place null last in anyOf for optional unions in JSON schema - #1028

Thanks for the quick turnaround on the tagged-union fix! I did a deep pass on b5b078a, including an empirical schema diff against 0.21.1 across ~25 type shapes. The ordering change itself looks right and minimal for most shapes, but two behavior changes go beyond reordering and need fixing before this can merge:

1. Meta metadata on a None union member is silently dropped

from typing import Annotated, Union
from msgspec import Meta, json

typ = Union[Annotated[None, Meta(description="explicit null means reset")], int]
json.schema(typ)

• 0.21.1: {"anyOf": [{"description": "explicit null means reset", "type": "null"}, {"type": "integer"}]}
• this PR: {"anyOf": [{"type": "integer"}, {"type": "null"}]} - the description (same for title/examples/extra_json_schema) is lost.

The unwrap loop detects NoneType through the Metadata wrapper but then discards the subtype and appends a hardcoded {"type": "null"}. Fix: remember the subtype (e.g. none_member = subtype) and append self.to_schema(none_member) instead of the literal.

2. Optional[tagged union] changes schema shape, not just order

For Union[A, B, None] where A/B are tagged structs and there are no other members:

• 0.21.1: {"anyOf": [{"type": "null"}, {"anyOf": [ref A, ref B], "discriminator": ...}]} - discriminator scoped to the struct members only.
• this PR: {"discriminator": ..., "anyOf": [ref A, ref B, {"type": "null"}]} - discriminator hoisted onto a composite that now contains a null branch.

Problems:

• OpenAPI 3.x expects every schema under a discriminator-bearing composite to carry the discriminator property; an inline {"type": "null"} can't, and it has no mapping entry. Strict tooling (openapi-generator, spectral lints) chokes on this.
• It's inconsistent within this same PR: Union[A, B, int, None] keeps the nested {anyOf + discriminator} member form, so the same tagged union renders in two different shapes depending on its neighbors.
• pydantic emits the nested form here, so this diverges exactly where #1027 asks for parity.

Fix: in the len(structs) >= 2 branch with empty options, keep the nested form and emit {"anyOf": [struct_schema, {"type": "null"}]} instead of schema.update(struct_schema) plus a top-level append. test_struct_tagged_union_with_none currently pins the flat shape and would need updating along with this.

Smaller notes:

• Test gaps worth closing while you're here: Union[A, B, int, None] (the mixed branch isn't covered by any test) and the Annotated[None, Meta(...)] case from point 1.
• The PR description still mentions the if options or has_none: guard that's no longer in the diff - worth refreshing once the above lands.

Everything else checked out clean: no lost union members, no duplicated nulls, $defs/recursive refs intact, plain unions byte-identical to main, and all outputs pass JSON Schema Draft 2020-12 meta-validation.

Both change requests are resolved. Re-checked empirically against current main across the shapes from the last pass:

• None-member Meta is preserved now: Union[Annotated[None, Meta(...)], int] keeps description (and title/examples/extra_json_schema) on the null branch, just moved last.
• Optional[tagged union] keeps the nested form: Union[A, B, None] emits {"anyOf": [{"anyOf": [refs], "discriminator": ...}, {"type": "null"}]} with no top-level discriminator, and Union[A, B, int, None] renders the same nested member, so the two are consistent again.

No regressions: plain unions and tagged unions without None are byte-identical to main, null is only relocated (never duplicated or dropped), $defs/refs intact, and every output passes JSON Schema Draft 2020-12 meta-validation. test_schema.py is green (115 passed) and the branch merges cleanly on current main. Thanks for the thorough follow-up!