Typescript: Wrong type inferred by returning a tuple

[MastroLindus]

TypeScript Version: 3.0.0-dev.20180707

Code

  const a: Array<[string, boolean]> = ["foo", "baz"].map(item => [item, !!item]);
  const b: Array<[string, boolean]> = ["foo", "baz"].map(item => { const value: [string, boolean] = [item, !!item]; return value;} );

Expected behavior:
Both expressions should be allowed
Actual behavior:
assignment for b works, assignment for a doesn't, as the compiler starts infering that the type for [item, !!item] is Array.

[RyanCavanaugh]

It's not wrong, it's just less precise than it should be.

I think this is fixable by improving the contextual type -> return type expression inference?

[jscheiny]

Another example where a tuple inference would be desirable is in f0 below:

declare function f0<T extends any[]>(args: T): T;
declare function f1<T extends any[]>(...args: T): T;

f0(["a", 1]); // (string | number)[]
f1("a", 1); // [string, number]

This is currently blocking a typesafe implementation of Promise.all without overloads.

[wjohnsto]

👍

Yes, the update to tuples to allow for f1 in @jscheiny's comment is great! Unfortunately for some of the Promise functions it doesn't work since the argument is actually an array argument, not rest arguments.

[rrousselGit]

This is not only related to functions. Variables are affected too:

    let test1: [number, string];
    const a = [42, ''];

    test1 = a; // error

Type '(string | number)[]' is not assignable to type '[number, string]'.
Property '0' is missing in type '(string | number)[]'.

[RyanCavanaugh]

@rrousselGit that is the intended behavior, because this code is legal:

    let test1: [number, string];
    const a = [42, ''];
    a[0] = "hello"; // Does not affect type system

    test1 = a; // Unsound to allow

[CorayThan]

This issue prevents normal creation of ES6 maps. This valid JS:

var kvArray = [['key1', 'value1'], ['key2', 'value2']];
var myMap = new Map(kvArray);

Doesn't compile due to Argument of type 'string[][]' is not assignable to parameter of type 'ReadonlyArray<[any, any]>'..

But if you just cast it to the right type it works fine:

const kvArray = [['key1', 'value1'], ['key2', 'value2']]
const myMapFromAny = new Map(kvArray as Array<[string, string]>)

At least I think this is the same issue. Didn't want to make a duplicate issue.

[MattiasMartens]

@rrousselGit By default, array literal declarations are given an array type, not a tuple type. There is a proposal to change this, and if you have a strong opinion to share I'd recommend chiming in because it is still in the feedback stage: #16896

[austonpramodh]

+1
for different return types for different promises in promise.all