

template <class ... T> struct voider { using type = void; };
template <class ... T>
using void_t = typename voider<T...>::type;

From http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3911.pdf section 2.3:

Alas, we have encountered implementation divergence (Clang vs. GCC) while working with the above very simple definition. We (continue to) conjecture that this is because of CWG issue 1558: “The treatment of unused arguments in an alias template specialization is not specified by the current wording of 14.5.7 [temp.alias].”