What is p in (0, 0) is (int x, int y) p for?

Case A

Here x and y become scalar variables of values 0 each.

if ((0, 0) is (int x, int y))
{
    Console.WriteLine($"{x + y}");
}

if ((object)(0, 0) is (int x, int y))
{
    Console.WriteLine($"{x + y}");
}

Case B

The following is compilable but I don't understand what p is for. x, y are scalar variables and p is a tuple with fields Item1 and Item2 (rather than x and y).

if ((0, 0) is (int x, int y) p)
{
    Console.WriteLine($"{x + y} and {p.Item1 + p.Item2}");
}

And the following does not compile due to the existence of p.

if ((object)(0, 0) is (int x, int y) p)
{
    Console.WriteLine($"{x + y} and {p.Item1 + p.Item2}");
}