[Bug libstdc++/106802] New: Comparators in <functional> don't work with orderings in <compare>

[Bug libstdc++/106802] New: Comparators in <functional> don't work with orderings in <compare>

[aaron at aarongraham dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106802

            Bug ID: 106802
           Summary: Comparators in <functional> don't work with orderings
                    in <compare>
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: aaron at aarongraham dot com
  Target Milestone: ---

gcc does not allow this to compile:

    std::less<>{}(std::strong_ordering::less, 0);

Even though `std::strong_ordering::less < 0` is perfectly legal and
well-formed.

It will compile this (but clang will not):

    std::less<>{}(std::strong_ordering::less, nullptr);

Godbolt link: https://gcc.godbolt.org/z/9ed16KbhP

[Bug libstdc++/106802] Comparators in <functional> don't work with orderings in <compare>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106802

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Aaron Graham from comment #0)
> Even though `std::strong_ordering::less < 0` is perfectly legal and
> well-formed.

I disagree.

[cmp.categories.pre] p3 says:

  The relational and equality operators for the comparison category types are
specified
  with an anonymous parameter of unspecified type. This type shall be selected
by the
  implementation such that these parameters can accept literal 0 as a
corresponding
  argument.

When the comparison happens inside std::less<>::operator() it's not being
compared with a literal 0, it's an lvalue of type int by that point. That isn't
required to work.

The version with nullptr isn't required to work either (so libc++ is correct to
reject that).

[Bug libstdc++/106802] Comparators in <functional> don't work with orderings in <compare>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106802

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

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

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #1)
> When the comparison happens inside std::less<>::operator() it's not being
> compared with a literal 0, it's an lvalue of type int by that point. That
> isn't required to work.

In fact, it isn't just not required to work, it's explicitly undefined. The
rest of p3 is:

"In this context, the behavior of a program that supplies an argument other
than a literal 0 is undefined."