deduction guides for std::array

One deduction guide is provided for std::array to provide an equivalent of std::experimental::make_array for construction of std::array from a variadic parameter pack.

The program is ill-formed if ( std:: is_same_v < T, U > && ... ) is not true. Note that ( std:: is_same_v < T, U > && ... ) is true when sizeof... ( U ) is zero.

Example

#include <array>
#include <cassert>
 
int main()
{
    int const x = 10;
    std::array a{1, 2, 3, 5, x}; // OK, creates std::array<int, 5>
    assert(a.back() == x);
 
//  std::array b{1, 2u}; // Error, all arguments must have the same type
 
//  std::array<short> c{3, 2, 1}; // Error, wrong number of template args
    std::array c(std::to_array<short>({3, 2, 1})); // C++20 alternative,
                                                   // creates std::array<short, 3>
}