我遇到了以下问题：

type NonUndefined<T> = T extends undefined ? never : T;

enum TestEnum {
    T1 = "T1",
    T2 = "T2",
}

interface Data {
    Val: TestEnum;
}

type AltFormData<T> = T extends object
    ? {
          readonly [P in keyof T]-?: AltFormData<NonUndefined<T[P]>>;
      } & AltFormValue<NonUndefined<T>>
    : AltFormValue<NonUndefined<T>>;

type Alt1FormData<T> = AltFormValue<NonUndefined<T>>;

type Alt2FormData<T> = T extends object ? AltFormValue<NonUndefined<T>> : AltFormValue<NonUndefined<T>>;

type AltFormValue<T = any> = {
    readonly $setValue: (value: T | undefined) => void;
};

const x2: AltFormData<Data> = {};
x2.Val.$setValue(TestEnum.T1); // ERROR - x2.Val is of type AltFormValue<TestEnum.T1> | AltFormValue<TestEnum.T2> and should be of type AltFormValue<TestEnum>

const x3: AltFormValue<TestEnum> = {};
x3.$setValue(TestEnum.T1); // GOOD

const x4: AltFormData<TestEnum> = {};
x4.$setValue(TestEnum.T1); // ERROR - x4 is of type AltFormValue<TestEnum.T1> | AltFormValue<TestEnum.T2> and should be of type AltFormValue<TestEnum>

const x5: AltFormValue<NonUndefined<TestEnum>> = {};
x5.$setValue(TestEnum.T1); // GOOD

const x6: Alt1FormData<TestEnum> = {};
x6.$setValue(TestEnum.T1); // GOOD

const x7: Alt2FormData<TestEnum> = {};
x7.$setValue(TestEnum.T1); // ERROR - x7 is of type AltFormValue<TestEnum.T1> | AltFormValue<TestEnum.T2> and should be of type AltFormValue<TestEnum>

似乎类型检查T extends object改变了结果。同样的事情发生在使用类型"T1" | "T2"而不是枚举时。您可以将此代码复制并粘贴到TS Playground中，或者只需单击链接。