struct Hoge<T> { } extension Hoge where T: Sequence { func fuga() -> T.Iterator.Element { // <- Error: 'Element' is not a member type of 'T' fatalError() } } これは通るねstruct Hoge<T> { } extension Hoge where T: Sequence { func fuga() -> T.Iterator.Element { // <- Error: 'Element' is not a member type of 'T' fatalError() } } これは通るね