a

error[E0391]: cycle detected when processing `Bar::{{impl-Trait}}`
  --> atb.rs:32:47
   |
32 | existential type Bar: Debug + TraitB<AssocB = impl Send>;
   |                                               ^^^^^^^^^
   |
note: ...which requires processing `Bar::{{impl-Trait}}`...
  --> atb.rs:32:47
   |
32 | existential type Bar: Debug + TraitB<AssocB = impl Send>;
   |                                               ^^^^^^^^^
note: ...which requires processing `Bar::{{impl-Trait}}`...
  --> atb.rs:32:47
   |
32 | existential type Bar: Debug + TraitB<AssocB = impl Send>;
   |                                               ^^^^^^^^^
   = note: ...which again requires processing `Bar::{{impl-Trait}}`, completing the cycle
note: cycle used when processing `Bar`
  --> atb.rs:32:1
   |
32 | existential type Bar: Debug + TraitB<AssocB = impl Send>;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

thread 'rustc' panicked at 'encountered error with `-Z treat_err_as_bug', src/librustc_errors/lib.rs:490:13
query stack during panic:
#0 [explicit_predicates_of] processing `Bar::{{impl-Trait}}`
#1 [predicates_defined_on] processing `Bar::{{impl-Trait}}`
#2 [predicates_of] processing `Bar::{{impl-Trait}}`
#3 [explicit_predicates_of] processing `Bar`
#4 [predicates_defined_on] processing `Bar`
#5 [predicates_of] processing `Bar`
#6 [typeck_tables_of] processing `a`
#7 [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