func testCollection() -> (T...) { // All your `map`, `filter`, etc are here! let arr1: [String] = someTs.map { "\($0)" } let arr2: [P1] = someTs.filter { $0 is Int } let first: P1? = someTs.first return (someTs...) }someTs... だからtomeTs... が (T...) なのは、まあそうかも。var someTs: T において (someTs...) の型が (T...) だというのは