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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96213] New: GCC doesn't complain about ill-formed
 non-dependent template default argument
Date: Thu, 16 Jul 2020 00:06:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96213

            Bug ID: 96213
           Summary: GCC doesn't complain about ill-formed non-dependent
                    template default argument
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

Possibly related (although these seem to complain about the opposite of what
I'm complaining about):
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D12672
- https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D58071


    // https://godbolt.org/z/EYzxx9
    template<class> int g;

    template<int =3D g<int>(0,1,2)>
    void h() { }

    int main() {
        h<1>();
    }

MSVC and Clang both reject template `h` as ill-formed, because `g<int>(0,1,=
2)`
is nonsense -- `g<int>` is an `int` and thus cannot be called like a functi=
on.

GCC accepts template `h` as well-formed, and in fact will treat this as a
SFINAE situation:

    template<class> int g;
    template<int =3D g<int>(42)> void h() {} // #1
    template<class =3D void> void h() {}     // #2

    int main() {
        h<>();  // unambiguously calls #2, because #1 has a deduction failu=
re
    }

My guess is that MSVC and Clang are closer to correct here.=