castFilter

class X
{
    this(int x) { this.x = x; }
    int x;
}
class Y : X
{
    this(int x) { super(x); }
}
auto y = castFilter!Y([new X(42), new Y(43)]);
auto yf = y.front;
static assert(is(typeof(yf) == Y));
y.popFront();
assert(y.empty);