[Bug libstdc++/69721] [6 Regression] math.h is broken, unqualified use of __is_integer

[redi at gcc dot gnu.org]

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).