choose a default constructor based on template argument, C++ (vc++)

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//Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23506 for x64

#include <iostream>
#include <type_traits>

template < typename T > struct my_literal_str {

// default constructor when T == char (with two template parameters)
     template< typename U = T, // 1
               typename = typename std::enable_if< std::is_same<U,char>::value >::type > // 2
     constexpr my_literal_str() : ptr( "Hello (char)" ) {}

// default constructor when T == wchar_t (with three template parameters)
     template< typename U = T, // 1
               typename = typename std::enable_if< std::is_same<U,wchar_t>::value >::type, // 2
               typename = U > // 3
     constexpr my_literal_str() : ptr( L"Hello (wchar_t)" ) {}

// default constructor when T != char && T != wchar_t (with four template parameters)
     template< typename U = T, // 1
               typename = typename std::enable_if< !std::is_same<U,char>::value >::type, // 2
               typename = typename std::enable_if< !std::is_same<U,wchar_t>::value >::type, // 3
               typename = U > // 4
     constexpr my_literal_str() : ptr(nullptr) {}

constexpr auto c_str() const { return ptr ; }

private: const T* ptr ;
};

int main() {

constexpr my_literal_str<char> a ;
    std::cout << a.c_str() << '\n' ; // Hello (char)

constexpr my_literal_str<wchar_t> b ;
    std::wcout << b.c_str() << L'\n' ; // Hello (wchar_t)

constexpr my_literal_str<int> c ;
    std::cout << c.c_str() << '\n' ; // nullptr
}

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Compilation time: 1,71 sec, absolute running time: 0,32 sec, absolute service time: 2,04 sec