Index type distributes over intersections too eagerly

🔎 Search Terms

index keyof indexed access deferred deferral instantiation

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20250202#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-hheppEC41UQPFk2SORE3x5VEFkc5zXPcyAvJ8vyIFgkrl2i6QUPQngsJw05xJeKTrGgcjBFEBjOEubQEBAGj6WLD9aMYIA

💻 Code

type Values<T> = T[keyof T];

// transforms from "keyed object map" to a union
type ExtractEventsFromPayloadMap<T> = Values<{
  [K in keyof T]: T[K] & { type: K };
}>;

type EnqueueObject<TEmittedEvent extends { type: PropertyKey }> = {
  // creates proxy methods
  emit: {
    [K in TEmittedEvent["type"]]: (
      payload: Omit<TEmittedEvent & { type: K }, "type">,
    ) => void;
  };
};

type Exec<TEmitted extends { type: PropertyKey }> = (
  enq: EnqueueObject<TEmitted>,
) => void;

declare function createStore<TEmitted>(definition: {
  emits: {
    [K in keyof TEmitted]: (payload: TEmitted[K]) => void;
  };
  exec: Exec<ExtractEventsFromPayloadMap<TEmitted>>;
}): any;

createStore({
  emits: {
    increased: (_: { upBy: number }) => {},
    decreased: (_: { downBy: number }) => {},
  },
  exec: (enq) => {
    enq.emit.increased({
      upBy: "bazinga", // this should error!
    });

    const fn = enq.emit.increased;
    //    ^?

    // this is just a copy of what is displayed as `enq.emit.increased`'s type
    const exactSameTypeAsAbove: (
      payload: Omit<
        { upBy: number } & { type: "increased" } & { type: "increased" },
        "type"
      >,
    ) => void = () => {};

    exactSameTypeAsAbove({
      upBy: "bazinga", // this one errors correctly
    });
  },
});

🙁 Actual behavior

There is no error on the annotated call

🙂 Expected behavior

It should error

Additional information about the issue

1. keyof (T & { type: K }) gets normalized to keyof T & "type"
2. that passed through Exclude<X, "type"> (within Omit) is left as Exclude<keyof T, "type">.
3. then T gets instantiated with ExtractEventsFromPayloadMap<{ increased: { upBy: number; }; decreased: { downBy: number; }; }
4. but now keyof ... returns only shared keys of this union
5. the only shared key is type so this Exclude gets computed as never (Exclude<"type", "type">)
6. and the final parameter type of enq.emit.increased gets computed as just {} (from { [K in never]: ... })

Commencez par la reproduction TypeScript Playground liée et suivez le chemin du checker pour indexed access, keyof, intersections et Omit. Comparez l’appel du callback annoté avec le type de fonction équivalent écrit explicitement. C’est terminé lorsque l’appel inféré enq.emit.increased refuse une chaîne pour upBy, avec un test de régression couvrant le cas signalé.