Readonly open-ended tuples have writable length property

[ASDFGerte]

Bug Report

In general, readonly tuples appear to not have a readonly length property.

type Test = Pick<[number], 'length'>;
//   ^?
// type Test = { length: 1; }
// no readonly modifier
declare const x: readonly [number?];
x.length = 0; // no error
declare const y: readonly [number, ...number[]];
y.length = 0; // no error
declare const z: readonly number[];
z.length = 0; // error, as expected

🔎 Search Terms

readonly tuple length property

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about common "Bugs" that aren't bugs

⏯ Playground Link

Playground link with relevant code

🙁 Actual behavior

A readonly tuple's length property is not readonly, and can as such we overwritten.

🙂 Expected behavior

A readonly tuple's length property should be readonly, similar to arrays, and disallow writes.

[jcalz]

(Linking to https://stackoverflow.com/questions/70501410/why-is-a-readonly-tuples-length-writable where this presumably came from)