[Bug c++/65656] New: __builtin_constant_p should be constexpr

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65656

            Bug ID: 65656
           Summary: __builtin_constant_p should be constexpr
           Product: gcc
           Version: 4.8.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: scovich at gmail dot com

Consider the following program compiled with `gcc -std=c++11'
===== bug.cpp =====
#include <cstdio>
int main(int argc, char *argv[]) { 
  constexpr bool x = __builtin_constant_p(argc);
  std::printf("x=%d\n", x);
}
===================

With optimizations disabled, it correctly treats __builtin_constant_p() as
constexpr and prints "0" as expected (because the value of argc is not a
compile-time constant).

With optimizations enabled (-O1 or higher), compilation fails:
bug.cpp: In function âint main(int, char**)â:
bug.cpp:3:48: error: âargcâ is not a constant expression
    constexpr bool x = __builtin_constant_p(argc);
                                                ^
Clang 3.4 handles the case just fine. 

While I can 100% understand that the return value of __builtin_constant_p()
might change depending on what information the optimizer has available, I'm
pretty sure __builtin_constant_p() should always return a value computable at
compile time.

NOTE: this issue is *NOT* the same as Bug #54021, in spite of the two sharing
the same title. The latter is mis-named: It actually requests support for
constant folding for ternary expressions involving __builtin_constant_p, even
when optimizations are disabled and such folding would not normally occur.