Hang : next-solver hang with recursive trait obligations involving type_alias_impl_trait

The bug is found by a fuzzer.
I tried this code:

#![feature(type_alias_impl_trait)]
use std::fmt::Display;
type Opaque<X: Trait> = impl Sized + 'static;

#[define_opaque(Opaque)]
fn define<X>() -> Opaque<X> {}

trait Trait {
    type Assoc: Display;
}
impl<'a> Trait for Opaque<&'a str> {
    type Assoc = &'a str;
}

fn extend<T: Trait + 'static>(s: T::Assoc) -> Box<dyn Display> {
    Box::new(s)
}
fn main() {
    let val = extend::<Opaque<&'_ str>>(&String::from("blah blah blah"));
    println!("{}", val);
}

The compiler behavior differs depending on the release channel and trait solver used:

I expected to see this happen: The compiler should behave consistently across solvers. In particular, when using
-Znext-solver

Instead, this happened: The compiler hangs.

Meta

rustc --version --verbose:

rustc 1.94.0-nightly (f57eac1bf 2026-01-10)
binary: rustc
commit-hash: f57eac1bf98cb5d578e3364b64365ec398c137df
commit-date: 2026-01-10
host: x86_64-unknown-linux-gnu
release: 1.94.0-nightly
LLVM version: 21.1.8

error[E0277]: the trait bound `X: Trait` is not satisfied
 --> results/hang/case_iter_1357_var_6/after.rs:5:19
  |
5 | fn define<X>() -> Opaque<X> {}
  |                   ^^^^^^^^^ the trait `Trait` is not implemented for `X`
  |
note: required by a bound in `Opaque`
 --> results/hang/case_iter_1357_var_6/after.rs:3:16
  |
3 | type Opaque<X: Trait> = impl Sized + 'static;
  |                ^^^^^ required by this bound in `Opaque`
help: consider restricting type parameter `X` with trait `Trait`
  |
5 | fn define<X: Trait>() -> Opaque<X> {}
  |            +++++++

error[E0277]: the trait bound `&'a str: Trait` is not satisfied
 --> results/hang/case_iter_1357_var_6/after.rs:9:20
  |
9 | impl<'a> Trait for Opaque<&'a str> {
  |                    ^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `&'a str`
  |
help: the trait `Trait` is implemented for `Opaque<&'a str>`
 --> results/hang/case_iter_1357_var_6/after.rs:9:1
  |
9 | impl<'a> Trait for Opaque<&'a str> {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
 --> results/hang/case_iter_1357_var_6/after.rs:9:6
  |
9 | impl<'a> Trait for Opaque<&'a str> {
  |      ^^ unconstrained lifetime parameter

Then the compiler hangs;

Start with the reproducer in results/hang/case_iter_1357_var_6/after.rs and run it on nightly with -Znext-solver=globally, comparing it with the default solver behavior reported in the issue. Trace the next-solver handling of the recursive trait obligations involving type_alias_impl_trait; done means the reproducer no longer hangs and the solver behavior is consistent across the listed modes.