打字稿`可以用不同的约束子类型实例化`错误

Question

当我返回的结果应该符合函数的预期类型时，我不断收到could be instantiated with a different subtype of constraint 错误，我无法理解我做错了什么。这是我的代码，删除了所有不相关的细节：

type User = { }

interface BasePageProps {
    user: User
}

type GetServerSidePropsResult<P> =
  | { props: P }

type PropsWithUser = <TProps extends BasePageProps>(
    callback: (user: User) => Promise<Omit<TProps, 'user'>>
) => Promise<GetServerSidePropsResult<TProps>>

export const testFunc: PropsWithUser = async (callback) => {
    const user = {}
    return { props: {
            user,
            ...(await callback(user))
        } }
}

这是我得到的错误：

'{ 用户：{}; } & Omit' 可分配给类型为“TProps”的约束，但可以使用约束“BasePageProps”的不同子类型来实例化“TProps”。

Here's this example on TS Playground.

为什么用不同子类型实例化的潜在TProps 是个问题？我该如何解决？

P.S.：您可能会注意到这些类型取自 Next.js。不过这个问题跟 Next.js 本身没有关系，所以请不要给这个问题打标签。

Answer 1

你的函数的有效签名是：

type PropsWithUser = <TProps extends BasePageProps>(
    callback: (user: User) => Promise<Omit<TProps, 'user'>>
) => Promise<GetServerSidePropsResult<BasePageProps & Omit<TProps, 'user'>>>

或者你可以删除一些重复：

type PropsWithUser = <TProps extends BasePageProps, TO = Omit<TProps, 'user'>(
    callback: (user: User) => Promise<TO>
) => Promise<GetServerSidePropsResult<BasePageProps & TO>>

证明编译器正确引发代码错误的反例。让我们定义一个扩展 BasePageProps 但不通过函数结果类型检查的 TProps 类型的具体示例：

type User = { }

type AnotherUser = { a: string }

interface TProps {
    user: AnotherUser
}

interface BasePageProps {
    user: User
}

type Eq = TProps extends BasePageProps ? true : false // type Eq = true

type PropsWithUser = <TProps extends BasePageProps>(
    callback: (user: User) => Promise<Omit<TProps, 'user'>>
) => Promise<GetServerSidePropsResult<TProps>>

TS playground

所以你的函数应该返回类型为Promise<{ props: { user: AnotherUser, ... } }> 的结果。它显然没有。

正如错误所说：'{ user: {}; } & Omit<TProps, "user">' is assignable to the constraint of type 'TProps', but 'TProps' could be instantiated with a different subtype of constraint 'BasePageProps'

type User = { }

interface BasePageProps {
    user: User
}

type AnotherUser = { a: string }

interface ConcreteTProps {
    user: AnotherUser
}

// { user: {}; } & Omit<TProps, "user"> is assignable
// to the constraint of type TProps (i.e. BasePageProps)
const a: BasePageProps = { user: {} }

// but 'TProps' could be instantiated with a different 
// subtype of constraint 'BasePageProps' (i.e. ConcreteTProps)
// Which is a subtype of BasePageProps by cannot be 
// assigned with { user: User }
const b: ConcreteTProps = { user: {} } // error