This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
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. ... An importable header is a member of an implementation-defined set of headers that includes all importable C++ library headers (184.108.40.206 [headers]). H shall identify an importable header. ...
This text seems to recognize that header units may be synthesized from non-headers (i.e. source files), yet a module-import-declaration of the form import header-name is required to name an (importable) header, not an arbitrary source file turned into header unit.
Change in 10.3 [module.import] paragraph 5 as follows:
H shall identify an importable header.