Certain homomorphic mappings break assignability with exactOptionalPropertyTypes

🔎 Search Terms

eopt exactOptionalPropertyTypes homomorphic Omit unassignable

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://tsplay.dev/w28Qzm

💻 Code

// @exactOptionalPropertyTypes

type T = {
  foo?: true;
  bar?: true;
};

type WrappedT<t extends T> = [t];

// Since Omit is normally treated as homomorphic, should work but errors with:
// Type 'true | undefined' is not assignable to type 'true'.
type OmitBarFromWrapped<t> =
  t extends WrappedT<infer inner> ? WrappedT<Omit<inner, "bar">> : never;

// Even if we directly rewrite Omit to be homomorphic, we still lose assignability with EOPT
type OmitHomomorphicFromWrapped<t> =
  t extends WrappedT<infer inner>
    ? WrappedT<HomomorphicOmit<inner, "bar">>
    : never;

type HomomorphicOmit<t, keyToOmit> = {
  [k in keyof t as k extends keyToOmit ? never : k]: t[k];
};

// Normally in this situation, I have a conform utility to force TS to allow the type
// but this is maybe the first time I've also seen that not work?
type OmitHomomorphicFromWrappedConformed<t> =
  t extends WrappedT<infer inner>
    ? WrappedT<conform<HomomorphicOmit<inner, "bar">, T>>
    : never;

type conform<t, base> = t extends base ? t : base;

🙁 Actual behavior

Applying a homomorphic mapping to a type can break property assignability with EOPT

🙂 Expected behavior

Should preserve assignability

Additional information about the issue

No response

Reproduzieren Sie das gemeldete Verhalten im verlinkten TypeScript-Playground mit aktiviertem exactOptionalPropertyTypes unter Verwendung der Beispiele WrappedT, Omit und HomomorphicOmit. Untersuchen Sie, wie homomorphe gemappte Typen und bedingte Inferenz die Zuweisbarkeit beeinflussen, und überprüfen Sie anschließend, dass die gemappten Ergebnisse weiterhin T zuweisbar sind, ohne den gemeldeten true | undefined-Fehler.