Mundane HRTB causes constraint satisfaction failure

[CJKay]

I'm trying to understand an error I'm seeing with some project code, which I've minimised down to a test case that I'm trying to get my head around.

The following two snippets build perfectly fine, as I would expect:

trait A {
    type X;
}

trait B: A<X = u32> {
    /* ... */
}

impl<T> A for T where T: B {
    type X = u32;
}

(https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=052cefcd91b153d1958b8452a9ddbfe7)

trait A {
    type X;
}

trait B: for<'a> A {
    /* ... */
}

impl<T> A for T where T: B {
    type X = u32;
}

(https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=302cd29650a304e40d8ca65bb9d7521d)

However, this third snippet, which combines both the mundane HRTB and the associated type requirement, fails with an obscure error:

trait A {
    type X;
}

trait B: for<'a> A<X = u32> {
    /* ... */
}

impl<T> A for T where T: B {
    type X = u32;
}

(https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=56ad9194355f412f7b22c121ff6479d3)

Giving:

error[[E0284]](https://doc.rust-lang.org/nightly/error-index.html#E0284): type annotations needed: cannot satisfy `<T as A>::X == _`
  --> src/lib.rs:10:14
   |
10 |     type X = u32;
   |              ^^^ cannot satisfy `<T as A>::X == _`

For more information about this error, try `rustc --explain E0284`.
error: could not compile `playground` due to previous error

What is it about the HRTB that causes the compiler to suddenly become unable to determine that the type of X must be u32? Am I fundamentally misunderstanding HRTBs?

I ran into this because I needed to impl a supertrait with GATs on a type with an existing impl of the sub-trait, a la:

#![feature(generic_associated_types)]

trait A {
    type X<'a>;
}

trait B: for<'a> A<X<'a> = Self::Y<'a>> {
    type Y<'a>;
}

impl<T> A for T where T: B {
    type X<'a> = T::Y<'a>;
}

(https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=74172d3948e66032f917f18fc8cfaf2a)