fix(v4): preserve z.preprocess input narrowing

[devareddy05]

Closes #5966.

z.preprocess(fn, schema) lost the input-type narrowing in v4.4.3. #5929 changed the return from ZodPipe<ZodTransform<A, B>, U> to ZodPreprocess<U>, which extends ZodPipe<ZodTransform, U> with ZodTransform used bare — so its I slot defaulted to unknown and core.input<A> came out unknown too, dropping the annotated arg type.

Adds a second I = unknown generic to $ZodPreprocess / ZodPreprocess (and the matching def/internals), threads it into $ZodTransform<unknown, I>, and binds it from the fn arg in the preprocess() factory. Pure type-level change — no runtime behavior shift, optionality plumbing from #5929 is preserved.

Two regression tests in index.test.ts cover the narrowed and unannotated cases.

[pullfrog]

TL;DR — Restores z.preprocess input-type narrowing that regressed in v4.4.3, where the annotated fn arg type was being collapsed to unknown in z.input<>. Pure type-level fix threading a new I generic through $ZodPreprocess and friends.

Key changes

• Thread I generic through $ZodPreprocess — Adds a second I = unknown generic to $ZodPreprocess, $ZodPreprocessDef, and $ZodPreprocessInternals, and plugs it into the inner $ZodTransform<unknown, I> so the input slot is no longer bare.
• Bind I from the preprocess() factory fn arg — preprocess<A, U, B>(fn, schema) now returns ZodPreprocess<U, B>, propagating the annotated arg type through to z.input<>.
• Regression tests for Narrowing the input type of z.preprocess no longer works after updating to latest version (v4.4.3) #5966 — Two expectTypeOf cases in index.test.ts cover both the annotated arg case (string | null | undefined) and the unannotated fallback (unknown).

_{Summary ｜ 3 files ｜ 1 commit ｜ base: main ← fix/preprocess-input-narrowing}

Before: z.input<typeof z.preprocess((v: string | null | undefined) => ..., z.string())> resolved to unknown.
After: it resolves to string | null | undefined, matching the annotated fn arg.

#5929 rewrote z.preprocess to return ZodPreprocess<U> (extending ZodPipe<ZodTransform, U>) instead of ZodPipe<ZodTransform<A, B>, U>. Because $ZodTransform was used bare, its I slot defaulted to unknown and core.input<A> collapsed to unknown, dropping the narrowed arg type that users relied on. Threading a second generic through the preprocess type chain and binding it from fn in the factory recovers the v4.4.2 behavior while keeping the optionality plumbing from #5929 intact.

Why is this type-only?

The runtime construction in `preprocess()` is unchanged — it still builds a `ZodPreprocess` with a `ZodTransform` input and the user schema as output. Only the declared generics on the interfaces and the factory return annotation move.

packages/zod/src/v4/classic/schemas.ts · packages/zod/src/v4/core/schemas.ts · packages/zod/src/v4/classic/tests/index.test.ts

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[pullfrog]

Reviewed — no issues found.

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