/* @flow */
declare const iterator: Iterator<number>;
declare function mapper(x: number): $ReadOnlyArray<string>

// Basic usage
(iterator.flatMap(mapper): Iterator<string>); // OK

// Return type is discarded: https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.flatmap
// "ii. If value is done, return undefined."
declare const iteratorWithReturn: $Iterator<number, number, void>;
(iteratorWithReturn.flatMap(mapper): $Iterator<string, void, void>); // OK
