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From: "Darrell.Wright at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/97134] New: partial specialization not more specialized
 when using CNTTP
Date: Mon, 21 Sep 2020 05:52:40 +0000
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X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97134

            Bug ID: 97134
           Summary: partial specialization not more specialized when using
                    CNTTP
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Darrell.Wright at gmail dot com
  Target Milestone: ---

g++, tried trunk on CE along with 10.2 and 10.1 will say the following code=
 is
not more specialized when JSONNAMETYPE is a CNTTP(json_name).  When it's a
NTTP(char const *) it is ok with it.  The code works in MSVC's C++20 mode.
https://godbolt.org/z/7v4nx5

#include <cstddef>
#include <iostream>
#include <string_view>
#include <utility>

template <std::size_t N>
struct json_name {
  static_assert(N > 0);
  char const m_data[N]{};

 private:
  template <std::size_t... Is>
  constexpr json_name(char const (&ptr)[N], std::index_sequence<Is...>)
      : m_data{ptr[Is]...} {}

 public:
  constexpr json_name(char const (&ptr)[N])
      : json_name(ptr, std::make_index_sequence<N>{}) {}

  // Needed for copy_to_iterator
  [[nodiscard]] constexpr char const *begin() const { return m_data; }

  // Needed for copy_to_iterator
  [[nodiscard]] constexpr char const *end() const {
    return m_data + static_cast<ptrdiff_t>(size());
  }

  [[nodiscard]] static constexpr std::size_t size() noexcept { return N - 1=
; }

  template <std::size_t M>
  constexpr bool operator=3D=3D(json_name<M> const &rhs) const {
    if (N !=3D M) {
      return false;
    }
    for (std::size_t n =3D 0; n < N; ++n) {
      if (m_data[n] !=3D rhs.m_data[n]) {
        return false;
      }
    }
    return true;
  }

  constexpr operator std::string_view() const {
    return std::string_view(m_data, size());
  }
};
template <typename... Chars>
json_name(Chars...) -> json_name<sizeof...(Chars)>;

template <std::size_t N>
json_name(char const (&)[N]) -> json_name<N>;

#ifdef USE_CNTTP
#define JSONNAMETYPE json_name
#else
#define JSONNAMETYPE char const *
#endif

template <JSONNAMETYPE Name, typename U>
struct X : std::false_type {
  static constexpr std::string_view name =3D Name;
};

template <JSONNAMETYPE Name>
struct X<Name, bool> : std::true_type {
  static constexpr std::string_view name =3D Name;
};

int main() {
  static constexpr char const A[] =3D "A";=20=20
  static constexpr char const B[] =3D "B";=20=20
  auto a =3D X<A, int>::name;
  auto b =3D X<B, bool>::name;
  std::cout << a << b << '\n';
}=