[Bug c++/72837] New: -Wundef is not being ignored with pragma

["TrevorJamesHickey at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72837

            Bug ID: 72837
           Summary: -Wundef is not being ignored with pragma
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: TrevorJamesHickey at gmail dot com
  Target Milestone: ---

Given the following code:  

    #if MACRO_WITHOUT_A_VALUE
    int var;
    #endif

    int main(){}
When compiled with, `g++ -std=c++1z -Wundef -o main main.cpp`,  
it produces the following warning:  

    main.cpp:1:5: warning: "MACRO_WITHOUT_A_VALUE" is not defined [-Wundef]
     #if MACRO_WITHOUT_A_VALUE
         ^
I'd like to keep the warning flag enabled, but suppress this particular
instance.  
I apply the following:

    #ifdef __GNUC__
    #pragma GCC diagnostic ignored "-Wundef"
    #pragma GCC diagnostic push
    #endif

    #if MACRO_WITHOUT_A_VALUE
    int var;
    #endif

    #ifdef __GNUC__
    #pragma GCC diagnostic pop
    #endif

    int main(){}
This only solves the problem in `clang++`.  

The command `clang++ -std=c++1z -Wundef -o main main.cpp` builds without
warnings.  
The command `g++ -std=c++1z -Wundef -o main main.cpp` builds with the same
`[-Wundef]` warning as before.

**How can I suppress `-Wundef` warnings in `g++`?**  

    g++ (Ubuntu 5.1.0-0ubuntu11~14.04.1) 5.1.0
    clang version 3.8.0

http://stackoverflow.com/questions/38831058/wundef-is-not-being-ignored-with-pragma-in-g