[Bug c++/119388] New: [12/13/14/15 Regression] -isystem does not disable warnings coming from system headers

[dangelog at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119388

            Bug ID: 119388
           Summary: [12/13/14/15 Regression] -isystem does not disable
                    warnings coming from system headers
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dangelog at gmail dot com
  Target Milestone: ---

When upgrading from GCC 11 to 13/14, some warnings start to appear from headers
that have been included via -isystem.

A minimal example would be creating a header containing this:

#include <iostream>

class Stuff {
    int x;
public:
    Stuff() { std::cout << x; }
};



And then including it via -isystem, and creating a Stuff object somewhere.
Under -O2 -Wall -Wextra this yields on GCC 14:


/app/my_includes/stuff.h:9:28: warning: 's.Stuff::x' is used uninitialized
[-Wuninitialized]
    9 |     Stuff() { std::cout << x; }


Whereas nothing was emitted on GCC 11.


I've assembled a somehow more flexible example here on Godbolt:
https://gcc.godbolt.org/z/c1Ts3ze87 

You can play with:
- different GCC versions / compiler option (anything above 11 will trigger the
warning; 11 will as well, if disabling system headers)
- the `#pragma GCC system_header`
- setting SYSTEM in the CMakeLists.txt (so that CMake uses -isystem)
- disabling inlining of f() into main() (which will suppress a second warning)


In projects using Qt we're randomly seeing this for optimization-related
warnings: -Wnull-dereference, -Wstrict-overflow, -Wmaybe-uninitialized. Some
degree of optimization seems to be necessary.


Apologies, it's highly probable that there could be duplicates of this, but
bugzilla's search isn't really the best.