[Bug libstdc++/96269] optional comparison with nullopt fails

["sshannin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96269

--- Comment #2 from sshannin at gmail dot com ---
(In reply to Jonathan Wakely from comment #1)
> Your operator== should be const-qualified.

I don't disagree. I can also fully remove the operator== and it compiles as
well (why should the presence of the non-const operator== cause the comparison
with nullopt in one direction to instantiate it).

But yeah, I lost something in my reduction in there. I think the main point is
that one direction of the comparison instantiates the templated operator== and
the other doesn't.

Consider this version of X instead, which avoids the const issues:
struct X {
  int y;

  template <typename T>
  bool operator==(const T&o) const {
    return y == o.summary();
  }
};

We again end up instantiating X's operator== (even though we wouldn't call it)
in the FLIP case (and thus fail to compile), but in the non-flip case
everything seems fine.