[Bug libstdc++/128429] libstdc++ headers not considered system headers in the build tree

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128429

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Right, this is intentional, so that we see warnings for bugs while building and
testing libstdc++.

You can test with -D_GLIBCXX_SYSHDR to simulate the behaviour of testing
installed headers, which might help to analyze and improve tests that are
sensitive to it.