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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/112632] New: [14 Regression]  Non-type template parameter
 created with converting constructor sometimes has original type
Date: Mon, 20 Nov 2023 05:08:02 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: needs-bisection, rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: pinskia at gcc dot gnu.org
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112632

            Bug ID: 112632
           Summary: [14 Regression]  Non-type template parameter created
                    with converting constructor sometimes has original
                    type
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: needs-bisection, rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
    template<typename T>
    inline constexpr bool C =3D true;

    struct n {
        constexpr n(int a) : i(a) {}
        int i;
    };

    template<n N>
    using get_n_i_type =3D decltype(N.i);

    template<int X>
    int f() {
        using iii =3D get_n_i_type<X>;
    #if 1  // Change to 0 and this compiles
        static_assert(C<iii>);
    #endif
        return iii{};
    }

    template int f<3>();
```

This is rejected on the trunk with:
```
<source>: In instantiation of 'int f() [with int X =3D 3]':
<source>:22:23:   required from here
   22 |     template int f<3>();
      |                       ^
<source>:11:37: error: request for member 'i' in '3', which is of non-class
type 'int'
   11 |     using get_n_i_type =3D decltype(N.i);
      |                                   ~~^
```

But was accepted in GCC 13.  Note the difference between this and PR 112594=
 is
that C here is declared an `inline constexpr` rather than a concept.=