Proposed resolution (approved by CWG 2023-06-13):

Change in 10.3 [module.import] paragraph 5 as follows:

[Note 2: A header unit is a separate translation unit with an independent set of defined macros. All declarations within a header unit are implicitly exported (10.2 [module.interface]), and are attached to the global module (10.1 [module.unit]). —end note]

[Accepted as a DR at the June, 2023 meeting.]

Subclause 10.3 [module.import] paragraph 5 specifies:

A module-import-declaration that specifies a header-name H imports a synthesized header unit, which is a translation unit formed by applying phases 1 to 7 of translation (5.2 [lex.phases]) to the source file or header nominated by H, which shall not contain a module-declaration. [Note 2: All declarations within a header unit are implicitly exported (10.2 [module.interface]), and are attached to the global module (10.1 [module.unit]). —end note]

It is unclear whether the contents of header units can vary depending on the set of defined macros at the point where the import (or #include) appears.