Correctly handle generic functions (e.g.: `Object.freeze`) passed as callbacks to generic functions (e.g.: `Array.prototype.map`)

[ExE-Boss]

Suggestion

🔍 Search Terms

• array map generic callback
• object.freeze array map

✅ Viability Checklist

My suggestion meets these guidelines:

• [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [x] This wouldn't change the runtime behavior of existing JavaScript code
• [x] This could be implemented without emitting different JS based on the types of the expressions
• [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [x] This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A generic transforming function (e.g.: Object.freeze) passed as a callback to a generic mapping function (such as Array.prototype.map), should be handled similarly to an arrow function with inferred types.

📃 Motivating Example

From engine262/src/engine.mjs:

// @target: ES2015
// @filename: index.ts
export const FEATURES = Object.freeze([
//                 ^?
    {
        name: 'Top-Level Await',
        flag: 'top-level-await',
        url: 'https://github.com/tc39/proposal-top-level-await',
    },
    {
        name: 'Hashbang Grammar',
        flag: 'hashbang',
        url: 'https://github.com/tc39/proposal-hashbang',
    },
    {
        name: 'RegExp Match Indices',
        flag: 'regexp-match-indices',
        url: 'https://github.com/tc39/proposal-regexp-match-indices',
    },
    {
        name: 'FinalizationRegistry.prototype.cleanupSome',
        flag: 'cleanup-some',
        url: 'https://github.com/tc39/proposal-cleanup-some',
    },
    {
        name: 'Arbitrary Module Namespace Names',
        flag: 'arbitrary-module-namespace-names',
        url: 'https://github.com/tc39/ecma262/pull/2154',
    },
    {
        name: 'At Method',
        flag: 'at-method',
        url: 'https://github.com/tc39/proposal-item-method',
    },
].map(Object.freeze));

Workbench Repro

Expected type:

export const FEATURES: readonly {
    readonly name: string;
    readonly flag: string;
    readonly url: string;
}[];

Like when using an arrow function:

```ts repro // @target: ES2015 // @filename: index.ts export const ARROW_FEATURES = Object.freeze([ // ^? { name: 'Top-Level Await', flag: 'top-level-await', url: 'https://github.com/tc39/proposal-top-level-await', }, { name: 'Hashbang Grammar', flag: 'hashbang', url: 'https://github.com/tc39/proposal-hashbang', }, { name: 'RegExp Match Indices', flag: 'regexp-match-indices', url: 'https://github.com/tc39/proposal-regexp-match-indices', }, { name: 'FinalizationRegistry.prototype.cleanupSome', flag: 'cleanup-some', url: 'https://github.com/tc39/proposal-cleanup-some', }, { name: 'Arbitrary Module Namespace Names', flag: 'arbitrary-module-namespace-names', url: 'https://github.com/tc39/ecma262/pull/2154', }, { name: 'At Method', flag: 'at-method', url: 'https://github.com/tc39/proposal-item-method', }, ].map((feature) => Object.freeze(feature))); ``` [Workbench Repro](https://www.typescriptlang.org/dev/bug-workbench/?target=2&lib=#code/PTAEAEBcEMCcHMCmkBcoCiBlATABgIwCsAUCBAGYCWANogHbQC2ialdAJogB4B0kAzsW4AHAPaxIoAMai6-SQEEASkoDyAdQD6AMXQKAKgFUlWUAF5QqgEYArRFMg9ysRIgBeiABQBtUmFABgUHBIaAAegD8xACQAN4x0QzMaADk+qLCALQAMogAbojUoAoA7tCUkCkANAnk1NDwqZAZmbQF1JnQZRXVCQCusNSpABaQkML8KCDwFcN9VjwyjMCQUgDMAJzAwrAZovzQHc1ZbYWd3ZU10QC+V-HRiUwsoCkAEtD8w1bQdPCgAOKwJiMOC9B51BojD5fH7wMHRAZDF6jcaTaazeaLUTLVabba7MQHDrDaHfX5g24xe6PZIvJSIeDoLjCUAAWWgq2GoAAkhxKFJEPx4RDGi8XEhmZkQZzMmx2PzBfDESMxhMpsAZpA5gslit1lsdnsiZlxSIpRypMNZXyBUKrpS4gkks8Uto2IdKG4OZRZPSZvJYABPHiG5qQQPCRCLWg-PrCTDYxDC+qilJSGN0OOZfiJpWDFWo9Wa7VYnH6-FGw6ZdOIWNZHPMCl3J1PVIKWBWCpAoNs0TsPq0UAAOSe-GE0AFw9HychLzgncg3cDUr7A8QmWdY4n683eaRKRRavRWsxutxW3sIOwADZsNsB9RgNgiAAWJtUlu0lIKSSs5DDPsZ1TDkpX-QCrgRfNkVVNENQxHVsT1PFDUJKsKkQRhQK1cCYkpABdHgQWETxPHIWtIAGRAAEpzAAPksWx7EcZxXA8UjyMoqiuIAbmIIA)

Actual type:

export const FEATURES: readonly Readonly<unknown>[];

💻 Use Cases

Getting correct type inference for JavaScript code on the wild web.

Related issues

• https://github.com/microsoft/TypeScript/issues/10725

[jsmart523]

Another example:

type Widget = {name:string};

const widgets: Widget[] = [
  {name: 'gear'},
  {name: 'bearing'},
  {name: 'axle'}
];

const workingReadonlyWidgets = Readonly<Widget>[] = widgets.map(
  w => Object.freeze(w)
);

const brokenReadonlyWidgets = Readonly<Widget>[] = widgets.map(
  Object.freeze
); // this line complains

[typescript-bot]

:wave: Hi, I'm the Repro bot. I can help narrow down and track compiler bugs across releases! This comment reflects the current state of the repro in the issue body running against the nightly TypeScript.

---

Issue body code block by @ExE-Boss

:warning: Assertions:

• const FEATURES: readonly Readonly[]

Historical Information

Version	Reproduction Outputs
4.2.2, 4.3.2, 4.4.2, 4.5.2, 4.6.2	:warning: Assertions: const FEATURES: readonly Readonly[]