workaround rust invariant type

[HerringtonDarkholme]

Consider this code

struct N<'a>(&'a ());
trait Bound { type Inner; }
impl<'a> Bound for N<'a> { type Inner = (); }
struct Container<T: Bound>(T::Inner);
fn cast<'a>(m: &'a Container<N<'static>>) -> &'a Container<N<'a>> {
  m // error, Container is invariant over T
}

Changing this to below works

struct N<'a>(&'a ());
trait Bound { type Inner; }
impl<'a> Bound for N<'a> { type Inner = (); }
struct Container<
  T: Bound, 
  D = <T as Bound>::Inner
>(T, D);
fn cast<'a>(m: &'a Container<N<'static>>) -> &'a Container<N<'a>> {
  m
}

See

rust-lang/rust#115799

rust-ndarray/ndarray#1480

Rust compiler can be smarter to detect that Innver is covariant but it does not