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.

Reproduce the TypeScript 3.4.3 example with strict null checks enabled, comparing flatMap(mapFn) with the separate map and flat calls. Trace the library definition and type-checking behavior for callbacks returning number[] | undefined; done means the reported example accepts the callback and infers (number | undefined)[] as expected.