[Bug c++/100381] New: new static_assert((std::__is_complete_or_unbounded(...)) failure from g++ 11.1.0

["gilles.gouaillardet at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100381

            Bug ID: 100381
           Summary: new
                    static_assert((std::__is_complete_or_unbounded(...))
                    failure from g++ 11.1.0
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gilles.gouaillardet at gmail dot com
  Target Milestone: ---

Created attachment 50732
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50732&action=edit
a reproducer that fails from g++ 11.1.0

g++ 11.1.0 is no more able to compile GROMACS, as reported at
https://gitlab.com/gromacs/gromacs/-/issues/4039

I trimmed the offending code into the attached and self-contained reproducer,
that fails to build with the following error:

$ g++ -c reproducer.cpp
In file included from
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/move.h:57,
                 from
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/stl_function.h:60,
                 from
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/functional:49,
                 from reproducer.cpp:1:
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits: In
instantiation of 'struct std::is_invocable_r<void,
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>&, IndexGroupsAndNames>':
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits:3011:46:  
required from 'constexpr const bool std::is_invocable_r_v<void,
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>&, IndexGroupsAndNames>'
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/invoke.h:103:27:  
required by substitution of 'template<class _Res, class _Callable, class ...
_Args> constexpr std::enable_if_t<is_invocable_r_v<_Res, _Callable, _Args ...>,
_Res> std::__invoke_r(_Callable&&, _Args&& ...) [with _Res = void; _Callable =
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>&; _Args = {IndexGroupsAndNames}]'
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/std_function.h:291:30:
  required from 'static _Res std::_Function_handler<_Res(_ArgTypes ...),
_Functor>::_M_invoke(const std::_Any_data&, _ArgTypes&& ...) [with _Res = void;
_Functor =
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>; _ArgTypes = {IndexGroupsAndNames}]'
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/std_function.h:422:21:
  required from 'std::function<_Res(_ArgTypes ...)>::function(_Functor) [with
_Functor =
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>; <template-parameter-2-2> = void;
<template-parameter-2-3> = void; _Res = void; _ArgTypes =
{IndexGroupsAndNames}]'
reproducer.cpp:55:56:   required from here
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits:2954:7:
error: static assertion failed: each argument type must be a complete class or
an unbounded array
 2954 |       static_assert((std::__is_complete_or_unbounded(
      |       ^~~~~~~~~~~~~
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits:2954:7:
note:
'std::__is_complete_or_unbounded<std::__type_identity<IndexGroupsAndNames>
>((std::__type_identity<IndexGroupsAndNames>{},
std::__type_identity<IndexGroupsAndNames>()))' evaluates to false
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits: In
instantiation of 'struct std::is_nothrow_invocable_r<void,
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>&, IndexGroupsAndNames>':
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits:3016:54:  
required from 'constexpr const bool std::is_nothrow_invocable_r_v<void,
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>&, IndexGroupsAndNames>'
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/invoke.h:105:14:  
required from 'constexpr std::enable_if_t<is_invocable_r_v<_Res, _Callable,
_Args ...>, _Res> std::__invoke_r(_Callable&&, _Args&& ...) [with _Res = void;
_Callable =
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>&; _Args = {IndexGroupsAndNames};
std::enable_if_t<is_invocable_r_v<_Res, _Callable, _Args ...>, _Res> = void]'
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/std_function.h:291:30:
  required from 'static _Res std::_Function_handler<_Res(_ArgTypes ...),
_Functor>::_M_invoke(const std::_Any_data&, _ArgTypes&& ...) [with _Res = void;
_Functor =
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>; _ArgTypes = {IndexGroupsAndNames}]'
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/bits/std_function.h:422:21:
  required from 'std::function<_Res(_ArgTypes ...)>::function(_Functor) [with
_Functor =
{anonymous}::DensityFitting::subscribeToPreProcessingNotifications(MdModulesNotifier*)::<lambda(const
IndexGroupsAndNames&)>; <template-parameter-2-2> = void;
<template-parameter-2-3> = void; _Res = void; _ArgTypes =
{IndexGroupsAndNames}]'
reproducer.cpp:55:56:   required from here
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits:2992:7:
error: static assertion failed: each argument type must be a complete class or
an unbounded array
 2992 |       static_assert((std::__is_complete_or_unbounded(
      |       ^~~~~~~~~~~~~
/home/users/gilles/local/gcc-11.1.0/include/c++/11.1.0/type_traits:2992:7:
note:
'std::__is_complete_or_unbounded<std::__type_identity<IndexGroupsAndNames>
>((std::__type_identity<IndexGroupsAndNames>{},
std::__type_identity<IndexGroupsAndNames>()))' evaluates to false


The same code can be compiled with previous g++ versions (and many other
compilers such as clang++).

A "fix" is to replace
class IndexGroupsAndNames;
with
class IndexGroupsAndNames {};


git bisect points to the following commit

commit c1fc9f6e10e646f01194c8f150affbc1cfbc404a
Author: Antony Polukhin <antoshkka@gmail.com>
Date:   Thu Sep 24 18:51:37 2020 +0100

    libstdc++: assert that type traits are not misused with incomplete types
[PR 71579]


Based on the commit message and me being illiterate in modern C++, I cannot
tell whether the reproducer is valid C++, or g++ is right about rejecting it.


Could you please assess the validity of the reproducer and rule g++ behavior
w.r.t modern C++ standard?