GAT ICE: missing generic associated type in trait path

[juntyr]

This ICE is related to #44265:

Code

#![feature(generic_associated_types)]

// The cyclic dependency between trait A and B compiles as expected
trait A {
    type BType: B<AType = Self>;
}

trait B {
    type AType: A<BType = Self>;
}

// rustc crashes on the generic cyclic dependency between traits C and D
trait C {
    type DType<T>: D<T, CType = Self>;
}
trait D<T> {
    type CType: C<DType = Self>;
}

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (ffa2e7ae8 2020-10-24)
binary: rustc
commit-hash: ffa2e7ae8fbf9badc035740db949b9dae271c29f
commit-date: 2020-10-24
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly
LLVM version: 11.0

Error output

error: internal compiler error: compiler/rustc_middle/src/ty/subst.rs:528:17: type parameter `T/#1` (T/1) out of range when substituting, substs=[<Self as D<T>>::CType]

thread 'rustc' panicked at 'Box<Any>', /rustc/ffa2e7ae8fbf9badc035740db949b9dae271c29f/compiler/rustc_errors/src/lib.rs:891:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

Backtrace

error: internal compiler error: compiler/rustc_middle/src/ty/subst.rs:528:17: type parameter `T/#1` (T/1) out of range when substituting, substs=[<Self as D<T>>::CType]

thread 'rustc' panicked at 'Box<Any>', /rustc/ffa2e7ae8fbf9badc035740db949b9dae271c29f/compiler/rustc_errors/src/lib.rs:891:9
stack backtrace:
   0: std::panicking::begin_panic
   1: rustc_errors::HandlerInner::span_bug
   2: rustc_errors::Handler::span_bug
   3: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
   4: rustc_middle::ty::context::tls::with_opt::{{closure}}
   5: rustc_middle::ty::context::tls::with_opt
   6: rustc_middle::util::bug::opt_span_bug_fmt
   7: rustc_middle::util::bug::span_bug_fmt
   8: <rustc_middle::ty::subst::SubstFolder as rustc_middle::ty::fold::TypeFolder>::fold_ty
   9: rustc_middle::ty::fold::TypeFoldable::fold_with
  10: rustc_middle::ty::fold::TypeFoldable::fold_with
  11: rustc_middle::ty::subst::Subst::subst
  12: rustc_middle::ty::GenericPredicates::instantiate_into
  13: rustc_middle::ty::GenericPredicates::instantiate
  14: rustc_trait_selection::traits::wf::WfPredicates::nominal_obligations
  15: rustc_trait_selection::traits::wf::WfPredicates::compute_projection
  16: rustc_trait_selection::traits::wf::predicate_obligations
  17: rustc_typeck::check::wfcheck::check_associated_type_bounds
  18: rustc_infer::infer::InferCtxtBuilder::enter
  19: rustc_typeck::check::wfcheck::check_associated_item
  20: rustc_typeck::check::wfcheck::check_trait_item
  21: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::check_trait_item_well_formed>::compute
  22: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  23: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_query_system::query::plumbing::ensure_query_impl
  27: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  28: rustc_hir::hir::Crate::par_visit_all_item_likes
  29: rustc_session::session::Session::track_errors
  30: rustc_typeck::check_crate
  31: rustc_interface::passes::analysis
  32: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  33: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  34: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  35: rustc_middle::ty::query::plumbing::<impl rustc_query_system::query::QueryContext for rustc_middle::ty::context::TyCtxt>::start_query::{{closure}}::{{closure}}::{{closure}}
  36: rustc_query_system::query::plumbing::get_query_impl
  37: rustc_interface::passes::QueryContext::enter
  38: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  39: rustc_span::with_source_map
  40: rustc_interface::interface::create_compiler_and_run
  41: scoped_tls::ScopedKey<T>::set
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.49.0-nightly (ffa2e7ae8 2020-10-24) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type lib

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

query stack during panic:
#0 [check_trait_item_well_formed] checking that `D::CType` is well-formed
#1 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error; 1 warning emitted

Ideally, C<DType<T> = Self> would compile, but generic associated types in trait paths are currently not implemented. Maybe rustc could give a nice error message when the <T> is missing.

[jackh726]

Duplicate to #79768 and related heavily to others. But this has a nice minimal repro, so worth leaving open. Probably fixed by #79554.

[cynecx]

Probably fixed by #79554.

I've tested this locally and I can confirm that #79554 fixes this (with this change: C<DType<T> = Self>).