In module-file it is forbidden.

• 15.5 Header unit importation [cpp.import]

However, it is unclear whether private module fragment is included in that (module-file), and if it is, then global module fragment is included as well.

Looking at P1857R3, which introduced this clause, it says:

1. Change Log

• R2
  • Add wording that rejects #includes from turning into imports in the purview of a module.

Since it says "purview of a module", there seems to be no intention to include global module fragment. On the other hand, private module fragment seems to be included.

The current clause [cpp.import]/3 seems to either include both fragments or none of them.

At least I think private module fragment is included, but what about?

If a pp-import is produced by source file inclusion (including by the rewrite produced when a #include directive names an importable header) while processing the group of a module-file or pp-private-module-fragment
, the program is ill-formed.