C++11: Compile-time value checking of constructor arguments possible?

[Zack Weinberg <zackw at panix dot com>]

It's a long story, but I happen to be trying my hand at writing a
"safe bit flags" class for C++11 and it would be extremely helpful to
be able to do compile-time value checks on constructor arguments.  It
seems like this should be possible via 'constexpr' constructors that
use static_assert inside the constructor body, but it doesn't work
with either g++ 4.6 or 4.7.  Here's an example:

$ cat test.cc
// intent is for K instances to be initializable only with an integer
// less than or equal to std::numeric_limits<int>::digits
#include <limits>
struct K
{
  K() = delete;
  K(const K&) = default;

  constexpr K(int s) : value(s == 0 ? 0 : 1 << (s-1))
  { static_assert(s <= std::numeric_limits<unsigned int>::digits, "overflow"); }

  const unsigned int value;
};

const K good(1);   // should compile
const K bad(999);  // should not compile

$ g++-4.6 -std=c++0x -fsyntax-only test.cc
test.cc: In constructor âconstexpr K::K(int)â:
test.cc:10:5: error: non-constant condition for static assertion
test.cc:10:5: error: âsâ is not a constant expression

Same errors with g++ 4.7.  If I stick 'constexpr' on the constructor
argument as well, I get either a parse error (if 'constexpr' is before
'int') or 'error: a parameter cannot be declared "constexpr"' (if
'constexpr' is after 'int').  (Has C++11 broken the syntax invariant
that specifiers and qualifiers can go either before or after the base
type? Feh.)

Any advice?  Solutions that work with 4.6 *strongly* preferred.

zw