String template parameters in c++

When using templates in c++ I sometime need to pass strings as value template parameters.

I found quite difficult to understand why certain parameters are allowed and other are not.

For example a const char * can be given as template argument if static member of a class, can not if defined outside.

I did a small program to test all this, commenting lines that do not compile. I also did a couple of supposition based on compiler output but they might be wrong.

What are the rules of template param values. I saw that the object needed external linkage but a bool is authorized although it obviously doesn't have any kind of linkage.

#include <iostream>
using namespace std;

struct tag {
    static char array[];
    static const char carray[];
    static char *ptr;
    static const char *cptr;
    static const char *const cptrc;
    static string str;
    static const string cstr;
};
char tag::array[] = "array";
const char tag::carray[] = "carray";
char *tag::ptr = (char*)"ptr"; // cast because deprecated conversion
const char *tag::cptr = "cptr";
const char *const tag::cptrc = "cptrc";
string tag::str = "str";
const string tag::cstr = "cstr";


namespace ntag {
    char array[] = "array";
    const char carray[] = "carray";
    char *ptr = (char *)"ptr"; // cast because deprecated conversion
    const char *cptr = "cptr";
    const char *const cptrc = "cptrc";
    string str = "str";
    const string cstr = "cstr";
};

template <class T, T t>
void print() { cout << t << endl; };

int main()
{
    cout << "-- class --" << endl;
    // Works
    print<char *, tag::array>();
    print<const char *, tag::carray>();

    // Does not work because it is a lvalue ?
    // print<char *, tag::ptr>();
    // print<const char *, tag::cptr>();
    // print<const char *const, tag::cptrc>();

    // Template type param must be a basic type ?
    // print<string, tag::str>();
    // print<const string*, tag::cstr>();

    cout << "-- namespace --" << endl;
    // Works
    print<char *, ntag::array>();

    // No external linkage ?
    // print<const char *, ntag::carray>();

    // Does not work because it is an lvalue ?
    // print<char *, ntag::ptr>();
    // print<const char *, ntag::cptr>();
    // print<const char *const, ntag::cptrc>();

    // The type of a template value param must a basic type
    // print<string, ntag::str>();
    // print<const string*, ntag::cstr>();
}