microsoft / microsoft/TypeScript
Way to detect whether a callable has type parameters
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描述
### 🔍 Search Terms
conditional type detect callable type parameter
### ✅ 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
I'm looking for some way to detect if a callable type is generic and has type parameters.
LLMs are hallucinating an answer that would be nice if it worked:
```js
type HasTypeParameter any> =
T extends (...args: any[]) => any ? true : false;
```
### 📃 Motivating Example
I have a utility type that wants to map the return value of a callable. However because as far as I know it's not possible to map a type-parametrized callable keeping its parametrization, we prefer to bail out in this case and maintain the original type.
Roughly it looks like
```js
type MappedCallable = HasTypeParameter extends true
? T
: (...args: Parameters) => MappedResult>;
```
### 💻 Use Cases
I can achieve this detection in some case where the type of the callable is incompatible with a mapped callable were its type parameters inferred as `any`.
```js
type LimitedHasTypeArgument any> =
((...args: Parameters) => ReturnType) extends T ? false : true
```
However this doesn't work in some more complex cases (still trying to get a minimal repro).
I cannot think of other workarounds, which is forcing us to forego the result type mapping completely.
Of course having a way to keep the callable type-parametrization in the result mapping would be even better.
The specific use case is to map all callable in the result, including object methods, to callables returning a promise, while maintaining other parts of their signature.
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调研方向
该 issue 未指定源文件、测试或实现入口点,并说明最小复现仍在开发中。首先将泛型可调用示例缩减为编译器测试;完成的标准是具备可靠的方法来检测或保留可调用类型参数,或有文档记录的解决方案。
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