`generic_associated_types` fails on dyn dyn

[hrls]

Code

#![feature(generic_associated_types)]
#![allow(incomplete_features)]

trait RefCont<'a, T> {
    fn t(&'a self) -> &'a T;
}

impl<'a, T> RefCont<'a, T> for &'a T {
    fn t(&'a self) -> &'a T {
        self
    }
}

impl<'a, T> RefCont<'a, T> for Box<T> {
    fn t(&'a self) -> &'a T {
        self.as_ref()
    }
}

trait MapLike<K, V> {
    type VRefCont<'a>: RefCont<'a, V>;
    fn get<'a>(&'a self, key: &K) -> Option<Self::VRefCont<'a>>;
}

impl<K: Ord, V: 'static> MapLike<K, V> for std::collections::BTreeMap<K, V> {
    type VRefCont<'a> = &'a V;
    fn get<'a>(&'a self, key: &K) -> Option<&'a V> {
        std::collections::BTreeMap::get(self, key)
    }
}

struct Source;

impl<K, V: Default> MapLike<K, V> for Source {
    type VRefCont<'a> = Box<V>;
    fn get<'a>(&self, _: &K) -> Option<Box<V>> {
        Some(Box::new(V::default()))
    }
}

fn main() {
    let m = Box::new(std::collections::BTreeMap::<u8, u8>::new())
        as Box<dyn MapLike<u8, u8, VRefCont = dyn RefCont<'_, u8>>>;
}

Meta

rustc --version --verbose:

rustc 1.50.0-nightly (fe982319a 2020-11-19)
binary: rustc
commit-hash: fe982319aa0aa5bbfc2795791a753832292bd2ba
commit-date: 2020-11-19
host: x86_64-apple-darwin
release: 1.50.0-nightly

Error output

   Compiling ttt v0.1.0 (/Users/hrls/tmp/ttt)
error: internal compiler error: compiler/rustc_middle/src/ty/subst.rs:476:25: Region parameter out of range when substituting in region 'a (index=3)

thread 'rustc' panicked at 'Box<Any>', /rustc/fe982319aa0aa5bbfc2795791a753832292bd2ba/compiler/rustc_errors/src/lib.rs:904:9
note: run with `RUST_BACKTRACE=1` environment variable to display a 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 1.50.0-nightly (fe982319a 2020-11-19) running on x86_64-apple-darwin

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

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

query stack during panic:
#0 [object_safety_violations] determine object safety of trait `MapLike`
#1 [typeck] type-checking `main`
end of query stack
error: aborting due to previous error

error: could not compile `ttt`

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

Backtrace

error: internal compiler error: compiler/rustc_middle/src/ty/subst.rs:476:25: Region parameter out of range when substituting in region 'a (index=3)

thread 'rustc' panicked at 'Box<Any>', /rustc/fe982319aa0aa5bbfc2795791a753832292bd2ba/compiler/rustc_errors/src/lib.rs:904:9
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_region
   9: <smallvec::SmallVec<A> as core::iter::traits::collect::Extend<<A as smallvec::Array>::Item>>::extend
  10: rustc_middle::ty::fold::TypeFoldable::fold_with
  11: rustc_middle::ty::structural_impls::<impl rustc_middle::ty::fold::TypeFoldable for &rustc_middle::ty::TyS>::super_fold_with
  12: rustc_middle::ty::fold::TypeFoldable::fold_with
  13: rustc_middle::ty::fold::TypeFoldable::fold_with
  14: rustc_middle::ty::Predicate::subst_supertrait
  15: <core::iter::adapters::Map<I,F> as core::iter::traits::iterator::Iterator>::try_fold
  16: <smallvec::SmallVec<A> as core::iter::traits::collect::Extend<<A as smallvec::Array>::Item>>::extend
  17: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
  18: <core::iter::adapters::flatten::FlatMap<I,U,F> as core::iter::traits::iterator::Iterator>::next
  19: <smallvec::SmallVec<A> as core::iter::traits::collect::Extend<<A as smallvec::Array>::Item>>::extend
  20: rustc_middle::arena::Arena::alloc_from_iter
  21: rustc_trait_selection::traits::object_safety::object_safety_violations
  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
  24: rustc_data_structures::stack::ensure_sufficient_stack
  25: rustc_query_system::query::plumbing::get_query_impl
  26: rustc_middle::ty::<impl rustc_middle::ty::context::TyCtxt>::is_object_safe
  27: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  28: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  29: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::resolve_vars_with_obligations
  30: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::structurally_resolved_type
  31: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_call
  32: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  33: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  34: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  35: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  36: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_coercable_to_type
  37: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_decl_local
  38: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_stmt
  39: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  40: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  41: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  42: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  43: rustc_typeck::check::check::check_fn
  44: rustc_infer::infer::InferCtxtBuilder::enter
  45: rustc_typeck::check::typeck
  46: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck>::compute
  47: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  48: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  49: rustc_data_structures::stack::ensure_sufficient_stack
  50: rustc_query_system::query::plumbing::get_query_impl
  51: rustc_query_system::query::plumbing::ensure_query_impl
  52: rustc_typeck::check::typeck_item_bodies
  53: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck_item_bodies>::compute
  54: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  55: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  56: rustc_data_structures::stack::ensure_sufficient_stack
  57: rustc_query_system::query::plumbing::get_query_impl
  58: rustc_typeck::check_crate
  59: rustc_interface::passes::analysis
  60: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::analysis>::compute
  61: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  62: rustc_query_system::dep_graph::graph::DepGraph<K>::with_eval_always_task
  63: rustc_data_structures::stack::ensure_sufficient_stack
  64: rustc_query_system::query::plumbing::get_query_impl
  65: rustc_interface::passes::QueryContext::enter
  66: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
  67: rustc_span::with_source_map
  68: 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 1.50.0-nightly (fe982319a 2020-11-19) running on x86_64-apple-darwin

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

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

query stack during panic:
#0 [object_safety_violations] determine object safety of trait `MapLike`
#1 [typeck] type-checking `main`
#2 [typeck_item_bodies] type-checking all item bodies
#3 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `ttt`

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