Parameters created from tuple are treated as mutable instead of immutable, unless written as "...args"

[jakebailey]

Bug Report

🔎 Search Terms

const array tuple parameter spread

🕗 Version & Regression Information

• This is the behavior in every version I tried,.

⏯ Playground Link

Playground Link

💻 Code

declare function each<T extends ReadonlyArray<any>>(cases: ReadonlyArray<T>): (fn: (...args: T) => any) => void;

const cases = [
    [1, '1'],
    [2, '2'],
] as const;

const eacher = each(cases);

// Error.
eacher((a, b) => {
});

// No error?
eacher((...args) => {
    const [a, b] = args;
});

🙁 Actual behavior

Argument of type '(a: 1 | 2, b: "1" | "2") => void' is not assignable to parameter of type '(...args: readonly [1, "1"] | readonly [2, "2"]) => any'.
  Types of parameters 'a' and 'args' are incompatible.
    Type 'readonly [1, "1"] | readonly [2, "2"]' is not assignable to type '[a: 1 | 2, b: "1" | "2"]'.
      The type 'readonly [1, "1"]' is 'readonly' and cannot be assigned to the mutable type '[a: 1 | 2, b: "1" | "2"]'.(2345)

Obviously they're not assignable, but why did we come up with [a: 1 | 2, b: "1" | "2"] and not readonly [a: 1 | 2, b: "1" | "2"]?

🙂 Expected behavior

No error, as though it had been expanded separately.

See also: DefinitelyTyped/DefinitelyTyped#64739

[jakebailey]

Followup is here: #53398

[jakebailey]

Fix has been reverted.