Clang, constexpr, bitshift

#include <climits>
using namespace std;

template <typename T> constexpr unsigned bits() {
    return sizeof(T) * CHAR_BIT;
}

int main() {
    constexpr int a = 1; // obviously fine
    constexpr int b = bits<int>(); // fine
    constexpr int c = 1 << 3; // fine
    constexpr int d = 1 << bits<int>(); // error: constexpr variable 'd' must be initialized by a constant expression
    constexpr int e = 1 + bits<int>(); // fine
    return 0;
}