trait Trait {
    type Item;
}

fn foo<A: Trait, B: Trait>()
where
    A::Item: Trait<Item = u32>,
    B::Item: Trait<Item = i32>,
{}

this should compile but fails with

error[E0283]: type annotations needed: cannot satisfy `<A as Trait>::Item: Trait`
 --> <source>:7:14
  |
7 |     A::Item: Trait<Item = u32>,
  |              ^^^^^^^^^^^^^^^^^
  |

The underlying issue is trying to prove <A as Trait>::Item: Trait normalizes the self type:

• normalizes-to(<A as Trait>::Item)
  • candidate A::Item: Trait<Item = u32>
    • alias-relate(A::Item, A)
      • normalizes-to(<A as Trait>::Item) inductive cycle
  • candidate B::Item: Trait<Item = i32>
    • alias-relate(B::Item, A)
      • normalizes-to(<B as Trait>::Item)
        • candidate A::Item: Trait<Item = u32>
          • alias-relate(A::Item, B)
            • normalizes-to(<A as Trait>::Item) inductive cycle
        • candidate B::Item: Trait<Item = i32>
          • alias-relate(B::Item, B)
            • normalizes-to(<B as Trait>::Item) inductive cycle