JSDoc rest arguments can no longer be extracted with Parameters<T>

[Gerrit0]

Bug Report

According to the docs, {...string} can be used to define a rest parameter having type string[]. This works for the implementation, but does not construct a type that can be extracted using standard utility types.

🔎 Search Terms

JSDoc variadic arguments, var args, rest arguments

🕗 Version & Regression Information

This was discovered while upgrading TypeScript from 4.1.5 to 4.2.2 in TypeDoc. There is a test case for variadic parameters which changed in an unexpected way.

Please keep and fill in the line that best applies:

• This changed between versions 4.1.5 and 4.2.2

⏯ Playground Link

Bug Workbench links:

• Good - 4.2.0-dev.20201211
• Bad - 4.2.0-dev.20201219

💻 Code

// @filename: a.js

/**
 * @callback Foo
 * @param {...string} args
 * @returns {number}
 */

/** @type {Foo} */
export const x = () => 1

// @filename: b.ts
export {}

type Params = Parameters<typeof import("./a")['x']>
//   ^?

🙁 Actual behavior

TS 4.2 resolves Params = never

type Params = Parameters<typeof import("./a")['x']>
//   ^? - type Params = never

🙂 Expected behavior

TS 4.1 resolves Params = string[]

type Params = Parameters<typeof import("./a")['x']>
//   ^? - type Params = string[]

[sandersn]

The problem is with the interaction of @callback and @type; you can't even call x inside a.js like you'd expect: x('a', 'b'). Here's a smaller repro:

/**
 * @callback Foo
 * @param {...string} args
 * @returns {number}
 */

/** @type {Foo} */
export const x = () => 1
var res = x('a', 'b')

Note that changing x's definition to (...args) => 1 doesn't help.

Edit: however, switching to a TS annotation (which works fine in JS files) works.
Another interesting note: quickinfo gives the correct type for args when hovering x, but not 'a'.