タプル続き

class Animal {} class Cat: Animal {} let strinCat: (String, Cat) = ("xyz", Cat()) if let stringAnimal: (String, Animal) = strinCat as? (String, Animal) { print(stringAnimal) }

でエラー:

Playground execution failed: warning: 20170418-130912.xcplaygroundpage:1:50: warning: conditional cast from '(String, Cat)' to '(String, Animal)' always succeeds if let stringAnimal: (String, Animal) = strinCat as? (String, Animal) { ^ error: 20170418-130912.xcplaygroundpage:1:41: error: cannot express tuple conversion '(String, Cat)' to '(String, Animal)' if let stringAnimal: (String, Animal) = strinCat as? (String, Animal) { ^