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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/93628] ranges::equal_to doesn't work for types
 convertible to function pointers
Date: Tue, 09 Mar 2021 11:17:27 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D93628

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
LWG 3530 should mean we don't have to support such silly types.
https://wg21.link/lwg3530

However, we are still required to impose a total order on function pointers,
which means ranges::less/greater/less_equal/greater_equal are still
(potentially) broken, as for object pointers in PR 78420. We might get away
with it if the compiler doesn't do similar optimizations for function point=
ers,
but we used to say that about object pointers too, and it stopped being tru=
e.

As noted above, we don't have a way to detect that a relational comparison
would call a built-in operator on function pointers. Maybe what we want is a
__builtin_less_than(t, u) which is t < u but with a total order.=