[Bug c/79482] New: _Static_assert(__builtin_constant_p(x)):

[mocramis at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79482

            Bug ID: 79482
           Summary: _Static_assert(__builtin_constant_p(x)):
           Product: gcc
           Version: 6.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mocramis at gmail dot com
  Target Milestone: ---

Compiling the wollowing code with gcc -O1 main.c:
===== main.c =====
int main(int argc, char *argv[]) { 
    int x = 0;

    _Static_assert(__builtin_constant_p(x) ? 1 : 2, "error");

    return 0;
}
===================

fails with the error :
  error: expression in static assertion is not constant
With optimizations disabled (O0), it compiles fine.

Also, if we replace the '?' operator by __builtin_choose_expr, the compiler
seems to behave as expected :
===== main.c =====
int main(int argc, char *argv[]) { 
    int x = 0;

    _Static_assert(__builtin_choose_expr(__builtin_constant_p(x), 1, 2),
                   "error");

    return 0;
}
===================

This compiles fine with any level of optimisation (while __builtin_choose_expr
should expect the same 'constness' as _Static assert).

Clang 3.9 compiles both versions without errors (with O3).

Tested with gcc 6.3.1.