是否有用户定义的类型保护来要求数组中的元素？

Question

我正在寻找一个用户定义的类型保护，它将确定一个数组是否包含可为空的或可选的元素。我为RequiredArray写了一个辅助类型：

type RequiredArray<A extends readonly any[]> =
    A extends [infer P, ...infer R]
    ? [NonNullable<P>, ...RequiredArray<R>]
    : []

这似乎有效。但是，当我尝试为它编写类型保护时......

function arrIsFullyDefined<Arr extends readonly any[]>(arr: Arr): arr is RequiredArray<Arr> {
    return arr.every((e) => e !== null && e !== undefined);
}

Typescript 抱怨可能会实例化不同的类型：

A type predicate's type must be assignable to its parameter's type.
  Type 'RequiredArray<Arr>' is not assignable to type 'Arr'.
    'RequiredArray<Arr>' is assignable to the constraint of type 'Arr', but 'Arr' could be instantiated with a different subtype of constraint 'any[]'.
      Type '[] | [unknown]' is not assignable to type 'Arr'.
        '[] | [unknown]' is assignable to the constraint of type 'Arr', but 'Arr' could be instantiated with a different subtype of constraint 'any[]'.
          Type '[]' is not assignable to type 'Arr'.
            '[]' is assignable to the constraint of type 'Arr', but 'Arr' could be instantiated with a different subtype of constraint 'any[]'.
              Type '[]' is not assignable to type 'Arr'.
                '[]' is assignable to the constraint of type 'Arr', but 'Arr' could be instantiated with a different subtype of constraint 'any[]'.ts(2677)

起初我认为这是一个潜在的无限数组类型被分配给Arr 泛型的问题，但是在RequiredArray 类型中检查这个（使用来自typescript-tuple 的IsFinite）给出了相同的结果. 我的问题是：

1. 为什么会导致错误？
2. 如何为这个助手编写类型保护？

Answer 1

感谢 Linda 的解决方案：

type _RequiredArray<Arr extends readonly any[]> =
    Arr extends [infer P, ...infer R]
    ? [NonNullable<P>, ..._RequiredArray<R>]
    : [];

type RequiredArray<Arr extends readonly any[]> = Arr & _RequiredArray<Arr>;