// main function
int& result = square(4);
// sub function
int& square(int a) {
int temp = a;
temp *= temp;
return temp;
}
//why that it warn about to return the
//reference with the local variable sir ???

Why are you calling the function before you are declaring it?

Perhaps this is the problem. Move the line
int& result = square(4);

to the end of subfunction declaration.