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From: "jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libstdc++/97841] New: [C++17] is_invocable handling of
 incomplete return type is wrong
Date: Sun, 15 Nov 2020 17:01:51 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97841

            Bug ID: 97841
           Summary: [C++17] is_invocable handling of incomplete return
                    type is wrong
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jason at gcc dot gnu.org
  Target Milestone: ---

I already mentioned this to Jonathan directly, but thought I should probably
also put it in bugzilla.

is_invocable and invoke_result condition their result on whether

"The expression INVOKE(declval<Fn>(),declval<ArgTypes>()...) is well-formed
when treated as an unevaluated operand"

It seems that we currently test for this by checking whether decltype of the
above is well-formed, but that seems wrong to me, since decltype of a call =
of
incomplete type is well-formed, but that call in any other unevaluated cont=
ext
is ill-formed.  So we accept this testcase, but VC++ and clang/libc++ reject
it:

#include <type_traits>
struct A;
using fn =3D A(*)(int);
static_assert (std::is_invocable_v<fn,int>); // error, A is incomplete

Various other places in the library talk about an expression being "well-fo=
rmed
when treated as an unevaluated operand" and could probably use a check to m=
ake
sure they don't have the same problem.

I imagine that changing decltype(INVOKE(...)) to decltype(0,INVOKE(...)) in
various places would be a simple fix?=