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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/112642] ranges::fold_left tries to access inactive
 union member of string in constant expression
Date: Mon, 20 Nov 2023 15:35:07 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.2.1
X-Bugzilla-Keywords: rejects-valid
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X-Bugzilla-Who: redi at gcc dot gnu.org
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112642

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-11-20

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think the libstdc++ code is fine here, suggesting a compiler bug.

Slightly reduced:

#include <string>
#include <vector>

using namespace std;
using namespace std::literals;

struct Fold
{
  template<typename _Iter, typename _Sent, typename _Tp, typename _Fp>
      constexpr auto
      operator()(_Iter, _Sent, _Tp __init, _Fp) const
      {
        return std::move(__init);
      }

    template<typename _Range, typename _Tp, typename _Fp>
      constexpr auto
      operator()(_Range&& __r, _Tp __init, _Fp __f) const
      { return (*this)(__r.begin(), __r.end(), std::move(__init),
std::move(__f)); }

} fold;

constexpr auto foo() {
    const auto vec =3D vector{ "a"s, "b"s, "c"s };
    return fold(vec, ""s, plus{});
}

constexpr auto bar() {
    return foo().size();
}

int main() {
    constexpr auto _ =3D bar();
}=