Created attachment 35517 [details]
Repro case with description

When compiling the attached sample with g++ 5.1.0 or 5.1.1 and -Werror the suppression of strict-overflow works for the approaches taken in the testing2 and testing3 functions but the analogous one in the TESTING macro will still produce a strict-overflow warning. This warning will strangely - even though -Werror is set - not result in an error though. With g++ 4.9.2 the diagnostic ignored seems to have no effect at all and a warning with resulting error is created in the TESTING macro.

When compiling with gcc instead of g++ - as intended - none of the approaches leads to a warning or an error in any of the aforementioned versions.

A related bug might be https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53469 . I found it when trying to create the repro sample for this bug. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431 has the same g++ only behavior but - with my limited understanding of g++ internals - I wouldn't expect the strict-overflow warning to be created in that early processing stage causing that issue.

Repro sample:

// g++ -fstrict-overflow -Wstrict-overflow -Werror macro.c
// gcc -fstrict-overflow -Wstrict-overflow -Werror macro.c

// Under g++:
//   The TESTING macro still presents a strict-overflow warning
//   with gcc 5.1.0 and 5.1.1 even though it should be ignored.
//   With gcc 4.9.2 the ignore is completely ignored and the
//   compilation fails due to the incorrectly generated warning.
// Under gcc:
//   Works as expected in all versions.

#define TESTING(_Exp) { \
_Pragma("GCC diagnostic push") \
_Pragma("GCC diagnostic ignored \"-Wstrict-overflow\"") \
 int b = _Exp ; \
_Pragma("GCC diagnostic pop") \
}

void testing() {
    int i = 4;
    TESTING(i + 4 < i);
}

void testing2() {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstrict-overflow"
    int j = 4;
    int b = j + 4 < j;
#pragma GCC diagnostic pop
}

void testing3() {
_Pragma("GCC diagnostic push")
_Pragma("GCC diagnostic ignored \"-Wstrict-overflow\"")
    int k = 4;
    int b = k + 4 < k;
_Pragma("GCC diagnostic pop")
}

int main() {
    testing();
    testing2();
    testing3();

    return 0;
}