[Bug c++/60304] Including disables -Wconversion-null in C++ 98 mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60304

--- Comment #27 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The original testcase gets a warning even for -std=c++98 now, thanks to
dmalcolm's r268589 (included in GCC 9.1).