a

error[E0271]: type mismatch resolving `<i32 as TraitB>::AssocB == impl std::marker::Send`
  --> atb.rs:37:1
   |
37 | existential type Bar: Debug + TraitB<AssocB = impl Send>;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected u32, found opaque type
   |
   = note: expected type `u32`
              found type `impl std::marker::Send`
   = note: the return type of a function must have a statically known size

thread 'rustc' panicked at 'encountered error with `-Z treat_err_as_bug', src/librustc_errors/lib.rs:490:13
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
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::emit_db
   8: rustc_errors::diagnostic_builder::DiagnosticBuilder::emit
   9: rustc::infer::InferCtxt::probe
  10: rustc::traits::error_reporting::<impl rustc::infer::InferCtxt<'a, 'gcx, 'tcx>>::report_fulfillment_errors
  11: rustc_typeck::check::FnCtxt::select_obligations_where_possible
  12: rustc_typeck::check::FnCtxt::resolve_type_vars_with_obligations
  13: rustc_typeck::check::coercion::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::try_coerce
  14: <rustc_typeck::check::coercion::CoerceMany<'gcx, 'tcx, 'exprs, E>>::coerce_inner
  15: rustc_typeck::check::FnCtxt::check_return_expr
  16: rustc_typeck::check::check_fn
  17: rustc::ty::context::GlobalCtxt::enter_local
  18: rustc_typeck::check::typeck_tables_of
  19: rustc::ty::query::__query_compute::typeck_tables_of
  20: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  21: rustc::dep_graph::graph::DepGraph::with_task_impl
  22: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  23: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  25: rustc_typeck::collect::find_existential_constraints::ConstraintLocator::check
  26: rustc::hir::intravisit::Visitor::visit_nested_item
  27: rustc_typeck::collect::find_existential_constraints
  28: rustc_typeck::collect::type_of
  29: rustc::ty::query::__query_compute::type_of
  30: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::type_of<'tcx>>::compute
  31: rustc::dep_graph::graph::DepGraph::with_task_impl
  32: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  33: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  34: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  35: <rustc_typeck::collect::CollectItemTypesVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  36: rustc::hir::Crate::visit_all_item_likes
  37: rustc::util::common::time
  38: rustc_typeck::check_crate
  39: <std::thread::local::LocalKey<T>>::with
  40: rustc::ty::context::TyCtxt::create_and_enter
  41: rustc_driver::driver::phase_3_run_analysis_passes
  42: rustc_driver::driver::compile_input
  43: rustc_driver::run_compiler_with_pool
  44: <scoped_tls::ScopedKey<T>>::set
  45: rustc_driver::run_compiler
  46: <scoped_tls::ScopedKey<T>>::set
query stack during panic:
#0 [typeck_tables_of] processing `a`
#1 [type_of] processing `Bar`
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 1.33.0-dev running on x86_64-apple-darwin

note: compiler flags: -Z treat-err-as-bug