[Bug libstdc++/97841] New: [C++17] is_invocable handling of incomplete return type is wrong

["jason at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97841

            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 context
is ill-formed.  So we accept this testcase, but VC++ and clang/libc++ reject
it:

#include <type_traits>
struct A;
using fn = 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-formed
when treated as an unevaluated operand" and could probably use a check to make
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?