[Bug libstdc++/119050] New: libstdc++ headers are not `-Wsystem-headers -Wundef ` friendly

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119050

            Bug ID: 119050
           Summary: libstdc++ headers are not `-Wsystem-headers -Wundef `
                    friendly
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Created attachment 60605
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=60605&action=edit
warnings with -Wundef

A simple:
```
#include <iostream>
```

Causes the attached warnings when used with `-Wsystem-headers -Wundef `.

I am not 100% sure with -Wundef should ever be used with C++ because the
recommended way to using the C++ feature macros is just use them (without
testing if they are defined) and comparing with a specific date.

So I don't mind if this will be closed as won't fix either.

Or maybe -Wundef can be refinded not to warn about the C++ feature macros ...