ambiguous assoc types result in incorrect suggestion

[lcnr]

trait Super<T> {
    type Assoc;
}
trait Trait: Super<u32> + Super<i32> {}

fn foo<T: Trait<Assoc = u32>>() {}

error[[E0222]](https://doc.rust-lang.org/stable/error-index.html#E0222): ambiguous associated type `Assoc` in bounds of `Trait`
 --> src/lib.rs:6:17
  |
2 |     type Assoc;
  |     -----------
  |     |
  |     ambiguous `Assoc` from `Super<i32>`
  |     ambiguous `Assoc` from `Super<u32>`
...
6 | fn foo<T: Trait<Assoc = u32>>() {}
  |                 ^^^^^^^^^^^ ambiguous associated type `Assoc`
  |
  = help: consider introducing a new type parameter `T` and adding `where` constraints:
              where
                  T: Trait,
                  T: Super<i32>::Assoc = u32,
                  T: Super<u32>::Assoc = u32

and

trait Super<T> {
    type Assoc;
}
trait Trait: Super<u32> + Super<i32> {}

fn main() {
    let _: dyn Trait<Assoc = u32>;
}

error[[E0222]](https://doc.rust-lang.org/stable/error-index.html#E0222): ambiguous associated type `Assoc` in bounds of `Trait`
 --> src/main.rs:7:22
  |
2 |     type Assoc;
  |     -----------
  |     |
  |     ambiguous `Assoc` from `Super<i32>`
  |     ambiguous `Assoc` from `Super<u32>`
...
7 |     let _: dyn Trait<Assoc = u32>;
  |                      ^^^^^^^^^^^ ambiguous associated type `Assoc`
  |
  = help: consider introducing a new type parameter `T` and adding `where` constraints:
              where
                  T: Trait,
                  T: Super<i32>::Assoc = u32,
                  T: Super<u32>::Assoc = u32

error[[E0191]](https://doc.rust-lang.org/stable/error-index.html#E0191): the value of the associated type `Assoc` (from trait `Super`) must be specified
 --> src/main.rs:7:16
  |
2 |     type Assoc;
  |     ----------- `Assoc` defined here
...
7 |     let _: dyn Trait<Assoc = u32>;
  |                ^^^^^^^^^^^^^^^^^^ help: specify the associated type: `Trait<Assoc = u32, Assoc = Type>`

Trying to write T: Super<i32>::Assoc = u32 results in a syntax error.

[fmease]

For the first case (where-clauses, impl-Trait) it could suggest (duplicating the equality bound):

- Trait<Assoc = u32>
+ Trait + Super<u32, Assoc = u32> + Super<i32, Assoc = u32>

For dyn-Trait, it shouldn't suggest anything and maybe explain that what the user intends to write can't be expressed (we can't disentangle the bounds like above as dyn-Trait can only have one non-auto / principal trait).