struct MyIterator<T> : IteratorProtocol { mutating func next() -> T? { return nil } } struct MySequence<T> : Sequence { func makeIterator() -> MyIterator<T> { return MyIterator() } } print(MySequence<Int>().prefix(2)) // AnySequence<Int>(_box: Swift._SequenceBox<Swift._PrefixSequence<__lldb_expr_68.MyIterator<Swift.Int>>>)