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

I don't believe it's correct, or at least, not if we instantiate has_members with a type that has both type1 and type2 nested, the result would be two specializations that are

has_members 

which would not be valid. Until the code is instantiated I think it's ok, but clang is rejecting it early. On g++, your fails with this use-case, once instantiated:

struct X
{
    using type1 = int;
    using type2 = double;
};

int main() {
    has_members::value;
}

The error message is doesn't seem to describe the actual problem, but it at least is emitted:

:20:21: error: incomplete type 'has_members' used in nested name specifier
     has_members::value;
                     ^~~~~

If you instantiate it with a type that has only type1 or type2 but not both, then g++ compiles it cleanly. So it's objecting to the fact that the members are both present, causing conflicting instantiations of the template.

To get the disjunction, I think you'd want code like this:

template >
struct has_members : std::bool_constant {};

template 
struct has_members, has_member_type2>::value>> : 
    std::bool_constant {};

This assumes you have traits to determine has_member_type1 and has_member_type2 already written.