[RClaudiuM]

Fixes #1724

What

Unions discriminated on a nested path no longer drop the object checks from the branch matched by an undefined case:

const Thing = type({ k: "1", a: "2" }).or({ k: "3", a: "undefined" })
Thing.allows(0) // false (was TypeError: Cannot use 'in' operator to search for 'k' in 0)
Thing(0).toString() // "must be an object (was a number)"

const T = type({ a: "2" }).or({ a: "undefined" })
T.allows(0) // false (was true)
T.allows({}) // false (was true)
T.allows({ a: undefined }) // true

Why

A discriminant on a nested path compiles to switch (data?.a) (or typeof data?.a for a domain discriminant). For every other case, matching it means data is an object with an a key, so pruneDiscriminant removes the branch's domain: "object" and its required-key check. data?.a is also undefined when data is a primitive, null or undefined, or when the key is missing, so for the undefined case those checks are still needed. Without them the required check ran "k" in 0 and threw, and a branch with only the discriminant key compiled to return true, accepting any primitive or {}.

resolveCase now leaves the branches (including overlapping ones) unpruned when the case is undefined, as a unit or a domain, and the path isn't empty. Root-level undefined cases are exact, so they're still pruned. The jitless path didn't have this bug and is unchanged.

Tests

In ark/type/__tests__/discrimination.test.ts, each with JIT and jitless: true where it applies:

• the example from the issue rejects 0, "s", null and undefined with "must be an object", and still checks k
• { a: 2 } | { a: undefined } rejects 0 and {} and accepts { a: undefined }, also for a two-level path ({ a: { b: ... } })
• a domain discriminant with an "undefined" case ({ a: string } | { a: undefined } | { a: number }) keeps domain: "object" and the a key in that case

The three tests fail on main (the first throws the TypeError from the issue) and pass with the change. No existing snapshot changed.

Checks run locally (Node 24): pnpm lint, pnpm typecheckRepo, pnpm test (1816 passing), pnpm testFiles ark/type/__tests__/discrimination.test.ts with type checking, pnpm build, pnpm testV8, pnpm testIntegration and the @ark/attest tests. I didn't run the benches or testTsVersions from prChecks.

• Code is up-to-date with the main branch (based on 326cdce)
• There are new or updated unit tests validating the change