[Bug c/66711] New: GCC does not correctly restore diagnostic state after pragma GCC diagnostic pop with -Werror

["steffen.muething at iwr dot uni-heidelberg.de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66711

            Bug ID: 66711
           Summary: GCC does not correctly restore diagnostic state after
                    pragma GCC diagnostic pop with -Werror
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: steffen.muething at iwr dot uni-heidelberg.de
  Target Milestone: ---

Created attachment 35880
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35880&action=edit
MWE, compile with "gcc -Wextra -Werror wrong-diagnostic-pop.c"

GCC 5.1.0 fails to correctly restore the diagnostics state after a "#pragma GCC
diagnostic pop" if the program was compiled with -Werror: Instead of emitting
an error, the compiler insteads issues a warning after the pragma.

I've attached a simple example file. When compiled with "-Wextra -Werror", I'd
expect the output:

"""
error.cc: In function 'int main()':
error.cc:9:10: error: comparison between signed and unsigned integer
expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
error.cc:18:10: error: comparison between signed and unsigned integer
expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
cc1plus: all warnings being treated as errors
"""

But I get the following:

"""
error.cc: In function 'int main()':
error.cc:9:10: error: comparison between signed and unsigned integer
expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
error.cc:18:10: warning: comparison between signed and unsigned integer
expressions [-Wsign-compare]
   x += x < y ? 1 : 0;
          ^
cc1plus: all warnings being treated as errors
"""

The second diagnostic was turned into a warning. All older versions of GCC that
I have available and that support diagnostic push/pop (4.6.4, 4.7.4, 4.8.4,
4.9.2) handle this correctly and issue an error in both cases. Example output
from GCC 4.9.2:

"""
wrong-diagnostic-pop.c: In function 'main':
wrong-diagnostic-pop.c:9:10: error: comparison between signed and unsigned
integer expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
wrong-diagnostic-pop.c:18:10: error: comparison between signed and unsigned
integer expressions [-Werror=sign-compare]
   x += x < y ? 1 : 0;
          ^
cc1: all warnings being treated as errors
"""

The problem is also present in the C++ frontend.