Type trait for strings

I gave a shot at implementing such a trait, but I am not sure it is really robust. Any input will be appreciated.

template <typename T>
struct is_string
    : public mpl::or_< // is "or_" included in the C++11 library?
        std::is_same<       char *, typename std::decay< T >::type >,
        std::is_same< const char *, typename std::decay< T >::type >
     > {};

assert ( ! is_string< int >::value );

assert (   is_string< char       *       >::value );
assert (   is_string< char const *       >::value );
assert (   is_string< char       * const >::value );
assert (   is_string< char const * const >::value );

assert (   is_string< char       (&)[5] >::value );
assert (   is_string< char const (&)[5] >::value );

// We could add specializations for string classes, e.g.
template <>
struct is_string<std::string> : std::true_type {};

This should work in C++17.

#include <iostream>
#include <string>
#include <type_traits>
 
template<typename T>
struct is_string
        : public std::disjunction<
                std::is_same<char *, typename std::decay_t<T>>,
                std::is_same<const char *, typename std::decay_t<T>>,
                std::is_same<std::string, typename std::decay_t<T>>
        > {
};

int main()
{
    std::cout << std::boolalpha;
    std::string str = "i am a string";
    std::cout << is_string<decltype(str)>::value << std::endl; // "true"
    std::cout << is_string<decltype("i am a string literal")>::value << std::endl; // "true"
}