constexpr length of a string from template parameter

I am trying to obtain the length of a string passed as a template argument using C++11. Here is what I have found so far:

#include <iostream>
#include <cstring>
extern const char HELLO[] = "Hello World!!!";

template<const char _S[]>
constexpr size_t len1() { return sizeof(_S); }

template<const char _S[]>
constexpr size_t len2() { return std::strlen(_S); }

template<const char _S[], std::size_t _Sz=sizeof(_S)>
constexpr size_t len3() { return _Sz-1; }

template<unsigned int _N>
constexpr size_t len5(const char(&str)[_N])
{
    return _N-1;
}

int main() {
    enum {
        l1 = len1<HELLO>(),
        // l2 = len2<HELLO>() does not compile
        l3 = len3<HELLO>(),
        l4 = len3<HELLO, sizeof(HELLO)>(),
        l5 = len5(HELLO),
    };
    std::cout << l1 << std::endl;  // outputs 4
    // std::cout << l2 << std::endl;
    std::cout << l3 << std::endl; // outputs 3
    std::cout << l4 << std::endl; // outputs 14
    std::cout << l5 << std::endl; // outputs 14
    return 0;
}

I am not very surprised with the results, I understand that the size of the array is lost in the case of len1() and len2(), although the information is present at compile time.

Is there a way to pass the information about the size of the string to the template as well? Something like:

template<const char _S[unsigned int _N]>
constexpr size_t len6() { return _N-1; }