Infinite loop in type inference about rust HOT 3 OPEN

error[E0271]: type mismatch resolving `<impl Gen as Gen>::B == _`
  --> src/lib.rs:18:7
   |
18 |     b(F(c))
   |     - ^^^^ cyclic type of infinite size
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `b`
  --> src/lib.rs:24:24
   |
21 | fn b<G, B>(_: G)
   |    - required by a bound in this function
...
24 |     <G as Fun>::O: Gen<B = B>,
   |                        ^^^^^ required by this bound in `b`