The current standard doesn’t say so, but the intention is that y and z have the same type. There is an open Core Working Group issue for this: http://wg21.cmeerw.net/cwg/issue1286
Yes, it is apparently allowed. According to the latest draft of the upcoming standard I could find, it is stated that A template-argument for a template template-parameter shall be the name of a class template or an alias template […]. However, alias templates seems very seldomly supported at the moment, so you might have some … Read more
The problem relies on SFINAE. If you rewrite your member function to be value_t<S<T>>, like the outside declaration, then GCC will happily compile it: template<class T> struct S { using value_type = int; static const value_t<S<T>> C = 0; }; template<class T> const value_t<S<T>> S<T>::C; Because the expression is now functionally equivalent. Things like substitution … Read more
This compiles in GCC 4.8 but fails in GCC 4.9, which is evidence that it’s related to CWG 1430 and bug report #59498. The fix proposed by Roy Chrihfield is the exact same one as yours: Rewriting the code to use a struct succeeds: template <typename T, typename …> struct alias { using type = … Read more