User defined type trait gives an unexpected false type

Printing string that contains type info:

std::string demangle(const char* mangled_name) {
        size_t len = 0;
        int status = 0;

        std::unique_ptr<char, decltype(&std::free)> pointer (
            __cxxabiv1::__cxa_demangle(mangled_name, nullptr, &len, &status),
            &std::free);

        return pointer.get();
}

template <typename T>
std::string type_of(const T& arg) {
     return demangle(typeid(arg).name());
}

My custom is_string type trait:

template <typename T> struct is_string : std::false_type {};
template <> struct is_string<char*> : std::true_type {};
template <> struct is_string<char[]> : std::true_type {};
template <size_t N> struct is_string<char[N]> : std::true_type {};
template <> struct is_string<const char*> : std::true_type {};
template <> struct is_string<const char[]> : std::true_type {};
template <size_t N> struct is_string<const char[N]> : std::true_type {};

// Helper variable:
template <typename T>
constexpr bool is_string_v = is_string<T>::value;

Application:

std::cout << std::boolalpha;

std::cout << "Testing: " << '\n'
    << "char* = " << is_string_v<char*> << '\n'
    << "char[] = " << is_string_v<char[]> << '\n'
    << "char[10] = " << is_string_v<char[10]> << '\n'
    << "char[15] = " << is_string_v<char[15]> << '\n'
    << "const char* = " << is_string_v<const char*> << '\n'
    << "const char[] = " << is_string_v<const char[]> << '\n'
    << "const char[3] = " << is_string_v<const char[3]> << '\n'
    << "const char[6] = " << is_string_v<const char[6]> << "\n\n";

const char* string1 = "Hello";
const char string2[] = "Hello";
char string3[] = "Hello";
auto string4 = "Hello";

std::cout << "Testing with decltype(variable): " << '\n'
    << type_of(string1) << " = " << is_string_v<decltype(string1)> << '\n'
    << type_of(string2) << " = " << is_string_v<decltype(string2)> << '\n'
    << type_of(string3) << " = " << is_string_v<decltype(string3)> << '\n'
    << type_of(string4) << " = " << is_string_v<decltype(string4)> << "\n\n";

std::cout << "Testing with string literal: " << '\n'
    << type_of("Hello") << " = " << is_string_v<decltype("Hello")> << '\n';

Output:

Testing:
char* = true
char[] = true
char[10] = true
char[15] = true
const char* = true
const char[] = true
const char[3] = true
const char[6] = true

Testing with decltype(variable):
char const* = true
char [6] = true
char [6] = true
char const* = true

Testing with string literal:
char [6] = false

• Why is is_string_v<decltype(some string literal)> false when they have the same type as string3 or char [6] which has true value when it passes as template parameter?
• Is there something wrong with my demangling function?