...args should not be readonly within function body

[jakebailey]

Bug Report

🔎 Search Terms

readonly parameter inferred tuple array args variadic

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

Playground Link

💻 Code

declare function callFn<T extends readonly any[]>(args: T, fn: (...args: T) => void): void;

declare const input: readonly string[];

callFn(input, (...args) => {
 args; // readonly string[] ?
})

declare function callFnNonGeneric(args: readonly string[], fn: (...args: readonly string[]) => void): void;

callFnNonGeneric(input, (...args) => {
 args; // readonly string[] ?
})

🙁 Actual behavior

...args is readonly.

🙂 Expected behavior

...args is not readonly.

When this function is called, the input will be a brand new array which can be modified without affecting the caller:

> const fn = (...args) => { args[0] = "oops" }
undefined
> const arr = ["some", "values"]
undefined
> fn(...arr)
undefined
> arr
[ 'some', 'values' ]

Split out of #53258 (comment)

The "fix" here is to strip readonly from variadic args (at the top level). This also means the fix in #53258 can be reverted (as the real fix can be more general).

[fatcerberus]

Hmm, but args is typed as T in this example, which must necessarily be instantiated with a readonly array type, and args is required to be the same type as input… so how would this work?

[jakebailey]

Like, how would it be implemented? Or something else?

[fatcerberus]

in short: how could args not be readonly if input is, since both are typed as T? It seems like you’d need to manufacture a type parameter here.

[jakebailey]

Well, the idea I (rather Wesley Wigham (@weswigham)) had was to copy Mutable<T> into lib.d.ts and say that within the function, the type of the ...args parameter is always observed as Mutable<T>, dropping readonlyness.

So, the function is typed the same.

[jakebailey]

Sorta kinda the same as what I did for spread types in assignments, which implicitly drops certain properties (#47078).

[RyanCavanaugh]

Jake Bailey (@jakebailey) feel free to self-assign if you want to