...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).

リンクされている Playground の例から始めて、readonly の可変長引数がどのように推論されるかを再現し、その後、関連する修正について PR #53258 の議論を読んでください。関数本体の rest パラメーターが変更可能でありながら、呼び出し元の readonly 配列が保持されることを確認し、この issue の例に照らして期待される動作を検証してください。