Limited type matching?

[Namespace]

Code:

----
import std.stdio;

void foo(short x, short y) { }
void foo(short[2] xy) { }

void main()
{
	foo(1, 2); /// works
	foo([1, 2]); /// works
	
	ushort[2] xy = [1, 2];
	foo(xy); /// fails
	
	ushort x = 1, y = 2;
	foo(x, y); /// works
}
----

What is the problem? If the compiler is able to cast implicit 
from ushort to short, what is the problem of casting ushort[2] to 
short[2]?