[Bug c++/114600] New: [modules] redefinition errors when using certain std headers in GMF

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114600

            Bug ID: 114600
           Summary: [modules] redefinition errors when using certain std
                    headers in GMF
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: m.cencora at gmail dot com
  Target Milestone: ---

$ cat std.cpp
module;

#include <type_traits>

export module std;

$ cat main.cpp
import std;

#include <type_traits>

int main()
{
}

$ g++ -std=c++20 -fmodules-ts -c std.cpp
$ g++ -std=c++20 -fmodules-ts main.cpp -o main

In file included from main.cpp:3:
/usr/include/c++/14/type_traits:89:36: error: template definition of
non-template ‘constexpr const _Tp std::integral_constant<_Tp, __v>::value’
   89 |       static constexpr _Tp value = __v;
      |                                    ^~~
/usr/include/c++/14/type_traits:90:29: error: template definition of
non-template ‘using std::integral_constant<_Tp, __v>::value_type = _Tp’
   90 |       using value_type = _Tp;
      |                             ^
/usr/include/c++/14/type_traits:91:47: error: template definition of
non-template ‘using std::integral_constant<_Tp, __v>::type = struct
std::integral_constant<_Tp, __v>’
   91 |       using type = integral_constant<_Tp, __v>;
      |                                               ^
/usr/include/c++/14/type_traits:92:45: error: template definition of
non-template ‘constexpr std::integral_constant<_Tp, __v>::operator value_type()
const’
   92 |       constexpr operator value_type() const noexcept { return value; }
      |                                             ^~~~~~~~
/usr/include/c++/14/type_traits:95:47: error: template definition of
non-template ‘constexpr std::integral_constant<_Tp, __v>::value_type
std::integral_constant<_Tp, __v>::operator()() const’
   95 |       constexpr value_type operator()() const noexcept { return value;
}
      |                                               ^~~~~~~~
/usr/include/c++/14/type_traits:963:26: error: redefinition of default argument
for ‘class _Up’
  963 |   template<typename _Tp, typename _Up = _Tp&&>
      |                          ^~~~~~~~
In file included from std.cpp:3,
of module std, imported at main.cpp:1:
/usr/include/c++/14/type_traits:963:26: note: original definition appeared here
  963 |   template<typename _Tp, typename _Up = _Tp&&>
      |                          ^~~~~~~~
/usr/include/c++/14/type_traits:2470:10: error: redefinition of ‘template<class
_Tp> decltype (__declval<_Tp>(0)) std::declval()’
 2470 |     auto declval() noexcept -> decltype(__declval<_Tp>(0))
      |          ^~~~~~~
/usr/include/c++/14/type_traits:2470:10: note: ‘template<class _Tp> decltype
(__declval<_Tp>(0)) std::declval@std()’ previously declared here


===
Seems like merging of GMFs does not work sometimes.
Also if I create a preprocessed file for <type_traits>
$ g++- -std=c++20 -fmodules-ts -x c++-system-header -E type_traits -o
common.hpp

and include this "common.hpp" instead of <type_traits> in both cpp files, then
compilation succeeds.

This works fine on gcc-13 (at least for <type_traits> header)