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From: "xgao at nvidia dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/110594] New: std::variant's converting constructor
 does not resolve alternative correctly
Date: Sat, 08 Jul 2023 07:03:13 +0000
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X-Bugzilla-Version: 13.1.1
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110594

            Bug ID: 110594
           Summary: std::variant's converting constructor does not resolve
                    alternative correctly
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xgao at nvidia dot com
  Target Milestone: ---

Created attachment 55502
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D55502&action=3Dedit
minimum repro

According to https://en.cppreference.com/w/cpp/utility/variant/variant, I w=
ould
expect the attached program to output:

int64_t: 1
variant is int

However, I am getting:

int64_t: 1
variant is complex

gcc-9 seems to have the correct behavior for the attached program, however,=
 the
implementation of std::variant in gcc-9 has another problem that the follow=
ing
example in that cppreference.com page does not work:

std::variant<float, long, double> z =3D 0; // OK, holds long
                                         // float and double are not candid=
ates

Thanks to the help of jjsjann123 (https://github.com/jjsjann123), this is
potentially related to the SFINAE in the "variant" header:

  // Helper used to check for valid conversions that don't involve narrowin=
g.
  template<typename _Ti> struct _Arr { _Ti _M_x[1]; };

seems that this will block int64_t from being selected as shown in
https://godbolt.org/z/Gcr4j53rd. However, the following code is totally val=
id:

int64_t x[] =3D {size_t(1)};

so probably that SFINAE might be wrong.=