[Bug c++/114549] New: GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[Bug c++/114549] New: GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[cpeterson at mozilla dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114549

            Bug ID: 114549
           Summary: GCC >= 10.1 selects the wrong overload of C++20
                    reversed operator== function
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cpeterson at mozilla dot com
  Target Milestone: ---

While updating Firefox from -std=c++17 to -std=c++20, I found a case where
GCC's resolution of C++20 reversed operator== functions behaves differently
from the Clang, MSVC, and ICX compilers. This is Firefox bug
https://bugzilla.mozilla.org/show_bug.cgi?id=1880776

I believe this difference was a regression in GCC 10.1.

Here's a Godbolt test case comparing those compilers' output:

https://godbolt.org/z/qneax5oaW

```
#include <type_traits>

struct Thing {
    template <typename T>
    bool operator==(const T& rhs) const {
        /* This operator== is selected by:
         *   GCC versions >= 10.1 -std=c++17
         *   GCC version 9.5 -std=c++2a
         *   Clang 18.1 -std=c++2a
         *   MSVC 19.38 -std=c++20
         *   Intel's ICX 2024.0.0 -std=c++20
         */
        return false;
    }
};

template <typename T>
bool operator==(T const& lhs, Thing const& rhs) {
    /* This operator== is selected by:
     *   GCC versions >= 10.1 -std=c++2a
     */
    return true;
}

bool test() {
    Thing const v{};
    return v == 3;
}
```

[Bug c++/114549] [11/12/13 Regression] GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114549

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |10.1.0, 13.1.0, 13.2.0
            Summary|GCC >= 10.1 selects the     |[11/12/13 Regression] GCC
                   |wrong overload of C++20     |>= 10.1 selects the wrong
                   |reversed operator==         |overload of C++20 reversed
                   |function                    |operator== function
   Target Milestone|---                         |11.5
           Keywords|                            |needs-bisection
      Known to work|                            |14.0

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Looks to be fixed on the trunk though.

[Bug c++/114549] [11/12/13 Regression] GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[cpeterson at mozilla dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114549

Chris Peterson <cpeterson at mozilla dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #2 from Chris Peterson <cpeterson at mozilla dot com> ---
> Looks to be fixed on the trunk though.

Thanks for verifying! I see now on godbolt.org that this bug is fixed in GCC
trunk (though GCC 13.2.0 still has the bug). In that case, I will resolve this
bug as fixed.

[Bug c++/114549] [11/12/13 Regression] GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114549

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-04-01
         Resolution|FIXED                       |---
             Status|RESOLVED                    |NEW
     Ever confirmed|0                           |1

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Chris Peterson from comment #2)
> > Looks to be fixed on the trunk though.
> 
> Thanks for verifying! I see now on godbolt.org that this bug is fixed in GCC
> trunk (though GCC 13.2.0 still has the bug). In that case, I will resolve
> this bug as fixed.

It was a regression on the other releases so let's leave it open until it is
fixed on the release branches or decided it is too hard to do as such.

[Bug c++/114549] [11/12/13 Regression] GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114549

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
           Keywords|needs-bisection             |

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Looks like it's changed in r14-6221.  Doesn't look backportable.

[Bug c++/114549] [11/12/13 Regression] GCC >= 10.1 selects the wrong overload of C++20 reversed operator== function

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114549

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2