https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69721
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
17.6.2.2 [using.headers]
"A translation unit shall include a header only outside of any declaration or
definition,"
That includes namespace declarations ("header" in the standard means a standard
header, this rule doesn't apply to non-standard include files).
