[Bug c++/126869] New: Loss of character due to adding header

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126869

            Bug ID: 126869
           Summary: Loss of character due to adding header
           Product: gcc
           Version: 16.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: d7d1cd at mail dot ru
  Target Milestone: ---

I encountered the following problem when using module partitions. I have this
header:
    $ cat foo.h
    #pragma once
    class Foo {};


It's added to the M module partition:
    $ cat foo.ixx
    module;
    #include "foo.h"
    export module M : foo;
    export using ::Foo;


The module interface looks like this:
    $ cat module.ixx
    module;
    #include "foo.h"
    export module M;
    export import : foo;


The module is used as follows:
    $ cat main.cc
    import M;
    int main() {
        Foo f;
    }


When compiling these files, I get the following error:
    $ g++ -std=c++23 -fmodules foo.ixx module.ixx main.cc 
    main.cc: In function ‘int main()’:
    main.cc:3:5: error: ‘Foo’ was not declared in this scope
        3 |     Foo f;
          |     ^~~


If I remove the header inclusion from the interface, the compilation succeeds.
Here's an example in the online compiler: https://godbolt.org/z/Mo6oKW7vh