[Bug c++/80472] New: cannot use push/pop with #pragma GCC diagnostic warning "-Wsystem-headers"

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80472

            Bug ID: 80472
           Summary: cannot use push/pop with #pragma GCC diagnostic
                    warning "-Wsystem-headers"
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
            Blocks: 58876, 69769, 70692
  Target Milestone: ---

Given the following header:

#pragma GCC system_header
#pragma GCC diagnostic push
#pragma GCC diagnostic warning "-Wsystem-headers"
// want warnings here despite being in a system header
#pragma GCC diagnostic pop
// But don't want warnings here
int missing_return() { }


A file including it will warn with -Wreturn-type (or -Wall):

In file included from dep.cc:1:0:
dep.h: In function ‘int missing_return()’:
dep.h:8:24: warning: no return statement in function returning non-void
[-Wreturn-type]
 int missing_return() { }
                        ^

The warning should be disabled again by the "pop", but -Wsystem-headers stays
active once enabled.

This makes it very difficult for libstdc++ to selectively enable important
warnings, e.g. Bug 58876


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58876
[Bug 58876] No non-virtual-dtor warning in std::unique_ptr
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69769
[Bug 69769] arithmetic operation on pointer to a function
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70692
[Bug 70692] No warning when std::function<const int&(...)> binds a reference to
a temporary