Synthesized optional properties don't satisfy `Record<` target under EOPT: false

🔎 Search Terms

switch map array narrow

🕗 Version & Regression Information

I went back to TS 4 - still present

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/KYDwDg9gTgLgBDAnmYcBiBLANjYUAKAhlIQLYBqhWArqgLxwDOMUGAdgOZwA+cb1pAEZ4ecACKFco5q04BtALoBuALAAodaEiw47XFABmhAMaoAkphx4APABU4oXGwAmjJi3YcAfHADe6uEC+MmAALjhbVTUguDBiMkYAfnCAJWBjaGdrGU8AGnRsfSISCipaLyiAX3VNcGh4PTwjUzgzACFCRgxjS30AZWAOUmA2GDsHECdXd1lvCam3C0KbWx9-aKCkFHCAcgNlqB2qmrUteoRkc168AaGRsftHEemczh8GJasoW+HRgFFwFBgIwuhA2HYfLx2p1utdvoNfg8KrVtPAtqgfvcAPIoEgwaBwBg7QguHaiHbQI4o86NQwmK4HTH-QHA0Hgx6TZ5uV5zdYxdHhJkwHF4STQKIxMBA5zdSTA8KffoI+4QxTHDSnfikApfAByITcfgCQQ6UDMbiJgmIAH0MIwdrljYEOgAvc2EuA7K0u2329TVDUjATiDC-Nm2S5GjbO4ger3EB1OuCuuPenb+k5nHTo1piUMjNkevlBDDOcI8iWbS7hPNhjBgiMoAB0pqqomLgVL5Y8nErgSwhGEWG7sz7F22IbrDcuLcILqqUXUGTYzAQJBXBmg2oYAAoZVOVwrawX62xFABKBVwoXWOH64aMd5rJNAmDUKBsKMxfnVz0k5yJtG35SsAMrGHKjDhPuJ5gowTakIQYB7vmK6noSz5Ad+QSMAA7hgMDGAAFnAyEHk26Lnl+WHUYE4GMKgx6odOzamuEHY0Rxr7vp+7EcXx45hJ6+xfIB-FicEwzhHeBqzmajCOph4lYXEJSQVRSliVaZplnA0FMWwTalkmGnfgGJkxGZGmWUpdEMSh4Yzq6bHGRpXEfup5k-hOewHKJnnfmwIRSQc97ArObryS5nkqQkzmKf5QTemYOl6WyhnOFF5nWVlmU0dleWZZU55JgGlRAA

💻 Code

export type FilterParamValue = string | number | Date | string[];

export interface IFilter<T extends string> {
    name: T;
    params?: Record<string, FilterParamValue>;
}

export interface IBasicFilterSegment<T extends string> extends IFilter<T> {
    type: 'filter';
}

export type IFilterSegment<T extends string> = IFilterSegmentExpression<T> | IBasicFilterSegment<T>;
export type SegmentOperator = 'and' | 'or';
export interface IFilterSegmentExpression<T extends string> {
    type: SegmentOperator;
    predicates: IFilterSegment<T>[];
}

enum FilterNames  {
    BarIs = 'bar_is',
    BazIs = 'baz_is'
}

enum DimensionType {
    Bar = 'bar',
    Baz = 'baz'
}

export type IDimension = {
    id: string;
    type: DimensionType.Bar;
} | {
    id: string;
    label: string;
    type: DimensionType.Baz;
};

const transform = (dimensions: IDimension[]): IFilterSegment<FilterNames> => {
    return {
        type: 'and',
        predicates: dimensions.map(dimension => {
            switch (dimension.type) {
                case DimensionType.Bar: {
                    return {
                        type: 'filter',
                        name: FilterNames.BarIs,
                        params: {
                            barId: dimension.id
                        }
                    }
                }
                case DimensionType.Baz: {
                    return {
                        type: 'filter',
                        name: FilterNames.BazIs,
                        params: {
                            bazId: dimension.id
                        }
                    }
                }
            }
        })
    }
}

🙁 Actual behavior

A compile-time error is displayed. It appears that the fact that we might return in the first switch case is "carried over" to the second one.

🙂 Expected behavior

Type narrowing to work correctly as the mapping is correct. In fact, if one extracts this mapping to a separate function, the behaviour is fixed. Playground.

Additional information about the issue

No response

Reproduce the reported error using the provided TypeScript Playground link and code, comparing EOPT: false with the extracted-function variant. Start by tracing the type-checker behavior for the switch inside Array.map; done means the inline mapping narrows correctly without the compile-time error and regression coverage confirms the behavior.