↓こういうのはOKっぽい

do { let a: [String: Cat] = ["": Cat()] let b: [String: Animal] = a // OK } do { let a: [String: [Cat]] = ["": [Cat()]] let b: [String: [Animal]] = a // OK } do { let a: [String: Cat?] = ["": Cat()] let b: [String: Animal?] = a // OK }