import Foundation protocol Foo { associatedtype Bar func hoge() } extension Foo where Bar: Decodable { func hoge() { print("Decodable.hoge") } } extension Foo where Bar: Encodable { func hoge() { print("Encodable.hoge") } } struct FooImpl1: Foo { typealias Bar = String } struct FooImpl2: Foo { typealias Bar = String } let foo1: FooImpl1 = FooImpl1() let foo2: FooImpl2 = FooImpl2() foo1.hoge() foo2.hoge()