declare const map: Map<string, number>;


declare function forEachKey<K, T>(map: ReadonlySet<K> | ReadonlyMap<K, any>, callback: (key: K) => T | undefined): T | undefined;

forEachKey(map, (key) => {}) // Error passing "map", key type is wrong (number??)

forEachKey<string, void>(map, (key) => {}) // OK; no error, key type is string


// Now, flip the union order in the parameter's type.
declare function forEachKey2<K, T>(map: ReadonlyMap<K, any> | ReadonlySet<K>, callback: (key: K) => T | undefined): T | undefined;

forEachKey2(map, (key) => {}) // Okay?

forEachKey2<string, void>(map, (key) => {}) // Still okay.