[Bug c++/114990] Compiler errors in <type_traits> compiling a module-based app

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114990

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ppalka at gcc dot gnu.org
         Depends on|                            |99000

--- Comment #1 from Patrick Palka <ppalka at gcc dot gnu.org> ---
After preprocessing, it seems Main.cpp basically boils down to:

import CompilerVersions; // #1
#include <type_traits>   // #2
import TypeTraits;       // #3

int main()
{
    return 0;
}

The issue here seems to be the #include <type_traits> appearing after the
import of CompilerVersions which (indirectly) also includes <type_traits> (in
its GMF).  GCC currently supports import-after-include GMF merging (so #2
followed by #3 is OK) not but include-after-import.  See also
https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Modules.html

So one workaround would be to arrange that all #includes come before imports
which might include the same thing.  Another workaround would be to compile the
problematic headers (<type_traits> in this case) as a header unit so that
#includes thereof implicitly translated into imports, via e.g.

g++ -fmodules-ts -x c++-system-header type_traits


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99000
[Bug 99000] [modules] merging of textual redefinitions: declaration
std::__copy_move_a2 conflicts with import