[Bug c++/106851] New: [modules] Name conflict for exported using-declaration

[Bug c++/106851] New: [modules] Name conflict for exported using-declaration

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

            Bug ID: 106851
           Summary: [modules] Name conflict for exported using-declaration
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 103524
  Target Milestone: ---

$ cat lib.h
namespace outer {
  template<typename T> void any_of(T) { }

  namespace inner {
    struct A { };
    inline constexpr A any_of;
  }
}

$ gcc -c -std=c++23 -fmodules-ts -x c++-header lib.h


$ cat lib.cc
export module lib;

import "lib.h";

export using outer::any_of;
export using outer::inner::any_of;


$ gcc -c -std=c++23 -fmodules-ts lib.cc 
lib.cc:6:28: error: ‘constexpr const outer::inner::A outer::inner::any_of’
conflicts with a previous declaration
    6 | export using outer::inner::any_of;
      |                            ^~~~~~
In module ./lib.h, imported at lib.cc:3:
lib.h:2:29: note: previous declaration ‘void outer::any_of(T)’
    2 |   template<typename T> void any_of(T) { }
      |                             ^~~~~~
lib.cc:1:8: warning: not writing module ‘lib’ due to errors
    1 | export module lib;
      |        ^~~~~~


The names are in different scopes, shouldn't they be exported that way too?


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103524
[Bug 103524] [meta-bug] modules issue

[Bug c++/106851] [modules] Name conflict for exported using-declaration

[johelegp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

Johel Ernesto Guerrero Peña <johelegp at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |johelegp at gmail dot com

--- Comment #1 from Johel Ernesto Guerrero Peña <johelegp at gmail dot com> ---
Could https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106363 be related?

[Bug c++/106851] [modules] Name conflict for exported using-declaration

[nathanieloshead at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

Nathaniel Shead <nathanieloshead at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |nathanieloshead at gmail dot com

--- Comment #2 from Nathaniel Shead <nathanieloshead at gmail dot com> ---
This behaviour should be as expected right? The 'using' is trying to bring both
names into the same scope (the global namespace), irrespective of the fact that
we're also exporting that new declaration. (That is, removing the 'export'
keywords from this test case gives the exact some result.)

That said, perhaps it would be helpful for the error message to point to the
using-declaration it actually conflicts with, rather than the definition that
said using-declaration points to.

[Bug c++/106851] [modules] Name conflict for exported using-declaration

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-02-28
     Ever confirmed|0                           |1
           Keywords|rejects-valid               |diagnostic

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Nathaniel Shead from comment #2)
> This behaviour should be as expected right?

Quite possibly, I still don't know how to use modules. I was just trying to
figure out how to define a 'std' module that includes all the library headers
and then exports everything.

Let's reclassify this as a diagnostic bug then.

[Bug c++/106851] [modules] Name conflict for exported using-declaration

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I tried doing it this way instead

namespace std {
  export using std::vector;
  namespace pmr {
    export using std::vector;
  }
}

but that didn't work, nothing got exported. But maybe that will be fixed by
Nathaniel's https://gcc.gnu.org/pipermail/gcc-patches/2024-March/647088.html

[Bug c++/106851] [modules] Name conflict for exported using-declaration

[nshead at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

--- Comment #5 from Nathaniel Shead <nshead at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #4)
> I tried doing it this way instead
> 
> namespace std {
>   export using std::vector;
>   namespace pmr {
>     export using std::vector;
>   }
> }
> 
> but that didn't work, nothing got exported. But maybe that will be fixed by
> Nathaniel's https://gcc.gnu.org/pipermail/gcc-patches/2024-March/647088.html

Yup, looks to be this issue. This is merged into trunk now and with a quick
test appears to work: https://godbolt.org/z/T5zPfPGro

[Bug c++/106851] [modules] Name conflict for exported using-declaration

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106851

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Excellent! Thanks for the fix.