Specialization ICE - unwrap in

[withoutboats]

The unwrap in Graph::parent here is failing for me.

I am unable to reduce my code to a shareably small sample which hits this, but this is a sample of the approximate shape of my code (again, no ICE though) and this is the backtrace:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', ../src/libcore/option.rs:326
stack backtrace:
   1:     0x7fc4e559dcdd - std::sys::backtrace::tracing::imp::write::h29f5fdb9fc0a7395
   2:     0x7fc4e55aea71 - std::panicking::default_hook::_{{closure}}::h17d8437f66223ab1
   3:     0x7fc4e55aced8 - std::panicking::default_hook::hbbe7fa36a995aca0
   4:     0x7fc4e55ad6b8 - std::panicking::rust_panic_with_hook::h105c3d42fcd2fb5e
   5:     0x7fc4e55ad552 - std::panicking::begin_panic::hbf62ea4a5ff3f9de
   6:     0x7fc4e55ad480 - std::panicking::begin_panic_fmt::h20f5943904e5791d
   7:     0x7fc4e55ad401 - rust_begin_unwind
   8:     0x7fc4e5618c2c - core::panicking::panic_fmt::h19323e466869c656
   9:     0x7fc4e5618b5b - core::panicking::panic::hbfac80217e56ecbe
  10:     0x7fc4e1db2955 - _<rustc..traits..specialize..specialization_graph..Ancestors<'a, 'tcx> as core..iter..iterator..Iterator>::next::ha4b603294de54f87
  11:     0x7fc4e1c07c9e - _<core..iter..FlatMap<I, U, F> as core..iter..iterator..Iterator>::next::hd671a17475304ea4
  12:     0x7fc4e1d9ebbf - rustc::traits::project::assoc_ty_def::hd2c32c511f927ac7
  13:     0x7fc4e1d9bb0f - rustc::traits::project::opt_normalize_projection_type::hac8607daffaabac5
  14:     0x7fc4e1d99776 - rustc::traits::project::normalize_projection_type::hc8d921334194224d
  15:     0x7fc4e1d9948a - _<rustc..traits..project..AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc..ty..fold..TypeFolder<'gcx, 'tcx>>::fold_ty::he80336df5fe7f90c
  16:     0x7fc4e1e0bb5a - rustc::ty::structural_impls::_<impl rustc..ty..fold..TypeFoldable<'tcx> for rustc..ty..Predicate<'tcx>>::super_fold_with::h02a2f90a72fe499e
  17:     0x7fc4e1bfe801 - _<collections..vec..Vec<T> as core..iter..traits..FromIterator<T>>::from_iter::ha48c52bee1e74668
  18:     0x7fc4e1d97a95 - rustc::traits::project::normalize_with_depth::h31c4e05e4c5432fb
  19:     0x7fc4e1dbc38f - rustc::traits::fully_normalize::hab6692619a95ed52
  20:     0x7fc4e1e4284d - rustc::infer::InferCtxtBuilder::enter::_{{closure}}::h14a7e5fe5b1e8e90
  21:     0x7fc4e1b730c9 - _<std..thread..local..LocalKey<T>>::with::h18b62112a5b30cb7
  22:     0x7fc4e1dfed1b - rustc::ty::context::GlobalCtxt::enter_local::hce46ccddc38b6c66
  23:     0x7fc4e1dbbbcc - rustc::traits::normalize_param_env_or_error::hb6194ed5c833924b
  24:     0x7fc4e1e2e88c - rustc::ty::_<impl rustc..ty..context..TyCtxt<'a, 'gcx, 'tcx>>::construct_parameter_environment::h358f05100bbd5e3b
  25:     0x7fc4e1db3081 - rustc::traits::specialize::specializes::h14f0f43092eacdc1
  26:     0x7fc4e1db099b - rustc::traits::specialize::specialization_graph::Graph::insert::hd0f3feb4c7004213
  27:     0x7fc4e1ded4ca - rustc::ty::trait_def::TraitDef::add_impl_for_specialization::ha3c0fa5ee23a0807
  28:     0x7fc4e3a121ed - _<rustc_typeck..coherence..overlap..OverlapChecker<'cx, 'tcx> as rustc..hir..intravisit..Visitor<'v>>::visit_item::h9e259f38cb9c748b
  29:     0x7fc4e3a18ddd - rustc_typeck::coherence::check_coherence::h22ce515e0e650788
  30:     0x7fc4e3a214b3 - rustc_typeck::check_crate::h3586447e34ddb139
  31:     0x7fc4e5b5c833 - rustc_driver::driver::phase_3_run_analysis_passes::_{{closure}}::h7f2cec505064b4bf
  32:     0x7fc4e5abee9e - rustc::ty::context::TyCtxt::create_and_enter::he5dca3f2a7a0810a
  33:     0x7fc4e5b234f8 - rustc_driver::driver::compile_input::hb4cc34cf85dc1edf
  34:     0x7fc4e5b4d602 - rustc_driver::run_compiler::h50f95674bd902ab5
  35:     0x7fc4e5a94ddd - std::panicking::try::call::h31fc30b58c55d6c3
  36:     0x7fc4e55bcf16 - __rust_maybe_catch_panic
  37:     0x7fc4e5aadac0 - _<F as alloc..boxed..FnBox<A>>::call_box::h24f3eb0b42327962
  38:     0x7fc4e55ab202 - std::sys::thread::Thread::new::thread_start::h8f3bd45211e9f5ea
  39:     0x7fc4dca60483 - start_thread
  40:     0x7fc4e51f66dc - clone
  41:                0x0 - <unknown>

[withoutboats]

Here's an example of the ICE https://is.gd/iFtKxe

#![feature(specialization)]

fn main() {}

pub trait Alpha<T> { }

pub trait Beta {
    type Event;
}

pub trait Delta {
    type Handle;
    fn process(&self);
}

pub struct Parent<A, T>(A, T);

impl<A, T> Delta for Parent<A, T>
where A: Alpha<T::Handle>,
      T: Delta,
      T::Handle: Beta<Event = <Handle as Beta>::Event> {
    type Handle = Handle;
    default fn process(&self) {
        unimplemented!()
    }
}

impl<A, T> Delta for Parent<A, T>
where A: Alpha<T::Handle> + Alpha<Handle>,
      T: Delta,
      T::Handle: Beta<Event = <Handle as Beta>::Event> {
      fn process(&self) {
        unimplemented!()
      }
}

pub struct Handle;

impl Beta for Handle {
    type Event = ();
}

EDIT: reduced further

[withoutboats]

I've pinpointed the issue more. If you replace <Handle as Beta>::Event in both Delta impls with the correct concrete type, this code compiles.

[Aatch]

Some digging suggests that specialisation graph isn't being created (or being incorrectly created) for Beta, at least as it pertains to the Beta for Handle impl. The problem is that there's no parent for that impl, I have a feeling there should be one though, but I'm not familiar enough with the code to be sure.

/cc @aturon @rust-lang/compiler

[dtolnay]

This looks a lot like #33187 which produces the same panic without even using specialization.

[arielb1]

Added A-specialization.

[kennytm]

All examples no longer ICE, they can all be compiled successfully.