Interface that extends another no longer constrains types like the original

🔎 Search Terms

interface
interface constrain
extend interface

🕗 Version & Regression Information

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

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/PQKhAIGUFEBUFUAK4C8b0c1z4TAFABmArgHYDGALgJYD2p4lAngA4CmAPLAHwAUAlAC5wscAG9wAJzaVikhqWIAbJeACGAZ3BkA1qVoB3BppEBucAF98zduGgA3NqUoBZNS1TgASm3K1JACYcGpSS1KQA5gA0do7O3KbWrGzgADLUIU5sklwOTpTgbAAelE4BWnnxnry8bPaUwrCVlPyo3OD2tNQBrQA+4uAAFmqkAUpszby0AEYAVr4NIs1CHV0Blon4xSz+BeGlkoRq5CmwyQHN0AC21JQHuXGu7oUlZRWPbiztYvjgf+oBC6PdKZUjZB75M62YqlUZaHRsJi0QhLD7uPi-f5YmxsRrNKFsKKYrF-JQZWHZYQgik5JpolgAbTpkOSAF1uESSf9aCwaPQNAB+YQAQUBzWpWUkAHleXRSFp+tNaLRxiNOf8Vp1uoksdIrrRHOLyZKIc4CS9YeVwAikSjmc5PhiuX8cXjHgT1VyyaDKWljWDac1Pkz8WyOcSSTy+fKhbF8hKAzLowrwEqVWw1RHNWtElZQBBoAA5AAiUDgSFQ2Cr1ZQuAI+HzvwggFByKAjW5MRhsEJNgg4rjnS43O7giSEZXCZqWbh8VoaNQ0DSEajdkSDx7XW73McTuPOafmYDAcDN-AN48AAUoGgAtNsFnfJJJ-El2AP2ED8puRzkJNM1JIk6PNOs7gPOi7LquBKfs437buA460EB+QHuAR4nmeja4Ce4DCmSJjIowgwpP41AROEaiqDivavikADCypKEOW7gvaTweDCbx7ux7S1tBzE-qaPGbP2DEqgJ8GIch+4WDOAhgQuGSQVo-EbsOkm7lOsmHsep4Rs6BmGUZxkmaZJnoVet73lQj7PpItEcGJTFqSxv6pgB0kFLJoHgUpK4qeuX7qaOCGacB2lobpmFgE24CADLk4AAJLONkRwnERC5gdQVwsEonacXCREkWE5GkJRjDJDR+ypcc9GMclBxpRMLmCWxnwWlxQboh1hWqUFrlCY6AxWA5TkNTVJwSSFUncSB8m+Uu-lrh+U1uTNWkJJFGH6WZu17ftB1GRZ153kU7A2dkdmjfVKWHLVq0cH+Hmzd582KYtUGBbBwVrWFKERehcXRSAsUJeNd3pZQwwFAVVpQ8VZEUVRyTqKMAJWiExCEIQVW3U14BjXjtWIEoxAaMWGQsAu5CDA9bXPLD7z5ENjPLWwMGUHBrFdV84gRgElPU7TjxCearM2oR9NfLU9husz7ghu6bLZtq+AjaJN2NcTpPk4LlA0w9O5IS9clzu9ylsxzXO-cbG06dth2O07zsu38x1WWdD6XS+GsquDTUk2TFMaFT+vC-1glPYBJs+ebS19d9A1G55qGA8DsWtgxLCdlTIThBERXgH4AQpOEmRqOshGw-nhfVRDKQGP4OgaLjWvpVnTCIJopRWz9g0M68hU898EYVxzCbc0r0KD1aEt2sPvA7S6yRy2aySeiS3o0lS-qT-LjJsQS7Ib1iUZyoKIpisCu-SrK-LgIqjEZqQG8qwEOr-HqBrNfGN+iyj4tESSwXkvCq7BV6UA9KArekod4+kDPSRWLJ2DH1AWffksYjTwKTOfB+qYn6ZixG-D+fwBYhyFs0f+09LTwiAfPekfA6gQODIfZWwgtTvzVmeDW2cu6ZF7knUKttwqmwUhBeOX1OZ92Tibe2elXYKMUftd2p1zqUFsj7ZIjkeSd27uzQ27lo4bVjuIz6K0WoaWEf9Ta6FmxAA

