Sized handling leads to incorrect trait overlaps

[tjhance]

trait Tr {
    spec fn f() -> bool;
}

impl<X: Sized> Tr for X {
    spec fn f() -> bool { true }
}

impl<X> Tr for [X] {
    spec fn f() -> bool { false }
}

fn test() {
    assert(<[u8] as Tr>::f() == true);
    assert(<[u8] as Tr>::f() == false);
    assert(false);
}

Related: #1047 (comment)