[Bug preprocessor/81794] "would be stringified in traditional C" warning should be controlled by -Wtraditional

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81794

--- Comment #2 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
Please can you turn it into a patch that contains both the fix *and* the new
testcase?  (e.g. gcc.dg/pragma-diag-8.c)