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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99374] C++17/20 mode fails to recognise pointer-to-member
 functions of incomplete types in conditional expression
Date: Thu, 04 Mar 2021 10:44:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99374

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

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The change (and the reason it works pre-C++17) is noexcept being part of the
type. Apparently G++ thinks the implicit conversion from R (C::*)() noexcep=
t to
R(C::*)() depends on C being complete.

So it has nothing to do with conditional expressions and can be reduced to:

struct S;
using F1 =3D int (S::*)();
using F2 =3D int (S::*)() noexcept;
F1 f1 =3D F2();=