Incorrect type inference of tuple when underlying array is extended

[anfedorov]

TypeScript Version: 2.1.5

Code

let x: [string, number];

x = ["hello", 10];
x[3] = 'hi';

let y = x[2];
if (y == null) {
    console.log('yes ever');
    y;
}

$ node static/build/js/test.js 
yes ever

Expected behavior:

Before reading the docs, I would expect the type checker to find an error on line 4.

Given the feature set defined in the docs, I would expect the type at line 4 to be let y : undefined.

Actual behavior:

Type is incorrectly inferred as let y : never.

[aluanhaddad]

Using --strictNullChecks and typescript@2.1.5, the type of y is string | number, outside of the if statement, and never inside of it.
The inference of never is correct and expected because y is not not nullable, and therefore the block will never execute.
With respect to the inferred type of the array element, I believe it is covered by the proposal in #6229

[anfedorov]

But the block does execute and the code compiles without warnings or errors, even with --strictNullChecks:

code $ cat test.ts
let x: [string, number];

x = ["hello", 10];
x[3] = 'hi';

let y = x[2];
if (y == null) {
    console.log('yes ever');
    y;
}
code $ tsc --strictNullChecks test.ts 
code $ node test.js
yes ever

Thanks for the reference to the proposal — that is indeed a little closer to the behavior I would have expected.

[aluanhaddad]

@anfedorov Ah, I see your point now. I misread it as y === null which would not be taken. It seems there may be a bug here, but I am not sure.

[anfedorov]

Yup, y is undefined, and the proposal you linked might be the most elegant fix.

[mhegazy]

unfortunately there is no way to do this correctly in the general case, e.g.: for (var z = 0, z<10, z++) x[z];. you can also find more details in #13778 on why the index signature intentionally, does not include undefined.

[anfedorov]

copying tuple behavior from Python seems like one way to handle it. i.e. x[3] = 'hi' could raise an error. another option is for x[3] = 'hi' to mutate the type of x to [string, number, undefined, string], fixing the error inside the if block.

this seems to work fine in the playground: https://www.typescriptlang.org/play/index.html#src=let%20x%3A%20%5Bnumber%2C%20string%2C%20undefined%2C%20number%5D%3B%0D%0Ax%20%3D%20%5B1%2C%20'1'%2C%20undefined%2C%202%5D%3B%0D%0A%0D%0Ax%5B3%5D%20%3D%202%0D%0Ax%5B2%5D%20%3D%203%0D%0A

regardless, x[z] just like x[prompt()] could be string | number | undefined?

[mhegazy]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.