[Bug c++/105985] New: The case of "indirectly" exported by the primary module interface unit is not accepted

[xmh970252187 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105985

            Bug ID: 105985
           Summary: The case of "indirectly" exported by the primary
                    module interface unit is not accepted
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xmh970252187 at gmail dot com
  Target Milestone: ---

// translation unit C 
export module M :C;

// translation unit U
export module M :U;
import :C;

// translation unit T
export module M;
export import :U;  // #1

[module.unit] says
> All module partitions of a module that are module interface units shall be directly or indirectly exported by the primary module interface unit ([module.import]). No diagnostic is required for a violation of these rules.



[module.import] p7 says

> When a module-import-declaration imports a translation unit T, it also imports all translation units imported by exported module-import-declarations in T; such translation units are said to be exported by T. Additionally, when a module-import-declaration in a module unit of some module M imports another module unit U of M, it also imports all translation units imported by non-exported module-import-declarations in the module unit purview of U. These rules can in turn lead to the importation of yet more translation units.


It is clear that "U" is exported by "T" since an exported
module-import-declaration at #1 imports "U" in "T", which is exactly said in
the first part of [module.import] p7.

Since "T" and "U" are both module units of "M", the second part of
[module.import] p7 should apply to the "module-import-declaration" at #1, since
the "non-exported module-import-declaration"("import :C;") appears in U, the
module-import-declaration at #1 should also import "C". So, "C" is arguably
said that it is indirectly exported by "T". All partition module interface
units of M are exported by the primary module interface unit of M. There is no
violation here.  

However, If we compile the example through GCC, it reports an error that:

> interface partition is not exported