💻 Code

/** SETUP ========================== */
function type<T>(): T { return null as unknown as T; }
type EventMap = Record<string, Event>;
type Listener<TEvent extends Event> = ((evt: TEvent) => void) | { handleEvent(object: TEvent): void };

export interface TypedEventEmitter<TEventMap extends EventMap> {
    addEventListener<TEventType extends keyof TEventMap>(
        type: TEventType,
        listener: Listener<TEventMap[TEventType]>,
        options?: AddEventListenerOptions | boolean,
    ): void;
    removeEventListener<TEventType extends keyof TEventMap>(
        type: TEventType,
        listener: Listener<TEventMap[TEventType]>,
        options?: EventListenerOptions | boolean,
    ): void;
}
/** END SETUP ========================== */

/**
 * ✅ Sanity test
 */
type<TypedEventEmitter<{ foo: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

// @ts-expect-error
type<TypedEventEmitter<{ bar: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

/**
 * ✅ Alias of the original type
 */
type CoolEventEmitter<TEventMap extends EventMap> = TypedEventEmitter<TEventMap>;

type<CoolEventEmitter<{ foo: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

                                                                                      // @ts-expect-error
type<CoolEventEmitter<{ bar: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

/**
 * ❌ Interface that simply extends the original type
 */
interface CoolInterfaceEventEmitter<TEventMap extends EventMap> extends TypedEventEmitter<TEventMap> { }

type<CoolInterfaceEventEmitter<{ foo: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

                                                                                               // @ts-expect-error
type<CoolInterfaceEventEmitter<{ bar: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ❌

/**
 * ❌ Interface that extends the original type and adds stuff
 */
interface CoolInterfacePlusDispatchEventEmitter<TEventMap extends EventMap> extends TypedEventEmitter<TEventMap> {
    dispatchEvent<TEventType extends keyof TEventMap>(ev: TEventMap[TEventType]): void;
}

type<CoolInterfacePlusDispatchEventEmitter<{ foo: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

                                                                                                           // @ts-expect-error
type<CoolInterfacePlusDispatchEventEmitter<{ bar: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ❌

/**
 * ✅ Copy pasting the code instead of extending the interface works
 */
interface CopyPastedEventEmitter<TEventMap extends EventMap> {
    addEventListener<TEventType extends keyof TEventMap>(
        type: TEventType,
        listener: Listener<TEventMap[TEventType]>,
        options?: AddEventListenerOptions | boolean,
    ): void;
    removeEventListener<TEventType extends keyof TEventMap>(
        type: TEventType,
        listener: Listener<TEventMap[TEventType]>,
        options?: EventListenerOptions | boolean,
    ): void;
    dispatchEvent<TEventType extends keyof TEventMap>(ev: TEventMap[TEventType]): void;
}

type<CopyPastedEventEmitter<{ foo: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

                                                                                            // @ts-expect-error
type<CopyPastedEventEmitter<{ bar: Event }>>() satisfies TypedEventEmitter<{ foo: Event }>; // ✅

🙁 Actual behavior

As soon as the original interface is extended – whether the extender adds properties, does not add properties, uses the type parameter, or doesn't use the type parameter – I seem to lose the ability to constrain types in the same way as the original interface can.

🙂 Expected behavior

I would expect an interface that extends another to constrain types in exactly the same way as the original.

Additional information about the issue

No response

Reproduce the behavior using the linked TypeScript Playground and the code in the issue, comparing the original interface, type alias, and extending interfaces. Trace the type-checking path for generic constraints and add a regression test; done means an extending interface preserves the original constraint and the existing @ts-expect-error cases behave as expected.