Union of arrays doesn't get converted to array of unions

[reverofevil]

TypeScript Version: 3.8.0-dev.20191121

Search Terms: array union

Code

declare const x: 'a'[] | 'b'[];
x.forEach(v => { }); // FAIL

// it doesn't work as is, so we have to resort
// to enforcing the shape of the type
const shape = <T>(t: T[]): T[] => t;
const x1 = shape(x); // converts to ('a' | 'b')[]
x1.forEach(v => { }); // OK

declare const y: 'a'[] | 'b'[] | never[]
y.forEach(v => { }); // FAIL
const y1 = shape(y); // cannot convert anymore
y1.forEach(v => { }); // FAIL

declare const z: 'a'[] | never[]
z.forEach(v => { }); // OK. apparently 3 cases was too much

Expected behavior:
No error messages in either case.

Actual behavior:
Conversion of union of arrays to array of unions happens only if it's enforced manually, and only if there is no never[] cases.

Playground Link:
Link

[RyanCavanaugh]

Many duplicates of this - start at #34605 and explore from there

[reverofevil]

I thought the issue with call signatures was only related.

Closing as duplicate of #7294.

[reverofevil]

For other suffering people: it works in both cases if you use another shaping function. So at least there is some way to make code work without any.

const thiccShape = <T extends any[]>(t: T): T[number][] => t;