Function parameters are not inferable when defined via JSDoc using @type tag (with strict)

🔎 Search Terms

infer @type strict JSDoc Parameters Function

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type inference.

I've pulled the latest version of this repository and made a test case in the "fourslash" section, which demonstrates the problem I see. (I couldn't see how to do something like this in the playground, sorry. Commit is linked)

⏯ Playground Link

https://github.com/scottmcginness/TypeScript/commit/cda5366448cb994866517834e0c1ca78b10042cb

💻 Code

// In file func.js
export function func(/** @type {string} */ param) {};

// In file use-it.js
import { func } from "./func.js";
type FuncParam = (typeof func) extends (...args: infer P) => any ? P : never;
//   ^ never, but expected [param: string]

In tsconfig.json:

{
  "compilerOptions": {
    "allowJs": true,
    "checkJs": true,
    "noEmit": true,
    "strict": true
  }
}

(The above definition is obviously just Parameters<T>, but put in full for comparison with another example below)

🙁 Actual behavior

The type FuncParam resolved to never (i.e. it took the false branch of the ternary)

🙂 Expected behavior

The type FuncParam should be [param: string], as given by the JSDoc @type tag

Additional information about the issue

This seems like a bug because all other ways of specifying the func function with JSDoc or the FuncParam type seemed to work as expected:

• Using the @param tag instead:

/**
 * @param {string} param
 */
export function func(param) {};

yields FuncParam → [param: string]

• Using a union with undefined works, but is not what I want for the definition of func:

export function func(/** @type {string | undefined} */ param) {};

yields FuncParam → [param?: string | undefined]

• Trying the conditional without infer works (but is not usable for the intended purpose):

import { func } from "./func.js";
type FuncIsAFunc = (typeof func) extends (...args: any) => any ? 'good' : never;
//   ^ 'good'

• Pulling just the first parameter out also works (but obviously that's not how Parameters<T> works):

import { func } from "./func.js";
type FuncParam = (typeof func) extends (arg: infer A) => any ? A : never;
//   ^ string

---

This also only seemed to occur specifically with strict: true. I couldn't see this with any of the other strict... options (though I may have missed something here)

---

All other views on the function seems to show that it is happily a function with a string parameter. i.e. the tooltip hover over func, while inside use-it.ts shows:

(alias) function func(param: string): void
import func

---

The output I see from the single test linked above (using hereby runtests --tests=jsDocInferredFunctionParameters) is:

  1) fourslash tests
       tests/cases/fourslash/jsDocInferredFunctionParameters.ts
         fourslash test jsDocInferredFunctionParameters.ts runs correctly:

      AssertionError: At marker '': quick info text: expected 'type FuncParam = never' to equal 'type FuncParam = [param: string]'
      + expected - actual

      -type FuncParam = never
      +type FuncParam = [param: string]

      at _TestState.verifyQuickInfoString (src\harness\fourslashImpl.ts:1863:16)
      at Verify.quickInfoIs (src\harness\fourslashInterfaceImpl.ts:268:20)
      at eval (jsDocInferredFunctionParameters.js:13:8)
      at runCode (src\harness\fourslashImpl.ts:4618:9)
      at runFourSlashTestContent (src\harness\fourslashImpl.ts:4576:5)
      at runFourSlashTest (src\harness\fourslashImpl.ts:4559:5)
      at Context.<anonymous> (src\testRunner\fourslashRunner.ts:59:39)
      at processImmediate (node:internal/timers:476:21)

---

This also happens for arrow functions and class methods, e.g.

// In func.js
export const func = (/** @type {string} */ param) => {};
export class Cls{
  method(/** @type {string} */ param) {}
}

with similar code as for FuncParam.