struct Hoge<T> { } extension Hoge where T == Array<Any> { func fuga() { print("fuga!") } } let x = Hoge<[Int]>() x.fuga()struct Hoge<T> { } extension Hoge where T == Array<Any> { func fuga() { print("fuga!") } } let x = Hoge<[Int]>() x.fuga()Playground execution failed: error: HogeGround.playground:5:1: error: 'Hoge<[Int]>' (aka 'Hoge<Array<Int>>') is not convertible to 'Hoge<Array<Any>>' x.fuga()