Predicate `value is object` doesn't work in Array.prototype.filter()

从链接的 TypeScript Playground 重现开始，比较 Array.prototype.filter() 对 isNumber 和 isObject 应用类型谓词的方式。跟踪 filter 谓词的类型检查路径，然后添加覆盖测试，证明过滤混合数组会产生 { a: number }[]，并运行相关的编译器测试。

Bug Report

🔎 Search Terms

type narrowing, type predicate, value is object, filter(), lodash.isObject

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about type guards.

⏯ Playground Link

Playground link with relevant code

💻 Code

// isNumber and isObject are modeled after the functions by the same names in lodash.

function isNumber(value?: any): value is number {
  return typeof value === "number";
}

function isObject(value?: any): value is object {
  return typeof value === "object";
}

// Each element could be an object or a number, so we can't treat it as either.
const variousThings = [{ a: 1 }, 1, "abc"];
variousThings[0] + 1;
variousThings[0].a;

// Filtering for only numbers yields values we can add, but not get properties of.
const allNumbers = variousThings.filter(isNumber);
allNumbers[0] + 1;
allNumbers[0].a;

// But filtering for only object yields fails to remove number from the union type.
const allObjects = variousThings.filter(isObject);
allObjects[0] + 1;
allObjects[0].a;

// Yet isObject is able to narrow a single element in a simple conditional.
const value = variousThings[0];
if (isObject(value)) {
    value.a
}

🙁 Actual behavior

allObjects was typed as (string | number | { a: number })[], so both lines below const allObjects fail.

🙂 Expected behavior

allObjects should be typed as { a: number }[], so allObjects[0].a should pass the type checker. Notably, as seen above, isObject does manage to narrow the type of a single value with the union type to the only member of the union that's an object, it just can't narrow all of the elements using .filter() the way isNumber is able to.

I can't conceive of why they behave differently, but I also don't know the internals well. I think this difference is undesired, but I might be missing something deeper.