[Bug c++/61412] Warnings incorrectly suppressed when compiling previously preprocessed file

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61412

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm pretty sure there's an existing report about this.

Adding -Wsystem-headers makes it warn.