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Malte UblandClaude Opus 5.5
b767c38e9e
`{ ...base, ...override }` over union-typed sources has one field set per
combination of source arms, so the merged value can carry fields from
several arms at once. When the per-arm distribution (slice 1a) cannot pick
one arm, the merge was refused with SC2003 and the site did not compile
(provider-composer's all-optional compat unions, mcp's aggregate config).
Build the object Node builds instead: the checked-dynamic literal, whose
keys are written in source order through the existing dynamic-object path
(field chunks, spreads by copy-data-properties, computed keys by
propertyKey), then converted into the declared union by the existing
checked conversion. Reflection, JSON and inspection see the keys Node
keeps; a value the union cannot hold throws the catchable TypeError at the
conversion rather than being reinterpreted as one arm. The fallback stays
narrow: union-typed spread source, auto tier, no capture buffer, and a
union the checked extraction can validate — everything that compiles
today lowers exactly as before, and `--tier=static-only` keeps the refusal
that tests/diagnostics/union-spread-merge.ts pins.
Corpus union-spread-merge.ts pins key order, last-writer-wins, nested
identity, the accepting typed consumer and the snapshot semantics
differentially; dyncheck's "union spread merge" covers the refusing
single-arm reinterpretation scriptc-only, since Node never checks a cast.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>