blabla.tsx

interface MyCustomType<T> {
  prop: Promise<T>
}

interface MyBlaBlaType {
    prop: string
}

interface MyObj {
    [key: string]: (...args: any) => any
}

function fn<M extends MyObj>(bla: M): { 
  [N in keyof M]: ReturnType<M[N]> extends MyCustomType<infer T>
    ? (...args: Parameters<M[N]>) => Promise<T> // I would like to tell TS that this will return Promise<what ever MyCustomType has as a generic )
    : M[N]
} {
    // Implementation detail
    return 123 as any;
}

// usage:
var y = fn({ 
  x1: 
    (): MyCustomType<string> => ({ prop: Promise.resolve("asd") }),
  x2:
    (): MyCustomType<number> => ({ prop: Promise.resolve(123) }),
  x3: 
    (): MyBlaBlaType => ({ prop: "blabla" })
}); // I would want to enforce y to be: { x1: Promise<string>, x2: Promise<number>, x3: MyBlaBlaType }