https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79960
--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> --- This is a problem with deducing through what I'm calling "non-transparent" aliases in partial ordering. When partial ordering compares, say, const and const volatile, we deduce "volatile __has_tuple_size<T>" and then substitute it, producing the odd const volatile __has_tuple_size<volatile __has_tuple_size<T>> which compares as different from const volatile __has_tuple_size<T> so we decide that they are unordered. Oops. My alias-equiv proposal would prevent the deduction in the first place, as pointed out on the committee IRC the other day, which I guess would entirely prevent constraining partial specializations. It would be nice if you could write instead template <typename T, typename U = __has_tuple_size<T>> struct tuple_size <const T> this seems like an oversight, much like the former prohibition of default template arguments on functions. Hmm.
