An `impl Trait` type expands to a recursive type.
An `impl Trait` type must be expandable to a concrete type that contains no
`impl Trait` types. For example the following example tries to create an
`impl Trait` type `T` that is equal to `[T, T]`:
```compile_fail,E0720
fn make_recursive_type() -> impl Sized {
[make_recursive_type(), make_recursive_type()]
}
```