Autoconverting template to template

This piece below is supposed to be primarily for a string view with T={char, const char} being the primary intended template instantiation target.

The cmp function is supposed to compare the views analogously to strcmp. The problem is that while char* happily converts to const char* I don't know how to get SVec<char> to convert to SVec<const char> just as happily.

The last line (cout<<(cmp(rv, rvc));) won't compile. I have to do the convertion explicitly (cmp(SVec<const char>(rv), rvc)). Can it be automatic like with char* to const char*?

The code (much simplified):

template <typename T>
class SVec {
    protected:
        T* begin_;
        size_t size_;
    public:

        SVec(T* begin, size_t size)         : begin_(begin), size_(size)            {};
        SVec(T* begin, T* end)                  : begin_(begin), size_(end-begin) {};
        SVec(T* begin)                          : begin_(begin) { while (*(begin++)) {}; size_ = begin - 1 - begin_;    }
                                                //^null element indicates the end
        ///Conversion
        operator SVec<const T>() const { return SVec<const T>(begin_, size_); }
};

//General lexicographic compare
template <typename T>
inline int cmp(const SVec<const T>& l, const SVec<const T> & r){
    return 1;
}

//Char specialization
template <> inline int cmp<char>(const SVec<const char>& l, const SVec<const char>& r){
    return 1;
}
//Explicit instantiation
template int cmp<char>(const SVec<const char>& l, const SVec<const char>& r);

#include <iostream>

int main(){
    using namespace std;

    char ar[] = "st";
    SVec<char> sv = ar;
    SVec<const char> svc = "str";

    cout<<(cmp(SVec<const char>(sv), svc));

    cout<<(cmp(sv, svc));
}

(The name was supposed to mean Read-only vector but it's supposed to be a nonrezi