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From: "ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/94061] defaulted member operator <=> defined as deleted if
 a base has protected member operator <=>
Date: Sat, 08 Jan 2022 16:35:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94061

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org

--- Comment #2 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Frank Heckenbach from comment #1)
> I ran into the same problem.
>=20
> Interestingly, clang also seems to reject it, so maybe it is wrong by the
> letter of the standard? Though it would seem strange to me -- after all,
> when manually implementing B::operator<=3D> a protected operator in A wil=
l do
> fine.

How do you define it?  It works if we define it as

  auto operator <=3D> (const B& b) const {
    return A::operator<=3D>(b);
  }

but not if it's defined as

  auto operator <=3D> (const B& b) const {
    return static_cast<const A&>(*this) <=3D> static_cast<const A&>(b);
  }

According to [class.spaceship], IIUC the synthesized operator<=3D> looks mo=
re
similar to the latter invalid definition (invoking <=3D> recursively as an
operator expression on each pair of corresponding subobjects), so GCC/Clang
might be right to define it as deleted.=