[Bug preprocessor/117124] Header file contents get modified when macro defined before header file inclusion

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117124

--- Comment #4 from Sam James <sjames at gcc dot gnu.org> ---
"If the program defines a reserved identifier or attribute token described in
6.7.12.1 as a macro name,
or removes (with #undef) any macro definition of an identifier in the first
group listed above or
attribute token described in 6.7.12.1, the behavior is undefined."