Complete union variants

Partial discriminant patches can create an invalid shape, such as changing a cat's kind to dog without adding bark. Generic builders require complete replacement for such transitions. Native TypeBox adapters additionally provide fromTypeBoxVariant(union, index, options) to generate one complete branch first. The same function is exported by the modern and legacy adapter packages.

import Type from 'typebox';
import { fromTypeBoxVariant } from '@mimlet/typebox';

const Pet = Type.Union([
  Type.Object({ kind: Type.Literal('cat'), lives: Type.Integer({ default: 9 }) }),
  Type.Object({ kind: Type.Literal('dog'), bark: Type.Boolean({ default: true }) }),
]);
const dogs = fromTypeBoxVariant(Pet, 1);
const quietDog = dogs.with({ bark: false }).buildValidated();
// Input patches are restricted to the selected dog branch.
// dogs.with({ kind: 'cat' }) and dogs.with({ lives: 9 }) are type errors.

The index identifies an existing top-level anyOf entry. Literal indexes are checked against known tuples by TypeScript, and runtime indexes are checked before any generation. This does not search nested schemas, resolve discriminant strings, or reinterpret oneOf as anyOf. Use an explicit branch schema/provider for those representations. Native contexts and references are forwarded without network I/O.

Creation invokes the selected branch's native creator, then checks the original union too. Validated builds check membership in the selected branch before invoking the original union's validator/decoder. A root Codec or legacy Transform is therefore preserved and decoded once. Output retains the original union's output type rather than asserting that opaque root transformations preserve the branch. For overlapping branches, the native union decoder's selection rules still apply. No constraints are removed from the original native schema to force generation.

typeBoxVariantAdapter(union, index, options) exposes the original source, the selected variantSource, the index in metadata.variant, a branch-input-typed standard validator, and strict check, issues, create, decode, and encode operations. Encoding a different branch fails the selected-branch check, even if it is accepted by the original union. These operations use the native library's checking semantics; they do not invent constraints that library ignores.

For branches that native minimal creation cannot satisfy, provide an explicit factory to createSchemaBuilder(adapter.standard, factory). This supports both synchronous and asynchronous factories and preserves the selected input and original output types. There are no hidden retries, repairs, or double decodes. As with all native adapters, schema handles, codecs, and registries are trusted, caller-owned objects. Do not mutate them while builders use them.