Proposed resolution:

Change [res.on.headers], Headers, paragraph 1, as indicated:

A C++ header may include other C++ headers.^[footnote] A C++ header shall provide the declarations and definitions that appear in its synopsis ([basic.def.odr]). A C++ header shown in its synopsis as including other C++ headers shall provide the declarations and definitions that appear in the synopses of those other headers.

^[footnote] C++ headers must include a C++ header that contains any needed definition (3.2).

See Frankfurt notes of 1001.