ICE when returning generic abitrary self type in defining function for existential associated type

[matprec]

#![feature(arbitrary_self_types)]
#![feature(existential_type)]

use std::ops::Deref;

trait Route {
    type Extension: Route;

    fn extend(self: impl Deref<Target = Self>) -> Self::Extension;
}

impl<C> Route for C {
    existential type Extension: Route;

    fn extend(self: impl Deref<Target = Self>) -> Self::Extension {
        self
    }
}

Playground

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: src/librustc/ty/subst.rs:480: Type parameter `impl Deref<Target = Self>/#1` (impl Deref<Target = Self>/1) out of range when substituting (root type=Some(impl Deref<Target = Self>)) substs=[C]

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:526:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:482
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::span_bug
   8: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::span_bug_fmt
  14: <rustc::ty::subst::SubstFolder<'a, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  15: rustc::infer::InferCtxt::commit_if_ok
  16: <rustc::traits::query::type_op::custom::CustomTypeOp<F, G> as rustc::traits::query::type_op::TypeOp<'gcx, 'tcx>>::fully_perform
  17: rustc_mir::borrow_check::nll::type_check::TypeChecker::eq_opaque_type_and_type
  18: rustc_mir::borrow_check::nll::type_check::type_check
  19: rustc_mir::borrow_check::nll::compute_regions
  20: rustc_mir::borrow_check::do_mir_borrowck
  21: rustc::ty::context::GlobalCtxt::enter_local
  22: rustc_mir::borrow_check::mir_borrowck
  23: rustc::ty::query::__query_compute::mir_borrowck
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::mir_borrowck<'tcx>>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  27: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::par_body_owners
  28: rustc::util::common::time
  29: <std::thread::local::LocalKey<T>>::with
  30: rustc::ty::context::TyCtxt::create_and_enter
  31: rustc_driver::driver::compile_input
  32: rustc_driver::run_compiler_with_pool
  33: <scoped_tls::ScopedKey<T>>::set
  34: rustc_driver::run_compiler
  35: <scoped_tls::ScopedKey<T>>::set
query stack during panic:
#0 [mir_borrowck] processing `<C as Route>::extend`
end of query stack
error: aborting due to previous error

[dlrobertson]

This doesn't panic on f001287 or nightly for me. Has it been fixed?

[matprec]

Can't confirm currently, because my toolchain is stuck on nightly 2019-01-18, because of the toolstate changes in that PR. On that toolchain, it ICEs. Will try to reproduce on new nightly once available.

[dlrobertson]

👍 double checked on 0c0c585 and 846ea58. Still no ICE.

[matprec]

@dlrobertson If you run it on the playground, you see that it still ICEs

[dlrobertson]

@MSleepyPanda Aha! Running rustc +nightly ./src/lib.rs doesn't ICE, but cargo +nightly build does.

EDIT: After some research I realized that in order to reproduce the ICE --edition=2018 must be set or you must use #![feature(nll)].

[Arnavion]

This is the same as #53598 I believe, since the impl Deref<Target = Self> AST is essentially a trait method type parameter extend<S: Deref<Target = Self>>(self: S) being used to define the trait's assoc type.

[dlrobertson]

@Arnavion good catch. I agree.

[pnkfelix]

NLL triage. P-high, assigning to self for initial investigation.

[pnkfelix]

downgrading to P-medium under same reasoning as given in #53598

[Aaron1011]

This now produces an error instead of an ICE on the latest nightly.

[adamAndMath]

Fixing the compile error makes it compile successfully.
Playground