ICE: `assertion failed: !t.has_escaping_bound_vars()'`

auto-reduced (treereduce-rust):

#![feature(non_lifetime_binders)]

trait Trait<T: ?Sized> {
    type Assoc<'a> = i32;
}

fn produce() -> impl for<T> Trait<(), Assoc = impl Trait<T>> {
    16
}

original:

#![allow(incomplete_features)]
#![feature(non_lifetime_binders)]

trait Trait<T: ?Sized> {
    type Assoc<'a> = i32;

    fn with_assoc(f: impl FnOnce(Self::Assoc<'_>)) {
        f(5i32)
    }
}

fn produce() -> impl for<T> Trait<(), Assoc = impl Trait<T>> {
    //~^ ERROR associated type `Assoc` not found for `Trait`
    //~| ERROR associated type `Assoc` not found for `Trait`
    //~| the trait bound `{integer}: Trait<()>` is not satisfied
    16
}

fn main() {}

Version information

rustc 1.84.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.84.0-dev
LLVM version: 19.1.1

Command:
/home/gh-matthiaskrgr/.rustup/toolchains/local-debug-assertions/bin/rustc

@rustbot label +F-non_lifetime_binders

Start by compiling the reduced example with the reported rustc command and inspect compiler/rustc_trait_selection/src/traits/select/mod.rs:643:21 alongside the evaluate_obligation query stack. Trace why the example reaches the assertion, then add coverage for the reduced case and verify that compilation reports a diagnostic instead of panicking.