Tuple with element after spread not assignable to tuple with rest elements

TypeScript Version: 3.5.1, 3.6.3, Nightly (2019-09-24)

Search Terms:

Tuple, spread, assignability

Code

declare const src : [number, ...number[]];

const [head, ...tail] = src;

//Expected: OK
//Actual  : OK
const dst0 : [number, ...number[]] = [head, ...tail];

//Expected: OK
//Actual  : Error
//Property '0' is missing in type 'number[]' but required in type '[number, ...number[]]'.
const dst1 : [number, ...number[]] = [head, ...tail, 1337];

Expected behavior:

Assignment to dst1 should succeed.

It should succeed because head is number and ...tail, 1337 should correspond to ...number[]

Actual behavior:

Assignment to dst1 fails.

Playground Link:

3.5.1 Playground

Related Issues:

Possibly this,
#29248

It mentions spread arguments, and an extra argument at the end causes TS to not realize there are the right number of arguments.
However, this repro just has tuples not being assignable to other tuples.

This is my current workaround,

const src : readonly [number, ...number[]] = [1,2,3,4,5] as const;

interface ReadonlyArray<T> {
    concat (
        this : readonly [T, ...T[]],
        ...items: (T | ConcatArray<T>)[]
    ) : ([T, ...T[]])
}

//Expected: OK
//Actual  : OK
const dst2 : [number, ...number[]] = src.concat(1337);

console.log(src);
console.log(dst2);

3.5.1 Playground

If anyone is using this for a tuple-of-arrays, you'll need,

const src : readonly [[number, number], ...[number, number][]] = [[1,2], [3,4]];

interface ReadonlyArray<T> {
    concat (
        this : readonly [T, ...T[]],
        ...items: (T | ConcatArray<T>)[]
    ) : ([T, ...T[]])
}
interface Array<T> {
    concat (
        this : readonly [T, ...T[]],
        ...items: (T | ConcatArray<T>)[]
    ) : ([T, ...T[]])
}

//Expected: OK
//Actual  : OK
const dst2 : readonly [[number, number], ...[number, number][]] = src.concat([5,6]);

console.log(src);
console.log(dst2);

3.5.1 Playground

Should the above overloads for concat be added to the lib.d.ts files?

Ryan Cavanaugh (@RyanCavanaugh)

Ryan Cavanaugh (@RyanCavanaugh) this works OK today, the issue can be closed

Just looking at old issues; this was fixed in #39094.

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Tuple with element after spread not assignable to tuple with rest elements #33573

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Tuple with element after spread not assignable to tuple with rest elements #33573

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AnyhowStep commented on Sep 24, 2019

Andarist commented on Feb 18, 2023

jakebailey commented on Jul 9, 2023

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