Type instantiation for type level generic function

🔍 Search Terms

type instantiation
type level generic function
generic return type

Related: https://github.com/microsoft/TypeScript/issues/40542, https://github.com/microsoft/TypeScript/issues/61133 and maybe https://github.com/microsoft/TypeScript/issues/52035?

Edit: Just found https://github.com/microsoft/TypeScript/issues/55435 and https://github.com/microsoft/TypeScript/issues/40179

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

#47607 added support for instantiation expressions, but those do not apply at the type level. I understand it'd be ambiguous in some cases (https://github.com/microsoft/TypeScript/pull/47607#issuecomment-1058192563), but in some cases there really isn't a reasonable workaround.

📃 Motivating Example

I actually have a very similar case to the example provided in https://github.com/microsoft/TypeScript/issues/40542

const provideBox = () => {
    const box = <T>(value: T) => ({value});
    return box;
};

type BoxMaker = ReturnType<typeof provideBox>
// type BoxMaker = <T>(value: T) => { value: T; }

type Box1<T> = ReturnType<BoxMaker<T>>;
// Type 'BoxMaker' is not generic.(2315)

const box = undefined as unknown as BoxMaker;

type Box2<T> = ReturnType<typeof box<T>>;
//type Box2<T> = { value: T; }

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "target": "ES2017",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

Playground Link: Provided

💻 Use Cases

1. What do you want to use this for?

In our case we have a more complex provideBox that accept arguments. We also need to rely on type inference for the generic type of the box function and its return value;

3. What shortcomings exist with current approaches?

I cannot extract and export a generic type for a return value of a generic function if I don't have direct reference to the function.

5. What workarounds are you using in the meantime?

The only workaround I found is to use a dummy runtime value to apply the type, and use and instantiation expression on that dummy value (see Box2 above)

Comienza reproduciendo el ejemplo motivador del enlace de Playground proporcionado; después, lee las issues relacionadas y PR #47607 para comprender el comportamiento existente de las expresiones de instanciación y las ambigüedades señaladas. Se considera terminado cuando una forma a nivel de tipos pueda extraer el tipo de retorno genérico sin un valor ficticio en tiempo de ejecución, produciendo el resultado Box2 previsto y preservando el comportamiento existente.