Nested but non-conflicting projection predicates emerging from supertrait bounds leads to ambiguity

Zulip Chat thread for discussion

I tried this code:

Original test case

use std::marker::PhantomData;

pub trait Receiver {
    type Target: ?Sized;
}

pub trait Deref: Receiver<Target = <Self as Deref>::Target> {
    type Target: ?Sized;
    fn deref(&self) -> &<Self as Deref>::Target;
}

impl<T: Deref> Receiver for T {
    type Target = <T as Deref>::Target;
}

// ===
pub struct Type<Id, T>(PhantomData<(Id, T)>);
pub struct AliasRef<Id, T: TypePtr<Id = Id>>(PhantomData<(Id, T)>);

pub trait TypePtr: Deref<Target = Type<<Self as TypePtr>::Id, Self>> + Sized {
    // ^ the impl head here provides the first candidate
    // <T as Deref>::Target := Type<<T as TypePtr>::Id>
    type Id;
}

pub struct Alias<Id, T>(PhantomData<(Id, T)>);

impl<Id, T> Deref for Alias<Id, T>
where
    T: TypePtr<Id = Id> + Deref<Target = Type<Id, T>>,
    // ^ the impl head here provides the second candidate
    // <T as Deref>::Target := Type<Id, T>
    // and additionally a normalisation is mandatory due to
    // the following supertrait relation trait
    // Deref: Receiver<Target = <Self as Deref>::Target>
{
    type Target = AliasRef<Id, T>;

    fn deref(&self) -> &<Self as Deref>::Target {
        todo!()
    }
}

I expected to see this happen: rustc compiles

Instead, this happened:

error[E0284]: type annotations needed
  --> src/lib.rs:28:1
   |
28 | / impl<Id, T> Deref for Alias<Id, T>
29 | | where
30 | |     T: TypePtr<Id = Id> + Deref<Target = Type<Id, T>>,
   | |______________________________________________________^ cannot infer type
   |
   = note: cannot satisfy `<T as Deref>::Target == _`

Meta

Rust version is at 1.88.0-nightly @ 2025-04-07 e643f59f6da3a84f43e7.

<backtrace>

Start by compiling the reproducer in src/lib.rs with the reported nightly toolchain and compare it with the original test case linked from pull request 139955. Read the Zulip discussion for the intended projection behavior, then add or update a regression test so the example compiles without the type-inference error.