interface Foo {
    public int foo();
}

template fooImpl() {
    public override int foo() { return 4; }
}

class Goo : Foo {
    mixin fooImpl!();
    public override int foo() { return 7; }
}

void main() {
    Foo f = new Goo;
    assert (f.foo == 7);
}

The above assert fails. Problem is the declaration order matters -- when the mixin comes AFTER the hand-coded foo(), it's fine.