如何通过条件类型检查区分不同的函数签名？

问：

我想在条件类型检查中区分以下函数类型：

type SyncFn = () => void;
type AsyncFn = (data: number) => Promise<void>;
type SyncFnWithArg = (data: number) => void;

因此，我可以使用 @Titian Cernicova-Dragomir 发布的 KeyOfType，并在给定接口中获取与给定类型匹配的密钥。

我尝试了以下方法：

type SyncFn = () => void;
type AsyncFn = (data: number) => Promise<void>;
type SyncFnWithArg = (data: number) => void;

interface Foo {
    a?: string;
    b?: number;
    c: number;
    d: string;
    f1?: SyncFn;
    f2?: AsyncFn;
    f3?: SyncFnWithArg;
}

// note: `KeyOfType` from https://stackoverflow.com/questions/49752151/typescript-keyof-returning-specific-type
type KeyOfType<T, V> = keyof { [P in keyof T as T[P] extends V? P: never]: any }

type KeyOfTypeOptionalIncluded<T, Condition> = KeyOfType<T, Condition | undefined>


let onlyStrings: KeyOfTypeOptionalIncluded<Foo, string>;
onlyStrings = 'a' // ✅ working as expected 🎉
onlyStrings = 'b' // ✅ erroring out as expected 🎉
onlyStrings = 'd' // ✅ working as expected 🎉


let onlySyncFn: KeyOfTypeOptionalIncluded<Foo, SyncFn>;
onlySyncFn = 'f1' // ✅ working as expected 🎉
onlySyncFn = 'f2' // ✅ erroring out as expected 🎉
onlySyncFn = 'f3' // ✅ erroring out as expected 🎉

let onlyAsyncFn: KeyOfTypeOptionalIncluded<Foo, AsyncFn>;
onlyAsyncFn = 'f1' // ✅ erroring out as expected 🎉
onlyAsyncFn = 'f2' // ✅ working as expected 🎉
onlyAsyncFn = 'f3' // ✅ erroring out as expected 🎉

let onlySyncFnWithArg: KeyOfTypeOptionalIncluded<Foo, SyncFnWithArg>;
onlySyncFnWithArg = 'f1' // 😭 should error out 😭
onlySyncFnWithArg = 'f2' // 😭 should error out 😭
onlySyncFnWithArg = 'f3' // ✅ working as expected 🎉

TS游乐场

问题是它被输入为，而它应该是......onlySyncFnWithArg"f1" | "f2" | "f3""f3"

我还注意到，如果我修改并删除它的参数，那么我会遇到更多问题，因为 的类型定义现在是不正确的，因为现在它不仅仅是在上面的第一个 TS Playground 中。AsyncFnonlySyncFn"f1" | "f2""f1"

第二个 TS 游乐场

我想这与打字稿中的函数重载是如何完成的有关，但我真的不知道，所以这就是我寻求帮助的原因......也许它不相关，但是我们能够在 TS 中进行这样的函数类型区分吗？