为什么 Typescript 在解构时无法推断出可区分的类型

我试图用对象析构来缩小受歧视类型的范围，但没有成功.我以为打字本能从v4.4年前就推断出来.

但是，此代码中的I cannot do it(仅为IDEA的一个示例):

type WritableRecord = {
    someField: 'a'
}

type ReadonlyRecord = {
    otherField: 'b'
}

type Writable = {
    readonly: false;
} & WritableRecord;

type Readonly = {
    readonly: true;
} & ReadonlyRecord;

export type RecordProps = Readonly | Writable

type Fn = (props: RecordProps) => void

const fn: Fn = ({ readonly, ...props }) => {
    switch (readonly) {
        case true:
            const readonlyCheck: ReadonlyRecord = props

            break;
        case false:
            const writableCheck: WritableRecord = props
            break
    }
}

但在不 destruct it works的情况下:

type WritableRecord = {
    someField: 'a'
}

type ReadonlyRecord = {
    otherField: 'b'
}

type Writable = {
    readonly: false;
} & WritableRecord;

type Readonly = {
    readonly: true;
} & ReadonlyRecord;

export type RecordProps = Readonly | Writable

type Fn = (props: RecordProps) => void

const fn: Fn = (props) => {
    switch (props.readonly) {
        case true:
            {
                const { readonly, ...readonlyProps } = props
                const readonlyCheck: ReadonlyRecord = readonlyProps
                break;
            }


        case false:
            {
                const { readonly, ...writableProps } = props
                const writableCheck: WritableRecord = writableProps
                break;
            }
    }
}

也许，我的V4.4声明是错误的，还是有错误？