Frontend crash: ImplExprPredNotFound on bounds on opaque types

[W95Psp]

Reproducer:

fn uints() -> impl Iterator<Item = u64> {
    vec![0u64].into_iter()
}

fn main() {
    let mut hello = uints();
    hello.next();
}

Open this code snippet in the playground

Crashes with:

warning[E9999]: Could not find a clause for `Binder { value: <impl std::iter::Iterator<Item = u64> as std::iter::Iterator>, bound_vars: [] }` in the item parameters
 --> src/lib.rs:5:1
  |
5 | fn main() {
  | ^^^^^^^^^

Solution

We could use param_env_reveal_all_normalized, but that would tear down the opaque type abstraction.

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[W95Psp]

We need to discuss that, I'm not sure this is still an bug.

[franziskuskiefer]

Still seems to crash.

[W95Psp]

Ah, thanks for investigating.

[Nadrieril]

We could use param_env_reveal_all_normalized, but that would tear down the opaque type abstraction.

I am personally in favor of that

[W95Psp]

Yeah, maybe that's the best thing to do, let's try it.