do { let a: (Animal) -> () = { _ in } let b: (Cat) -> () = a // OK } do { let a: ([Animal]) -> () = { _ in } let b: ([Cat]) -> () = a // error: cannot convert value of type '([Animal]) -> ()' to specified type } do { let a: (Animal?) -> () = { _ in } let b: (Cat?) -> () = a // OK }