Type error for new Map from undefined Iterable

[JaredNeil]

TypeScript Version: 3.8.3

Search Terms: Map MapConstructor Iterable Iterator

Code

let undefinedArray: [string, string][] | undefined;
const map1 = new Map(undefinedArray);

let definedInterable: IterableIterator<[string, string]> = new Set<string>().entries();
const map2 = new Map(definedInterable);

let undefinedIterable: IterableIterator<[string, string]> | undefined;
const map3 = new Map(undefinedIterable); // Type error here

Expected behavior:
Passes type check because new Map(undefined) is a valid call to the Map constructor.
Actual behavior:

No overload matches this call.
  Overload 1 of 4, '(iterable: Iterable<readonly [string, string]>): Map<string, string>', gave the following error.
    Argument of type 'IterableIterator<[string, string]> | undefined' is not assignable to parameter of type 'Iterable<readonly [string, string]>'.
      Type 'undefined' is not assignable to type 'Iterable<readonly [string, string]>'.
  Overload 2 of 4, '(iterable: Iterable<readonly [string, string]>): Map<string, string>', gave the following error.
    Argument of type 'IterableIterator<[string, string]> | undefined' is not assignable to parameter of type 'Iterable<readonly [string, string]>'.
      Type 'undefined' is not assignable to type 'Iterable<readonly [string, string]>'.
  Overload 3 of 4, '(entries?: readonly (readonly [unknown, unknown])[] | null | undefined): Map<unknown, unknown>', gave the following error.
    Argument of type 'IterableIterator<[string, string]> | undefined' is not assignable to parameter of type 'readonly (readonly [unknown, unknown])[] | null | undefined'.
      Type 'IterableIterator<[string, string]>' is missing the following properties from type 'readonly (readonly [unknown, unknown])[]': length, concat, join, slice, and 14 more.(2769)

Playground Link: https://www.typescriptlang.org/play/#code/DYUwLgBArgdgJiAZgSxiOBBATlghgTwC4IBtAZzC1QHMAaCCqmagXRJYgB9p4lV0A3ACgAxgHsYFCAFtcABwCMEALwQ0AdwgBZeQApYCFGkw4CASmFDQkQ-zgBJGGBB4ARqGL3nb0F5e4wMSwAHnJKGnpGGhYAPhU1EE0AZXBgqOYY3TMAOhAnKhAyLOFxSUhZOQAmeI1tPVtjR29cdxALIStwHgb0Px8QT2bWvoCg0PS6BnDmWK5uvmMSiSkKgGYaxLq5fV4jXqHQdqA

Related Issues: None

Suggested Fix:
The MapConstructor interface in es2015.iterable.d.ts should accept a single optional argument.

diff --git a/src/lib/es2015.iterable.d.ts b/src/lib/es2015.iterable.d.ts
index 374aad9322..6f9b786c1d 100644
--- a/src/lib/es2015.iterable.d.ts
+++ b/src/lib/es2015.iterable.d.ts
@@ -137,7 +137,7 @@ interface ReadonlyMap<K, V> {
 }
 
 interface MapConstructor {
-    new <K, V>(iterable: Iterable<readonly [K, V]>): Map<K, V>;
+    new <K, V>(iterable?: Iterable<readonly [K, V]>): Map<K, V>;
 }
 
 interface WeakMap<K extends object, V> { }