https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89741
--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This needs to be verified by our C++ language lawyers, but if:
"If no valid specialization can be generated for a template definition, and
that template is not instantiated, the template definition is ill-formed, no
diagnostic required."
rule applies in this case, then what you are trying to do is not valid C++ and
you'll need to find some other way, e.g. one where there are instantiations
that could be instantiated. Say use
template<typename T1, typename T2>
struct Y {
static constexpr bool value = false;
};
template<>
struct Y<void, void> {
static constexpr bool value = true;
};
template<>
struct Y<void, float> {
static constexpr bool value = true;
};
template<int x, typename T, T y = 0>
struct X {
static_assert(Y<void, decltype(y)>::value, "1");
};
or something similar where all instantiations aren't invalid.
