Traits with associated types and wrapping

help
1

I have an Iterator trait defined like this:

    pub trait IteratorImpl : PartialEq {
        type distance_type_impl : Integer;
        fn increment_impl(&mut self);
        fn successor_impl(mut self) -> Self where Self : Sized {
            self.increment_impl();
            self
        }
    }

I need to define addition and subtraction on iterators, but as the Add and Sub traits are defined in a different module, I need to wrap this iterator. I am using:

    #[derive(Clone, PartialEq, Debug)]
    pub struct It<I>(pub I);

Now I want to define an Iterator that only works on It<_> types, so I have a another trait:

    pub trait Iterator : PartialEq + Add + Sub + Sized {
        type distance_type : Integer;
        fn increment(&mut self);
        fn successor(mut self) -> Self where Self : Sized {
            self.increment();
            self
        }
    }

and I then want to define a single implementation for this new Iterator trait:

    impl<I> Iterator for It<I> where I : IteratorImpl {
        type distance_type = I::distance_type_impl;
        fn increment(&mut self) {
            self.0.increment_impl();
        }
    }

However I get this error when compiling

type mismatch resolving `<I as elements::IteratorImpl>::distance_type_impl == elements::It<I>`:
 expected associated type,
    found struct `elements::It`type mismatch resolving `<I as elements::IteratorImpl>::distance_type_impl == elements::It<I>`:
 expected associated type,
    found struct `elements::It` [E0271 []E0271]

I don't understand why I cannot forward the associated type from Iterator to the wrapped associated type on the Iterator in the It<_> wrapper.

Edit: I copied and pasted these bits plus a few definitions into the Rust Playground, and I did not get an error, so I think some other definition must be causing the problem being reported.