New rules for type auto-detection from braced initializer list 

C++17 introduced new rules for auto-detection of the initializer list, which complements C++14 rules. The C++17 rule insists that the program is ill-formed if an explicit or partial specialization of std::initializer_list is declared:

#include <iostream>
using namespace std;

template <typename T1, typename T2>
class MyClass {
     private:
          T1 t1;
          T2 t2;
     public:
          MyClass( T1 t1 = T1(), T2 t2 = T2() ) { }

          void printSizeOfDataTypes() {
               cout << "\nSize of t1 is " << sizeof ( t1 ) << " bytes." << endl;
               cout << "\nSize of t2 is " << sizeof ( t2 ) << " bytes." << endl;
     }
};

int main ( ) {

    //Until C++14
    MyClass<int, double> obj1;
    obj1.printSizeOfDataTypes( );

    //New syntax in C++17
    MyClass obj2( 1, 10.56 );

    return 0;
}

The preceding code can be compiled and the output can be viewed with the following commands:

g++-7 main.cpp -std=c++17
./a.out

The output of the preceding program is as follows:

Values in integer vectors are ...
1 2 3 4 5 

Values in double vectors are ...
1.5 2.5 3.5