Infer a tuple type instead of array type when possible

Suggestion

I’d prefer typescript to be more aggressive about inferring tuple types, instead of always defaulting to Array.

This is related to https://github.com/microsoft/TypeScript/issues/6574. I’d like to open up a discussion about this again since that thread is locked, and conversation stopped after a workaround was provided without completely addressing the general issue.

🔍 Search Terms

Typescript, tuple, tuples, map, inference

✅ Viability Checklist

My suggestion meets these guidelines:

• [*] This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• [*] This wouldn’t change the runtime behavior of existing JavaScript code
• [*] This could be implemented without emitting different JS based on the types of the expressions
• [*] This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [*] This feature would agree with the rest of TypeScript’s Design Goals.

Existing typescript code could start emitting type errors from this change, so it would make sense to provide this as an option in tsconfig.

⭐ Suggestion

Typescript should infer a type to be a tuple instead of an array when it’s possible.

📃 Motivating Example

The specific example I have in mind is for Array.prototype.map(), but there are probably more.

const tuple: [number, number, number] = [1, 2, 3];
const result = tuple.map(value => value + 1);

Here, typescript infers the type of result to be number[] rather than [number, number, number]. This could result in some wonkiness when trying to access values inside the tuple:

const [a, b, c, d] = result; // Typescript doesn't detect an error since result is an Array type.
const e = result[10]; // Typescript doesn't detect an error since result is an Array type.

Both of these errors can be caught if Typescript instead inferred result to be the tuple type [number, number, number].

💻 Use Cases

The above example is a use case. I want typescript to tell me when I make errors in my code.

[RyanCavanaugh]

Related #32758

[echentw]

@whzx5byb seems like there may have been a regression.

const myTuple: [number, number] = [1, 2];

// result is inferred to be number[]
const result = myTuple.map(v => v + 1);

Playground link