error[E0391]: cycle detected when expanding type alias `Bar`
  --> $DIR/issue-110629-private-type-cycle-dyn.rs:1:38
   |
LL | type Bar<'a, 'b> = Box<dyn PartialEq<Bar<'a, 'b>>>;
   |                                      ^^^^^^^^^^^
   |
   = note: ...which immediately requires expanding type alias `Bar` again
   = note: type aliases cannot be recursive
   = help: consider using a struct, enum, or union instead to break the cycle
   = help: see <https://doc.rust-lang.org/reference/types.html#recursive-types> for more information
note: cycle used when collecting item types in top-level module
  --> $DIR/issue-110629-private-type-cycle-dyn.rs:1:1
   |
LL | / type Bar<'a, 'b> = Box<dyn PartialEq<Bar<'a, 'b>>>;
LL | |
LL | |
LL | | fn bar<'a, 'b>(i: &'a i32) -> Bar<'a, 'b> {
...  |
LL | |     assert!(bar(&meh) == bar(&muh));
LL | | }
   | |_^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0391`.