Array spreads should be contextually typed

[mournie]

TypeScript Version: 3.8

Search Terms:

• array literal type
• array spread type inference
• implicit array literal type
• array destructuring type inference [sic]

Response to Original Issue Post

We have "special" logic for unifying object literals in these cases; the spread syntax interferes with that.

If you post a more representative sample where it's not just spreading for the sake of spreading, we may be able to advise more.

Originally posted by @RyanCavanaugh in #30761 (comment)

Real Use Case

The common pattern of literal array assignment with conditional inclusion of elements is one use case.

Code

type t = 'a'

// The common pattern
const a:t[] = [
  ...(true?['a']:[]),
]

// Minimal repro of phenomenon
const a:t[] = [
  ...['a']
]

The error messages given above are both Type 'string[]' is not assignable to type '"a"[]'.

Of course if the type of the nested array is asserted or otherwise known, the spread doesn't get in the way of that - it only seems to interrupt the automatic inference of the literal. We get as specific as string, but not as specific as the actual value.

[DanielRosenwasser]

I think that issue is not quite related.

This seems pretty reasonable, though in the future please follow the template.

const a: ("foo" | "bar")[] = [
    ...(Math.random() < 0.5 ? ["foo"] : []),
];

Playground Link

[mournie]

This seems pretty reasonable, though in the future please follow the template.

Yeah sorry - I used the "reference in new issue" button and it pre-filled the above quote instead of the standard template, so I just went with that.

[mournie]

I think that issue is not quite related.

I guess object spread types are inferred in a way analogous to the way in which the array example isn't, so here's another example.

type a = {one: 1}
type b = 'b'[]

const a:a = {...{one:1}} // work
const b:b = [...['b']] // not work

[rentalhost]

Any updates on this case?

type Case = [string?]

const caseTrue: Case = [...(true ? ["abc" ] : [])];
const caseFalse: Case = [...(false ? ["abc"] : [])];