Key attributes (variant, field, entry type/schema) are refused when repeated. A CultUnion key
parses as the reference's Int32, negatives included; a key outside it is a compile error. Decode
classifies the key as the reference does: non-integer, outside Int32, undeclared (named), and a
pair of the wrong length is refused before its key is read; no value is echoed beyond the
undeclared key the reference names. The SlotRecord trait, implemented by the shared slot
generator, bounds an arm payload, so a String or Option arm is a compile error. compile_fail
fixtures use keys other than 0; the derive crate's unit tests assert each refusal's message.
Unverified: committed before the first build; the verification job runs next.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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