ICE "internal compiler error: cannot relate bound region" using associated type in method sig

[nikomatsakis]

use std::ops::{Shl, Shr};
use std::cell::RefCell;

pub trait Subscriber {
    type Input;
}

pub trait Publisher<'a> {
    type Output;

    fn subscribe(&mut self, Box<Subscriber<Input=Self::Output> + 'a>);

}

pub trait Processor<'a> : Subscriber + Publisher<'a> { }

impl<'a, P> Processor<'a> for P
where P : Subscriber + Publisher<'a> { }


struct MyStruct<'a> {
    sub: Box<Subscriber<Input=u64> + 'a>
}

impl<'a> Publisher<'a> for MyStruct<'a> {
    type Output = u64;

    fn subscribe(&mut self, t : Box<Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
        self.subscriber = t;
    }

}

fn main() {}

yields:

lunch-box. rustc ~/tmp/rovar.rs
/home/nmatsakis/tmp/rovar.rs:28:5: 30:6 error: internal compiler error: cannot relate bound region: ReFree(88, BrNamed(DefId { krate: 0u32, node: 96u32 }, 'a)) <= ReLateBound(DebruijnIndex { depth: 3u32
}, BrAnon(1))
/home/nmatsakis/tmp/rovar.rs:28     fn subscribe(&mut self, t : Box<Subscriber<Input=<MyStruct<'a> as Publisher>::Output> + 'a>) {
/home/nmatsakis/tmp/rovar.rs:29         self.subscriber = t;
/home/nmatsakis/tmp/rovar.rs:30     }
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'Box<Any>', /home/nmatsakis/versioned/rust-1/src/libsyntax/diagnostic.rs:123

Backtrace:

stack backtrace:
   1:     0x7f2b4a0904b0 - sys::backtrace::write::hc33c0f30f6bee48dCzt
   2:     0x7f2b4a0b26c0 - failure::on_fail::hb963d0c36b10514daPz
   3:     0x7f2b4a020240 - rt::unwind::begin_unwind_inner::hdae086d0b79f35712tz
   4:     0x7f2b450aa860 - rt::unwind::begin_unwind::h13484122622889777099
   5:     0x7f2b450aa7f0 - diagnostic::SpanHandler::span_bug::headb6283b29a226fIRF
   6:     0x7f2b4848f890 - middle::infer::region_inference::RegionVarBindings<'a, 'tcx>::make_subregion::h4c7a20a638c31a25Kfw
   7:     0x7f2b4843f920 - middle::infer::sub::Sub<'f, 'tcx>.Combine<'tcx>::regions::h6359f29cf9bb6e07Xoy
   8:     0x7f2b4843ddc0 - middle::infer::combine::Combine::substs_variances::h24213547863999897
   9:     0x7f2b4843dd00 - middle::infer::combine::Combine::substs::h12473875684778725248
  10:     0x7f2b4843da60 - middle::infer::combine::Combine::trait_refs::h5280693724941496106
  11:     0x7f2b48442d30 - middle::infer::combine::Combine::projection_tys::h14373726893306599401
  12:     0x7f2b48426930 - middle::infer::sub::Sub<'f, 'tcx>.Combine<'tcx>::tys::h22d46133309df1a2Wvy
  13:     0x7f2b48442bd0 - middle::infer::combine::Combine::projection_predicates::h11220478815809982305
  14:     0x7f2b48441560 - middle::infer::higher_ranked::C.HigherRankedRelations<'tcx>::higher_ranked_sub::unboxed_closure.73888
  15:     0x7f2b48440e70 - middle::infer::higher_ranked::C.HigherRankedRelations<'tcx>::higher_ranked_sub::h8741887278504938399
  16:     0x7f2b484a6130 - result::Result<V, E>.FromIterator<Result<A, E>>::from_iter::Adapter<Iter, E>.Iterator::next::h12275455710249568012
  17:     0x7f2b484a5eb0 - result::Result<V, E>.FromIterator<Result<A, E>>::from_iter::h4394336484059744225
  18:     0x7f2b484a5b60 - middle::infer::combine::Combine::existential_bounds::h1684376140490263861
  19:     0x7f2b48426930 - middle::infer::sub::Sub<'f, 'tcx>.Combine<'tcx>::tys::h22d46133309df1a2Wvy
  20:     0x7f2b48426930 - middle::infer::sub::Sub<'f, 'tcx>.Combine<'tcx>::tys::h22d46133309df1a2Wvy
  21:     0x7f2b48445960 - middle::infer::combine::Combine::args::h1520184800584367187
  22:     0x7f2b484457b0 - vec::Vec<T>.FromIterator<T>::from_iter::h7679049291035860243
  23:     0x7f2b484453a0 - middle::infer::combine::Combine::fn_sigs::h16564086657495642215
  24:     0x7f2b48443b10 - middle::infer::higher_ranked::C.HigherRankedRelations<'tcx>::higher_ranked_sub::unboxed_closure.73930
  25:     0x7f2b48443400 - middle::infer::higher_ranked::C.HigherRankedRelations<'tcx>::higher_ranked_sub::h8904585972080634262
  26:     0x7f2b484a6400 - middle::infer::combine::Combine::bare_fn_tys::h7458720669919803527
  27:     0x7f2b48426930 - middle::infer::sub::Sub<'f, 'tcx>.Combine<'tcx>::tys::h22d46133309df1a2Wvy
  28:     0x7f2b484a9320 - middle::infer::InferCtxt<'a, 'tcx>::sub_types::h0c797a62be34ef88KYA
  29:     0x7f2b484a8e00 - middle::infer::mk_subty::hc58e5329cc8f3127sqA
  30:     0x7f2b49832d10 - check::compare_method::compare_impl_method::ha3cace0054dac102ZAj
  31:     0x7f2b4983ebf0 - check::check_item::h94f685b036503098RNk
  32:     0x7f2b49903c40 - check_crate::unboxed_closure.30436
  33:     0x7f2b498fe780 - check_crate::h326f19254f8388eauSx
  34:     0x7f2b4a5dcee0 - driver::phase_3_run_analysis_passes::h21c032817652340ahwa
  35:     0x7f2b4a5c5c40 - driver::compile_input::h0fc2c0f3b0492b65wba
  36:     0x7f2b4a694ec0 - thunk::F.Invoke<A, R>::invoke::h7309714978540172785
  37:     0x7f2b4a693c40 - rt::unwind::try::try_fn::h11574143453518770554
  38:     0x7f2b4a117b10 - rust_try_inner
  39:     0x7f2b4a117b00 - rust_try
  40:     0x7f2b4a693f30 - thunk::F.Invoke<A, R>::invoke::h11037746254120548559
  41:     0x7f2b4a0a0570 - sys::thread::thread_start::h8705f831408525e7qrw
  42:     0x7f2b448c70c0 - start_thread
  43:     0x7f2b49cc3f89 - __clone
  44:                0x0 - <unknown>

Based on the backtrace, I think that @jroesch's PR will help here.