[Bug c++/99682] New: [modules] Module implementation unit is unable to indirectly import its corresponding interface unit

[Bug c++/99682] New: [modules] Module implementation unit is unable to indirectly import its corresponding interface unit

[ensadc at mailnesia dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99682

            Bug ID: 99682
           Summary: [modules] Module implementation unit is unable to
                    indirectly import its corresponding interface unit
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ensadc at mailnesia dot com
  Target Milestone: ---

$ cat src/foo.cpp
export module foo;

$ cat src/bar.cpp
export module bar;
import foo;

$ cat src/foo-impl.cpp
module foo;
import bar;

$ g++ -fmodules-ts -c src/foo.cpp
$ g++ -fmodules-ts -c src/bar.cpp
$ g++ -fmodules-ts -c src/foo-impl.cpp
In module imported at src/foo-impl.cpp:2:1:
bar: error: cannot import module in its own purview
In module imported at src/foo-impl.cpp:1:1:
foo: note: module ‘foo’ declared here
In module imported at src/foo-impl.cpp:2:1:
bar: error: failed to read compiled module: Bad import dependency
bar: note: compiled module file is ‘gcm.cache/bar.gcm’
bar: fatal error: returning to the gate for a mechanical issue
compilation terminated.

====

AFAIK, although a module implementation unit cannot *directly* imports its
module interface unit (http://eel.is/c++draft/module.import#9 ), *indirect*
import should not be prohibited (and should probably be a no-op, because the
primary module interface unit has been imported by `module foo;`).

MSVC compiles this test case without any errors.