[Bug c++/110948] New: Incorrect -Winvalid-constexpr on virtual defaulted operator==

[Bug c++/110948] New: Incorrect -Winvalid-constexpr on virtual defaulted operator==

[arthur.j.odwyer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110948

            Bug ID: 110948
           Summary: Incorrect -Winvalid-constexpr on virtual defaulted
                    operator==
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arthur.j.odwyer at gmail dot com
  Target Milestone: ---

Bug #98712 seems related.

// https://godbolt.org/z/eKKxcovEn
struct D;
struct B {
    bool operator==(const B&) const;
    virtual bool operator==(const D&) const;
};
struct D : B {
    bool operator==(const D&) const override = default;
};

GCC alone gives this bogus -Winvalid-constexpr warning:

<source>: In member function 'virtual constexpr bool D::operator==(const D&)
const':
<source>:10:10: warning: call to non-'constexpr' function 'bool
B::operator==(const B&) const' [-Winvalid-constexpr]
   10 |     bool operator==(const D&) const override = default;
      |          ^~~~~~~~
<source>:5:10: note: 'bool B::operator==(const B&) const' declared here
    5 |     bool operator==(const B&) const;
      |          ^~~~~~~~

This is obviously contrived code, but the symptom might indicate that GCC is
too eager to pretend that the user actually wrote `constexpr`, in situations
where the compiler is merely supposed to make an implicitly defined function
constexpr-friendly if possible.

The AFAICT-analogous situation with `operator=` instead of `operator==`
correctly compiles without warning: https://godbolt.org/z/Mof1qaadr