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

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

[jason at gcc dot 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?

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

[redi at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |redi at gcc dot gnu.org
   Last reconfirmed|                            |2020-11-19
             Status|UNCONFIRMED                 |ASSIGNED
   Target Milestone|---                         |11.0
     Ever confirmed|0                           |1

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

[redi at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|redi at gcc dot gnu.org            |unassigned at gcc dot gnu.org
             Status|ASSIGNED                    |NEW

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.0                        |11.2

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.1 has been released, retargeting bugs to GCC 11.2.

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

[rguenth at gcc dot gnu.org]

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

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.2                        |11.3

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11.2 is being released, retargeting bugs to GCC 11.3

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

[redi at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.3                        |11.4

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

[jakub at gcc dot gnu.org]

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

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.4                        |11.5

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 11.4 is being released, retargeting bugs to GCC 11.5.

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

[redi at gcc dot gnu.org]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|11.5                        |---