Proposed resolution (approved by CWG 2024-08-16):

Change in 10.2 [module.interface] paragraph 6 as follows:

A redeclaration of an entity X is implicitly exported if X was introduced by an exported declaration; otherwise it shall not be exported if it is attached to a named module.

Consider:

  export module mod;
  extern "C++" void func();
  export extern "C++" {
    void func();
  }

Per 10.2 [module.interface] paragraph 6, this is currently ill-formed, but implementation treatment varies and there seems to be little rationale why this should be ill-formed for entities not attached to a named module.

The rule also makes the following example ill-formed, which was intended to be well-formed:

  export module M;
  namespace N { // external linkage, attached to global module, not exported
    void f();
  }
  namespace N { // error: exported namespace, redeclares non-exported namespace
    export void g();
  }

The mental model here is that for an entity not attached to a named module, exportedness is not a meaningful property of that entity.