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

exactOptionalPropertyTypes を有効にした、リンク先の TypeScript playground で、WrappedT、Omit、HomomorphicOmit の例を使用して報告された動作を再現してください。ホモモーフィックな mapped type と条件付き推論が代入可能性にどのような影響を与えるかを調査し、その後、報告された true | undefined エラーなしで mapped result が引き続き T に代入可能であることを確認してください。