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

Reproduce the reported behavior in the linked TypeScript playground with exactOptionalPropertyTypes enabled, using the WrappedT, Omit, and HomomorphicOmit examples. Investigate how homomorphic mapped types and conditional inference affect assignability, then verify that the mapped results remain assignable to T without the reported true | undefined error.