Why Typescript cannot infer discriminated types when destructing-CodePudding

I try in vain to narrow discriminated types with object destructuring. I thought Typescript is able to infer them since v4.4.

However, I cannot do it in this code (just an example of idea):

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
    }
}

But without destructuring it works:

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;
            }
    }
}

Maybe, I get v4.4 announcement wrong, or is there an error?

CodePudding user response：

You've just discovered that currently (as of TS4.7) there's no support for control flow analysis for destructured rest elements. This is a known missing feature of TypeScript, requested at microsoft/TypeScript#46680.

If you want to see this supported you might want to go to that issue and give it a