upcastElementsTo

class X
{
    this(int x) { this.x = x; }
    int x;
}
class Y : X
{
    this(int x) { super(x); }
    int x;
}

void f2(X[2] xs) {}

Y[2] xy = [new Y(42), new Y(43)];
static assert(is(typeof(xy) == Y[2]));

// TODO: f2(xy);