[Bug c++/112594] New: Non-type template parameter created with converting constructor sometimes has original type

["mital at mitalashok dot co.uk" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112594

            Bug ID: 112594
           Summary: Non-type template parameter created with converting
                    constructor sometimes has original type
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mital at mitalashok dot co.uk
  Target Milestone: ---

The following code complains that `N` (which was declared `n N`) is `3` (of
type `int`) instead of `n(3)`:

<https://godbolt.org/z/rv3777Ycv>

    template<typename T>
    concept C = true;

    struct n {
        constexpr n(int) {}
        static int i;
    };

    template<n N>
    using get_n_i_type = decltype(N.i);

    template<int X>
    int f() {
        using iii = get_n_i_type<X>;
    #if 1  // Change to 0 and this compiles
        static_assert(C<iii>);
    #endif
        return iii{};
    }

    template int f<3>();

Compiler error given:

    <source>: In instantiation of 'int f() [with int X = 3]':
    <source>:21:19:   required from here
       21 | template int f<3>();
          |                   ^
    <source>:10:33: error: request for member 'i' in '3', which is of non-class
type 'int'
       10 | using get_n_i_type = decltype(N.i);
          |                               ~~^

This compiles fine if Clang and MSVC or if you remove the static_assert or
change `template<int X>` to `template<auto X>`.