ICE with trait object + associated type

[BurntSushi]

Probably not minimal example:

#[derive(Clone, Debug)]
struct Foo;

pub trait Bar {
    fn shrink(&self) -> Box<Iterator<Item=Self>>;
}

impl Bar for Foo {
    fn shrink(&self) -> Box<Iterator<Item=Foo>> {
        Box::new(None.into_iter())
    }
}

fn main() {
}

Version: rustc 1.3.0-nightly (6d71ce536 2015-07-06)

This code was working on the previous nightly---I think the 2015-07-06 nightly introduced this regression. In particular, this very badly breaks any non-trivial use of QuickCheck (which has the same signature as shrink above).

[BurntSushi]

I forgot to show the output of compilation:

[andrew@Liger qc] cargo run --verbose
   Compiling qc v0.1.0 (file:///tmp/qc)
     Running `rustc src/main.rs --crate-name qc --crate-type bin -g --out-dir /tmp/qc/target/debug --emit=dep-info,link -L dependency=/tmp/qc/target/debug -L dependency=/tmp/qc/target/debug/deps`
../src/libcore/iter.rs:804:35: 804:42 error: internal compiler error: Encountered error `Unimplemented` selecting `Binder(<Foo as core::cmp::PartialOrd>)` during trans
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', ../src/libsyntax/diagnostic.rs:176


Could not compile `qc`.

Caused by:                                                                                                        
  Process didn't exit successfully: `rustc src/main.rs --crate-name qc --crate-type bin -g --out-dir /tmp/qc/target/debug --emit=dep-info,link -L dependency=/tmp/qc/target/debug -L dependency=/tmp/qc/target/debug/deps` (exit code: 101)

[tomjakubowski]

Smaller example:

pub struct Foo;

impl Foo {
    pub fn shrink(&self) -> Box<Iterator<Item=Foo>> {
        Box::new(None.into_iter())
    }
}

fn main() {
}

[tomjakubowski]

This also seems to only happen if the associated type binding is the Self type in an impl. This works for example:

pub struct Foo;

impl Foo {
    pub fn shrink(&self) -> Box<Iterator<Item=String>> {
        Box::new(None.into_iter())
    }
}

fn main() {
}

[jroesch]

99% sure this is maybe due to the fulfillment context bug I committed that @eddyb fixed yesterday. The PR should be in tonights nightly. Here is the PR: #26807. I'll try it out on a build of master and report back.

[eddyb]

Incidentally, those examples are very close to the test case added in #26807.

[apasel422]

All three examples in this issue compile now.

[alexcrichton]

Yay!