Apply suggestions from code review

Co-authored-by: Harris Miller <harrismillerconsulting@gmail.com>

Author: davetapley

test/filter.test.ts

@@ -93,5 +93,5 @@ expectType<Record<string, string>>(filter(isNotNil, {} as Dict));
 
 
 // filter(() => narrow)(dist)
-expectType<Map<string, string>>(filter(isNotNil, new Map() as Map<string, string | undefined>));
-expectType<Map<string, number>>(filter(gt5, new Map() as Map<string, number>));
+expectType<Map<string, string>>(filter(isNotNil, new Map<string, string | undefined>()));
+expectType<Map<string, number>>(filter(gt5, new Map<string, number>()));

types/filter.d.ts

@@ -13,12 +13,15 @@ export function filter<A, P extends A>(
 // filter(() => boolean, list)
 export function filter<T>(
   pred: (value: T) => boolean,
-): <P extends T, C extends readonly P[] | Record<string, P>>(collection: C) => C;
+): {
+  <K>(map: Map<K, T>): Map<K, T>;
+  <P extends T, C extends readonly P[] | Record<string, P>>(collection: C): C;
+};
 
 // filter(() => narrow, map)
-export function filter<K, T, P extends T>(pred: (val: T) => val is P, map: readonly Map<K, T>): Map<K, P>;
+export function filter<K, T, P extends T>(pred: (val: T) => val is P, map: Map<K, T>): Map<K, P>;
 // filter(() => boolean, map)
-export function filter<K, T>(pred: (val: T) => boolean, map: readonly Map<K, T>): Map<K, T>;
+export function filter<K, T>(pred: (val: T) => boolean, map: Map<K, T>): Map<K, T>;
 
 // filter(() => narrow, list) - readonly T[] falls into Record<string T> for some reason, so list needs to come first
 export function filter<T, P extends T>(pred: (val: T) => val is P, list: readonly T[]): P[];