[Bug c++/54021] [c++0x] __builtin_constant_p should be constexpr

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54021

--- Comment #9 from Jason Merrill <jason at gcc dot gnu.org> 2012-09-09 
16:48:46 UTC ---
(In reply to comment #5)
>     // This causes "error: the value of 'x' is not usable in a constant 
> expression"
>     constexpr bool c = __builtin_constant_p(x);
> }

This is also fixed by my patch.