microsoft / microsoft/TypeScript
Type instantiation for type level generic function
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Description
### 🔍 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
- [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 isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] 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
```ts
const provideBox = () => {
const box = (value: T) => ({value});
return box;
};
type BoxMaker = ReturnType
// type BoxMaker = (value: T) => { value: T; }
type Box1 = ReturnType>;
// Type 'BoxMaker' is not generic.(2315)
const box = undefined as unknown as BoxMaker;
type Box2 = ReturnType>;
//type Box2 = { value: T; }
```
Compiler Options
```json
{
"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](https://www.typescriptlang.org/play/?jsx=0#code/MYewdgzgLgBADgJxANwJYBMCmAhEAPGAXhgAoBKIgPhgG8AoGRmUSWAI3yJgB4AVSksgCGAGwCumAFwxeFQtRI1h4zAF8yAbgZMEmKGIRgYHPFtVa6UAJ5xMMXHgCyQgNaYEXAEp6DYXjcxua1sQADN4JDQsB0o6AHo4mGC7B2c3D2I+AWUJaVkqWhgcqRkNGFU6SwD7fABGLK8fQ39bblTXdyzKLQSZaoBydvT+mFQIGDAQWABzTDB3VGAAOhIAJgBmWoBWMkqWaGNOYjEwLFDUefQYIXGTl0mAdyMbmqcOhAtk19WG4m99ZoBIIBMKHPBdHpxL4OH78Lg0IqiXKlcqVIA)
### 💻 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)
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by reproducing the motivating example in the provided Playground link, then read the related issues and PR #47607 to understand existing instantiation-expression behavior and the noted ambiguities. Done means a type-level form can extract the generic return type without a dummy runtime value, producing the intended Box2 result while preserving existing behavior.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100