[Bug c++/43167] Warnings should not be disabled when instantiating templates defined in system headers

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43167

--- Comment #18 from Jonathan Wakely <redi at gcc dot gnu.org> ---
From Matthias Kretz on IRC:

GCC needs to make a difference between warnings that apply when reading the
system headers and warnings that only manifest on a template instantiation or
constprop