Specifying associated type requires redundant constraints

[sgrif]

Reproduction script

Expected behavior: The file compiles successfully
Actual behavior: The function x requires an additional <T as Foo>::Baz: Bar<U>. This check is entirely redundant with the trait definition, and would always have to be true for T: Foo<Bar=U> to hold.

This only occurs if the constraint referencing the first associated type is a parameter to the trait, not an associated type, which is why this doesn't happen for IntoIterator. This appears to have been an unintended side effect of RFC 1214

[rkarp]

Edit: I think I've found the oldest issue about this: #24159

[estebank]

Current output:

error[E0277]: the trait bound `<T as Foo>::Baz: Bar<U>` is not satisfied
  --> src/main.rs:9:1
   |
1  |   trait Foo {
   |   --------- required by `Foo`
...
9  |   fn x<T: Foo<Bar=U>, U>(t: &T) {
   |   ^                            - help: consider further restricting the associated type: `where <T as Foo>::Baz: Bar<U>`
   |  _|
   | |
10 | | }
   | |_^ the trait `Bar<U>` is not implemented for `<T as Foo>::Baz`

[estebank]

This now compiles.