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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/111357] [11/12/13/14 Regression] __integer_pack fails to
 work with values of dependent type convertible to integers in noexcept
 context
Date: Mon, 11 Sep 2023 08:54:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111357

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced to remove <utility>:

namespace std {
  template<typename _Tp, _Tp... _Idx>
    struct integer_sequence
    { };

  template<typename _Tp, _Tp _Num>
    using make_integer_sequence
      =3D integer_sequence<_Tp, __integer_pack(_Num)...>;
}

using std::integer_sequence;
using std::make_integer_sequence;

template<int... V>
void g(integer_sequence<int,V...>)
{}

template<typename ...T>
struct c1
{
  static constexpr int value =3D 1;
  constexpr operator int() { return value; }=20
};
template<typename T>
struct R
{
        using S =3D make_integer_sequence<int,c1<T>{}>;

        R() noexcept(noexcept(g(S())))
        {}
};
int main()
{
        R<int>();
}

This certainly seems like a front end bug not a libstdc++ one, but the libr=
ary
can make it work by casting _Tp _Num to its own type:

  template<typename _Tp, _Tp _Num>
    using make_integer_sequence
      =3D integer_sequence<_Tp, __integer_pack(_Tp(_Num))...>;

This should not be needed, but makes it work.=