On Thu, Oct 31, 2019 at 10:14 AM connor horman via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
I propose to allow reinterpret_cast from pointer to integral, as well as the no-op/identity reintepret_cast in constant expressions. This particular version is useful in implementing hashcode algorithms at compile time, and to my knowledge bit_cast does not work for pointer types at compile time.

I think this is a non-starter. Consider:

// https://godbolt.org/z/Xd48ZV
const int i = 42;
constexpr int is_64_aligned(const int *p) { 
    return reinterpret_cast<intptr_t>(p) % 64 == 0;
constexpr bool x = is_64_aligned(&i);
int main(int argc, char **argv) {
    bool y = is_64_aligned(argc ? &i : nullptr);
    assert(x == y);

How is the compiler supposed to know at compile time what bits are in the runtime bit-pattern of `p`?