pointer non-type template parameter

I don't really understand why the code below does not compile:

template<const char*>
struct Foo{};

constexpr const char s1[] = "test1";
constexpr const char* const s2 = "test2";

int main()
{
    Foo<s1> foo1; // ok
    // Foo<s2> foo2; // doesn't compile
}

Uncommenting the last line in main() makes g++ and clang++ emit the errors

error: 's2' is not a valid template argument because 's2' is a
variable, not the address of a variable

and

error: non-type template argument for template parameter of
      pointer type 'const char *' must have its address taken

respectively.

My questions are:

1. Why is s1 instantiation OK and s2 not?


2. Is there any sane situation where such pointer non-type template parameter is of any use?