Copilot round 9: four findings, all of them places where round 8's fail-closed
inversion was the right idea applied incompletely. The rule was only enforced
at the positions I happened to be editing; it now holds everywhere a schema is
inspected.
1. `admitsNull` returned early on an explicit `type`, ignoring a sibling
`anyOf` — but the two are conjunctive, so `{ type: ["string", "null"],
anyOf: [{ type: "string" }] }` admits no null at all. A sibling union now
withholds the claim.
2. A null-typed branch could be unsatisfiable via its own nested `anyOf`
(`{ type: "null", anyOf: [{ type: "string" }] }`). The branch check tested
`not`/`allOf`/`oneOf` but not `anyOf`; it now treats a nested union as
equally opaque.
3. The `type: [T, "null"]` collapse path had no applicator guard at all —
`collapsed` clears `anyOf` unconditionally and the renderers ignore the
rest, so `{ type: ["string", "null"], anyOf: [{ const: "a" }] }` was widened
into an unconstrained string field. That path now declines the same way the
`anyOf` path does.
4. The `admitsNull` docstring still advertised `oneOf` support that round 8
deliberately removed. Rewritten to state what is recognized, what is
declined and why, and the asymmetry that settles the direction.
The three code fixes share one helper (`hasOpaqueApplicator`) rather than
repeating the condition, so the next position that needs it cannot drift.
Signed-off-by: cliffhall <cliff@futurescale.com>
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