Setting associated type of a trait breaks supertrait bounds

[WaffleLapkin]

This code compiles fine until you'll uncomment BAssoc = ().

pub trait A {
    type AAssoc: B</*BAssoc = ()*/>;
}

pub trait B
where
    Self: Into<<Self as B>::BAssoc> 
{
    type BAssoc;
}

fn test<T: A>(x: T::AAssoc) -> <T::AAssoc as B>::BAssoc {
    x.into()
}

With BAssoc = () the code fails to compile with the following error:

error[E0277]: the trait bound `(): From<<T as A>::AAssoc>` is not satisfied
  --> src/lib.rs:13:5
   |
5  | pub trait B
   |           - required by a bound in this
6  | where
7  |     Self: Into<<Self as B>::BAssoc> 
   |           ------------------------- required by this bound in `B`
...
13 |     x.into()
   |     ^^^^^^^^ the trait `From<<T as A>::AAssoc>` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `Into<()>` for `<T as A>::AAssoc`

error[E0308]: mismatched types
  --> src/lib.rs:13:5
   |
12 | fn test<T: A>(x: T::AAssoc) -> <T::AAssoc as B>::BAssoc {
   |                                ------------------------ expected `()` because of return type
13 |     x.into()
   |     ^^^^^^^^- help: try adding a semicolon: `;`
   |     |
   |     expected `()`, found associated type
   |
   = note:    expected unit type `()`
           found associated type `<<T as A>::AAssoc as B>::BAssoc`
   = help: consider constraining the associated type `<<T as A>::AAssoc as B>::BAssoc` to `()`
   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

That seems really weird - how making type less generic makes code that should work for any type fail?

Meta

Tested on stable 1.46.0 and 1.48.0-nightly (2020-09-20 1fd5b9d)

[WaffleLapkin]

This is fixed on nightly. Presumably by #73905