[Bug c++/93929] New: In copy elision cases, fallback to lvalue even if rvalue overload resolution succeeds

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93929

            Bug ID: 93929
           Summary: In copy elision cases, fallback to lvalue even if
                    rvalue overload resolution succeeds
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Example:

struct X {
    X();
    X(X const&);
    X(X&&) = delete;
};
X make() {
    X a;
    return a;
}

This should be ill-formed. The rules are:
> If the first overload resolution fails or was not performed, overload resolution is performed again, considering the expression or operand as an lvalue.
and
> If a best viable function exists and is unique, overload resolution succeeds and produces it as the result.

Overload resolution considering "a" as an rvalue finds the move constructor.
That's successful overload resolution - we should pick it, and then reject the
program (edg and msvc do this). But gcc (and clang) considers this as failing
overload resolution and falls back to pick the copy constructor.