ICE with existential associated type

[Seeker14491]

The following code causes an ICE:

#![feature(existential_type)]

extern crate futures; // 0.1.24

use futures::{future, Future};

pub struct AStruct;

pub trait ATrait {
    type FutureType: Future<Item = (), Error = Self::Error> + Send + 'static;
    type Error;

    fn a_function(&self) -> Self::FutureType;
}

impl ATrait for AStruct {
    existential type FutureType: Future<Item = (), Error = Self::Error> + Send;
    type Error = ();

    fn a_function(&self) -> Self::FutureType {
        future::ok(())
    }
}

Playground

Compiler output:

error: internal compiler error: broken MIR in DefId(0/0:14 ~ frontend[2c69]::{{impl}}[0]::a_function[0]) (Terminator { source_info: SourceInfo { span: src\lib.rs:19:9: 19:23, scope: scope[0] }, kind: _0 = const futures::future::result_::ok(move _2) -> [return: bb2, unwind: bb1] }): call dest mismatch (futures::future::result_::FutureResult<(), <AStruct as ATrait>::Error> <- futures::future::result_::FutureResult<(), ()>): NoSolution
  --> src\lib.rs:19:20
   |
19 |         future::ok(())
   |                    ^^

thread 'main' panicked at 'no errors encountered even though `delay_span_bug` issued', librustc_errors\lib.rs:334:17
stack backtrace:
   0: <std::sync::barrier::BarrierWaitResult as core::fmt::Debug>::fmt
   1: <std::path::Iter<'a> as core::convert::AsRef<std::path::Path>>::as_ref
   2: std::panicking::take_hook
   3: std::panicking::take_hook
   4: <rustc::ty::sty::Binder<rustc::ty::ProjectionPredicate<'tcx>> as rustc::ty::ToPredicate<'tcx>>::to_predicate
   5: std::panicking::rust_panic_with_hook
   6: <rustc_errors::emitter::ColorConfig as core::fmt::Debug>::fmt
   7: <rustc_errors::Handler as core::ops::drop::Drop>::drop
   8: <rustc_driver::derive_registrar::Finder as rustc::hir::itemlikevisit::ItemLikeVisitor<'v>>::visit_impl_item
   9: <rustc_driver::profile::trace::Query as core::fmt::Debug>::fmt
  10: <rustc_driver::profile::trace::Query as core::fmt::Debug>::fmt
  11: rustc_driver::target_features::add_configuration
  12: _rust_maybe_catch_panic
  13: <env_logger::filter::inner::Filter as core::fmt::Display>::fmt
  14: rustc_driver::main
  15: <unknown>
  16: std::panicking::update_panic_count
  17: _rust_maybe_catch_panic
  18: std::rt::lang_start_internal
  19: <unknown>
  20: <unknown>
  21: BaseThreadInitThunk
  22: RtlUserThreadStart
query stack during panic:
end of query stack

error: internal compiler error: unexpected panic

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: rustc 1.31.0-nightly (4efdc04a5 2018-10-06) running on x86_64-pc-windows-msvc

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

error: Could not compile `frontend`.

To learn more, run the command again with --verbose.

[eddyb]

cc @nikomatsakis @Zoxc @cramertj

[oli-obk]

related to #53678

both seem to not fully resolve all associated types

[Aaron1011]

This compiles successfully on the latest nightly.

[Aaron1011]

Can this be closed now that a test has been added?

[Seeker14491]

I don't believe a test has been added. Centril's commit above added tests for some related ICEs, but not this one.

[lcnr]

Considering that this issue used existential type and impl Trait has changed a lot since then I don't think trying to get an MVCE is worth it here, so I will close this without adding a regression test for this.

[vandenheuvel]

See also this short discussion on Zulip.