Why can't an interface be assigned to Record<string, unknown>?

Question

I just noticed that an interface cannot be assigned to Record<string, unknown> (playground link):

interface Foo {
    foo: number
}

const foo: Foo = { foo: 1 }
const bar: Record<string, unknown> = foo
//    |-> Error: Type 'Foo' is not assignable to type 'Record<string, unknown>'
//               Index signature is missing in type 'Foo'.(2322)

However the same is possible, when the type declaration for Foo is omitted (playground link):

const foo = { foo: 1 }
const bar: Record<string, unknown> = foo // no error here

Question: Why is there a difference between both examples? For me the reduced type of the variable foo is the same in both examples... Shouldn't the interface Foo be assignable to Record<string, unknwon>?!

In my understanding Record<string, unknown> is equivalent to object and thus any interface should be assignable to it. Also https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/ban-types.md suggests using Record<string, unknown> instead of object.

Remarks: The first example works when either object (playground link) or Record<string, any> (playground link) is used instead of Record<string, unknown>.

Answer 1

您遇到过类型中缺少索引签名（仅在接口上，而不是在类型别名上）#15300

当您将接口更改为类型时，代码将起作用：

type Foo = {
    foo: number
}

const foo: Foo = { foo: 1 };
const bar: Record<string, unknown> = foo;

Answer 2

编辑：@Lesiak 上面有正确的答案。我将其保留只是为了相关答案的链接。

诚然，我有点超出了我的深度，但我正在查看这个答案，我发现：

[A] TypeScript 安全性的很大一部分来自于这样一个事实：[...] 只有当它知道它明确打算作为字典时，它才会让您将对象视为字典。

这与我的测试一致。修改您的接口以显式地将Foo.foo其视为索引不会产生错误。（游乐场链接）

interface Foo {
    [foo: string]: number
}

const foo = { foo: 1 }
const bar: Record<string, unknown> = foo

这并不能完全回答您的问题，因为Record<string, any>与您的显式界面一起工作，但也许知识渊博的人可以从这里获取它。