[Bug preprocessor/85211] New: -isystem automatically adds extern "C" to includes

[blastrock at free dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85211

            Bug ID: 85211
           Summary: -isystem automatically adds extern "C" to includes
           Product: gcc
           Version: 7.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blastrock at free dot fr
  Target Milestone: ---

This is a very old problem documented in GCC 6:

https://gcc.gnu.org/onlinedocs/gcc-6.4.0/cpp/System-Headers.html#System-Headers

(see the last paragraph)

However that part was removed from the documentation in GCC 7:

https://gcc.gnu.org/onlinedocs/gcc-7.3.0/cpp/System-Headers.html#System-Headers

I would suggest putting it back since that legacy behavior is still present.
Also, it would be nice to have a documented way to opt out from it.

If it is of any importance, I have that problem with the g++ compiled in the
vitasdk (
https://github.com/vitasdk/autobuilds/releases/download/master-linux-v771/vitasdk-x86_64-linux-gnu-2018-04-02_17-48-36.tar.bz2
) that should have been built with the following script:
https://github.com/vitasdk/buildscripts/blob/master/CMakeLists.txt . The target
arch is arm-vita-eabi.