[Bug libstdc++/115145] New: Help lambda in rewritten std::variant comparisons does not specify return type

[Bug libstdc++/115145] New: Help lambda in rewritten std::variant comparisons does not specify return type

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115145

            Bug ID: 115145
           Summary: Help lambda in rewritten std::variant comparisons does
                    not specify return type
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

..which can be an regression:

#include <variant>

struct Bool {
  operator bool();
  Bool(const Bool&) = delete;
};

struct Elem {
  Bool& operator==(const Elem&) const;
};

int main() {
  std::variant<Elem> v;
  return v == v;
}

https://godbolt.org/z/14q14TrYW

[Bug libstdc++/115145] Help lambda in rewritten std::variant comparisons does not specify return type

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115145

--- Comment #1 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:b33f44ca6ce2dae7502ce138600e1631ffc9232c

commit r15-769-gb33f44ca6ce2dae7502ce138600e1631ffc9232c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 21 15:13:01 2024 +0100

    libstdc++: Ensure std::variant relops convert to bool [PR115145]

    Ensure that the result of comparing the variant alternatives is
    converted to bool immediately rather than copied.

    libstdc++-v3/ChangeLog:

            PR libstdc++/115145
            * include/std/variant (operator==, operator!=, operator<)
            (operator<=, operator>, operator>=): Add trailing-return-type to
            lambda expressions to trigger conversion to bool.
            * testsuite/20_util/variant/relops/115145.cc: New test.