flatMap errors when callback returns certain union types

TypeScript Version: 3.4.3

Search Terms: flat, flatmap

Code

// Map<string, number[]>
const myMap = new Map([
  ['foo', [1, 2, 3]],
  ['bar', [4, 5, 6]]
]);

// string[]
const myArray = ['foo', 'bar', 'baz'];

// (key: any) => number[] | undefined
const mapFn = key => myMap.get(key);

const flatMapped = myArray.flatMap(mapFn);

Actual behavior:

With strict null checks on:

Argument of type '(key: any) => number[] | undefined' is not assignable to parameter of type '(this: undefined, value: string, index: number, array: string[]) => number | readonly number[]'.
  Type 'number[] | undefined' is not assignable to type 'number | readonly number[]'.
    Type 'undefined' is not assignable to type 'number | readonly number[]'. ts(2345)

Turning strict null checks off prevents the error as you might expect, but that's obviously not desirable.

More generally, any callback that returns foo[] | bar (where bar is not compatible with foo) seems to run into the same issue. The Map.prototype.get example I've used here just seems like it would be a relatively common use case in particular (you'd typically follow it up by calling .filter(Boolean) or similar on the mapped result).

More code

Seeing if doing the operations separately will help:

// (number[] | undefined)[]
const mapped = myArray.map(mapFn);

// any[] !!
const flattened = mapped.flat();

Expected behavior:

flatMapped/flattened result to be inferred as (number | undefined)[].

Playground Link:

Doesn't seem to be able to load the appropriate library definition, even with /// <reference lib="es2019.array" />

Related Issues:

#29604 seems to be related to the .flat() part.

Das TypeScript 3.4.3-Beispiel mit aktivierten strict null checks reproduzieren und dabei flatMap(mapFn) mit den separaten Aufrufen von map und flat vergleichen. Die Bibliotheksdefinition und das Type-Checking-Verhalten für Callbacks verfolgen, die number[] | undefined zurückgeben; abgeschlossen ist die Aufgabe, wenn das gemeldete Beispiel den Callback akzeptiert und wie erwartet (number | undefined)[] ableitet.