ICE: never type as default associated type

[malbarbo]

This code generates an ICE

#![feature(never_type, specialization)]
#![allow(incomplete_features)]

use std::iter::{self, Empty};

trait Trait {
    type Out: Iterator<Item = u32>;

    fn f(&self) -> Option<Self::Out>;
}

impl<T> Trait for T {
    default type Out = !;

    default fn f(&self) -> Option<Self::Out> {
        None
    }
}

struct X;

impl Trait for X {
    type Out = Empty<u32>;

    fn f(&self) -> Option<Self::Out> {
        Some(iter::empty())
    }
}

fn f<T: Trait>(a: T) {
    if let Some(iter) = a.f() {
        println!("Some");
        for x in iter {
            println!("x = {}", x);
        }
    }
}

pub fn main() {
    f(10);
}

with this message.

error: internal compiler error: librustc/traits/codegen/mod.rs:169: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<! as std::iter::Iterator>)),depth=1),Unimplemented)]` resolving bounds after type-checking

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:499:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error

Replacing 1.f() by X.f() makes the code to compile and run fine, which is strange because ! does not implements Iterator .

By removing the specialization the compiler refuses to compile the code with the error:

error[E0277]: the trait bound `!: std::iter::Iterator` is not satisfied
  --> src/main.rs:10:9
   |
10 | impl<T> Trait for T {
   |         ^^^^^ `!` is not an iterator; maybe try calling `.iter()` or a similar method
   |
   = help: the trait `std::iter::Iterator` is not implemented for `!`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.

[JohnTitor]

The ICE has been fixed in the latest nightly, marked as E-needstest

[malbarbo]

Thanks @JohnTitor. Indeed, the example compiles fine in nightly (4f03f4a 2019-11-12), however, replacing the main function by these two

fn f<T: Trait>(a: T) {
    if let Some(iter) = a.f() {
        println!("Some");
        for x in iter {
            println!("x = {}", x);
        }
    }
}

pub fn main() {
    f(10);
}

generates the same ICE.

[JohnTitor]

@malbarbo ah, thanks for the update! I confirmed it (playground)

[JohnTitor]

Triage: the ICE doesn't occur anymore with the latest nightly, marking as E-needs-test.