class Animal {} class Cat: Animal {} let a: () -> Cat = { Cat () } let b: () -> Animal = a // OK let c: () -> [Cat] = { [Cat()] } let d: () -> [Animal] = c // NG: error: cannot convert value of type '() -> [Cat]' to specified type '() -> [Animal]' let e: [Cat] = [Cat()] e is [Animal] // true