Useless compile error on triple-equal 1|2 against 0

[rogierschouten]

TypeScript Version: 2.7.1

Search Terms: infer, compare, equals

Code

type T = [number] | [number, number];  // now t.length has type 1|2
let t: T;
if (t!.length === 0) { // compile error: Operator '===' cannot be applied to types '1' and '0'
}
if (t!.length > 100) { // no compile error
}
if (t!.length === 1 - 1) { // no compile error
}

Expected behavior:
No compile errors on any of these. It's plain inconsistent and not useful.

Actual behavior:
Compile error on t === 0 but not on other comparisons

Playground Link:
See above

Related Issues:

[ghost]

Tuples have a fixed length as of #17765. If you suspect that t could be a "tuple" of arbitrary length, you could simply use the type number[], since tuples are arrays.
It's true that we could give an error on t.length > 100 because all known lengths are less, but not sure if that would be worth the complexity to add. Same for evaluating constant expressions like 1 - 1; any arithmetic operation on numbers is just treated as a number for simplicity.

[krryan]

For what it's worth, I would love to see arithmetic constraints on things like lengths. But such an error only really becomes useful when users can write such constraints (I envision writing a function like f(valueThatIsNotEmpty: string & { length: number > 0; }) or something).

[typescript-bot]

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