...args should not be readonly within function body

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 https://github.com/microsoft/TypeScript/pull/53258#issuecomment-1470970487

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).

Start with the linked Playground example to reproduce how readonly variadic arguments are inferred, then read the discussion in PR #53258 about the related fix. Confirm that function-body rest parameters are mutable while preserving the caller's readonly array, and verify the expected behavior against the examples in this issue.