Multiple SFINAE class template specialisations using void_t

Question

Are multiple class template specialisations valid, when each is distinct only between patterns involving template parameters in non-deduced contexts?

A common exampl

Answer 1

There is a rule that partial specializations have to be more specialized than the primary template - both of your specializations follow that rule. But there isn't a rule that states that partial specializations can never be ambiguous. It's more that - if instantiation leads to ambiguous specialization, the program is ill-formed. But that ambiguous instantiation has to happen first!

It appears that clang is suffering from CWG 1558 here and is overly eager about substituting in void for std::void_t.

This is CWG 1980 almost exactly:

In an example like

template using X = T;
template X f();
template X f();

it appears that the second declaration of f is a redeclaration of the first but distinguishable by SFINAE, i.e., equivalent but not functionally equivalent.

If you use the non-alias implementation of void_t:

template  struct make_void { using type = void; };
template  using void_t = typename make_void::type;

then clang allows the two different specializations. Sure, instantiating has_members on a type that has both type1 and type2 typedefs errors, but that's expected.