[Bug c++/117896] Unexpected warning when specializing a template

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117896

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2024-12-03

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Seems like a bug in auto NTTP handling

Without the std::lib dependencies:

template <typename T, template <T> class other_type>
struct S {
};

namespace detail {
  template <typename protocol>
    struct ot_impl;

  template <typename T, template <T> class other_type>
    struct ot_impl<S<T, other_type>> {
      using type = T;
    };
}

template <auto Q>
using get_type = typename detail::ot_impl<decltype(Q)>::type;

namespace detail {
  template <typename Q, get_type<Q{}> V>
    struct getter_impl;

  template <typename T, template <T> class other_type, T V>
    struct getter_impl<S<T, other_type>, V> {
      using type = typename other_type<V>::type;
    };
}

template <auto Q, get_type<Q> V>
using get_other_type = typename detail::getter_impl<decltype(Q), V>::type;

template <unsigned char V>
struct M {
    using type = float;
};

template<>
struct M<0>{
    using type = int;
};

inline constexpr auto p = S<unsigned char, M>{};

template<typename, typename> constexpr bool is_same_v = false;
template<typename T> constexpr bool is_same_v<T,T> = true;

int main() {
    static_assert(is_same_v<get_other_type<p, 0>, int>);
    static_assert(is_same_v<get_other_type<p, 1>, float>);
}