static_assert on initializer_list::size()

Question

Why is std::initializer_list<_E>::size not allowable in a static_assert, even though it\'s declared as a constexpr in my libstdc++ (v. 4.6)?

For example, the foll

Answer 1

I haven't really figured out what's going on here.

If I say

const std::initializer_list li = { 1, 2.5, 3.7, 4.3 };
static_assert(li.size() == 4, "fail");

I get a complain that 'li' was not declared 'constexper'.

But if I say

constexpr std::size_t dSize(std::initializer_list di)
{
    return di.size();
}

then

static_assert(dSize({1, 2.5, 3.7, 4.3}) == 4, "failed");   // works

but

static_assert(dSize(li) == 4, "failed");

fails with "error: the value of 'li' is not usable in a constant expression"

This is all with -std=c++11

So, somehow, an initializer list passed into an arg list can be part of a constant expression, but an initializer list just declared as a const variable can't.