template< class T, class... U >
array( T, U... ) -> array<T, 1 + sizeof...(U)>;
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.
[edit] Example#include <algorithm>
#include <array>
#include <cassert>
#include <type_traits>
int main()
{
const int 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 facility
assert(std::ranges::equal(c, std::array{3, 2, 1}));
static_assert(std::is_same_v<short, decltype(c)::value_type>);
}
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