[Bug c++/100617] [modules] Exported namespace not visible from outside when the module imports another module that declares the same namespace

["asynts+bugs at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100617

Paul Scharnofske <asynts+bugs at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |asynts+bugs at gmail dot com

--- Comment #2 from Paul Scharnofske <asynts+bugs at gmail dot com> ---
I ran into the same issue and produced a slightly different example:

```c++
// foo.cpp
export module foo;

import bar;

namespace foo
{
    export void foo() { }
}
```
```c++
// bar.cpp
export module bar;

namespace foo
{

}
```
```c++
// baz.cpp
export module baz;

import foo;
import bar;

int main() {
    foo::foo();
}
```
```none
$ ~/.local/lib/gcc-trunk/bin/g++ --version
g++ (GCC) 12.0.1 20220211 (experimental)
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ ~/.local/lib/gcc-trunk/bin/g++ -Wall -Wextra -std=c++20 -fmodules-ts bar.cpp
foo.cpp baz.cpp
baz.cpp: In function 'int main()':
baz.cpp:8:5: error: 'foo' has not been declared
    8 |     foo::foo();
      |     ^~~
baz.cpp: At global scope:
baz.cpp:2:8: warning: not writing module 'baz' due to errors
    2 | export module baz;
      |        ^~~~~~
```
https://godbolt.org/z/8zMaq6eqr

-   In other words, the 'export' specifier doesn't even have to be mentioned.
    It is enough if the same namespace is mentioned even if it is completely
empty and not exported.

-   The issue disappears if the 'import bar' is removed from 'foo.cpp', in
other words, this only happens
    if that module is actually imported.  It doesn't matter that it's still
imported in 'baz.cpp'.