Compiler hang with recursive bound and generic impl on opaque type

[trait-Cong]

I tried this code :

#![feature(type_alias_impl_trait)]

trait Base {}
impl Base for () {}

trait Link {}

type X
where
    X: Link,
= impl Base;

impl<T> Link for X {}

fn main() {
    let _: X = ();
}

I expected to see this happen: The compiler should report an error.

Instead, this happened: The compiler hangs indefinitely (infinite loop).

Meta

rustc --version --verbose:

rustc 1.91.0-nightly (f34ba774c 2025-08-03)
binary: rustc
commit-hash: f34ba774c78ea32b7c40598b8ad23e75cdac42a6
commit-date: 2025-08-03
host: x86_64-pc-windows-msvc
release: 1.91.0-nightly
LLVM version: 20.1.8

Backtrace

error[E0282]: type annotations needed
 --> example2.rs:8:1
  |
8 | type X
  | ^^^^^^ cannot infer type for type parameter `T`

then the process hangs

[madhav-madhusoodanan]

@ywxt Is this issue similar to the one we saw in #142063 ?

[ywxt]

I didn't look into this yet.

Is it triggered under the parallel frontend? If not, I think it's irrelvant.

[madhav-madhusoodanan]

Not exactly on the parallel frontend (hangs even with -Zthreads=1) but I suspect it runs into a cycle ->

1. type X where X: Link is run; X is being checked for whether it implements Link
2. impl Link for X {} is run; X is checked for well-formedness
3. Step 1 is run again, completing the cycle

[ywxt]

The key to #142063 is inconsistent between -Zthreadn=1 and -Zthreads=n. It shouldn't get involved in bugs with single thread.