"coherence failed to report ambiguity" ICE Involving Lifetimes

[kyledewey]

The following causes an ICE:

trait Trait1<'l0, T0> {}
trait Trait0<'l0>  {}

impl <'l0, 'l1, T0> Trait1<'l0, T0> for bool where T0 : Trait0<'l0>, T0 : Trait0<'l1> {}

fn main() {}

Backtrace:

crash.rs:4:1: 4:89 error: internal compiler error: coherence failed to report ambiguity: cannot locate the impl of the trait `Trait0<'_>` for the type `T0`
crash.rs:4 impl <'l0, 'l1, T0> Trait1<'l0, T0> for bool where T0 : Trait0<'l0>, T0 : Trait0<'l1> {}

stack backtrace:
   1:        0x10e27099f - sys::backtrace::write::h65efba87d2ac2b9cGGC
   2:        0x10e297ae2 - panicking::on_panic::h7344421187b575a3mGI
   3:        0x10e1ce9f5 - rt::unwind::begin_unwind_inner::h0fb3967931f800c95nI
   4:        0x10d9ac11e - rt::unwind::begin_unwind::h14950887375606487128
   5:        0x10d9ac0aa - diagnostic::SpanHandler::span_bug::hce9df0bd0b4a6d6bsmB
   6:        0x10b3ae93e - middle::traits::error_reporting::report_fulfillment_errors::h914196447e3f8e39tRN
   7:        0x10aa7f283 - check::vtable::select_all_fcx_obligations_or_error::heb0bf2b308159b20P1b
   8:        0x10aaf585d - check::wf::CheckTypeWellFormedVisitor<'ccx, 'tcx>::check_item_well_formed::h8592f75fbcdfabe4kWk
   9:        0x10abe603e - check_crate::closure.38354
  10:        0x10abe1517 - check_crate::hc0c9219806b9606cnuC
  11:        0x10a934a62 - driver::phase_3_run_analysis_passes::h22c1ac11f5719067nGa
  12:        0x10a91677c - driver::compile_input::ha52ae5794027c2d3Qba
  13:        0x10a9dc873 - run_compiler::h88dac0540af48212F4b
  14:        0x10a9da39a - boxed::F.FnBox<A>::call_box::h782914981501029945
  15:        0x10a9d9837 - rt::unwind::try::try_fn::h11237440895302402829
  16:        0x10e31fa48 - rust_try_inner
  17:        0x10e31fa35 - rust_try
  18:        0x10a9d9b10 - boxed::F.FnBox<A>::call_box::h3896333940280334120
  19:        0x10e283e9d - sys::thread::create::thread_start::hb17c0976197a6554xnH
  20:     0x7fff8df11741 - _pthread_start

With:

% rustc --version
rustc 1.0.0-nightly (e6a812402 2015-04-14) (built 2015-04-13)

I can confirm that this bug has been around since 1.0.0-alpha (though with a slightly different assertion violation; I accidentally posted that version originally).

[kyledewey]

Removing either constraint in the where clause, or changing the lifetimes used in the where clause to be the same causes the ICE to go away.

[arielb1]

I guess we should change that error message to be different from "coherence failed to report ambiguity" to "trait selection failed".

Your use-case isn't currently supported (#21974). Not sure which error message to give, through.

[frewsxcv]

A couple more examples of this (including a nightly regression) can be found in #27989

[malbarbo]

With rustc 1.7.0, 1.8.0 and 1.9.0, the example fails to compile, but don't generate an ICE.

[frewsxcv]

Added a regression test in #34453.