こんな感じ？
protocol P { associatedtype AT } protocol HasStringProtocol {} protocol HasIntProtocol {} extension P where AT: HasStringProtocol { func po() -> String { return "string" } } extension P where AT: HasIntProtocol { func po() -> Int { return 1 } } struct StringAndInt: HasStringProtocol, HasIntProtocol {} struct S: P { typealias AT = StringAndInt } let s = S() let string = s.po() as String // "string" let int = s.po() as Int // 1