Hojun-Cho 4d44242363 wcc: reject transitive nested-tagged widen at type_assignable (#199 α) ...

cgen has no wrapped-slot layout — the tagged-union slot is universally
[tag:8B][payload:up_to_24B], single level. The recursive walk admitted
let r: (size|io.eof|io.error) = u for u: io.underread (transitively
in io.error.params); cg_tag_for_variant + taggedvariantindext don't
recurse, returned -1, defaulted to tag=0, and the slot read back as
variant 0 = size at runtime.

Restores SSoT inside the checker pair: is / as / match variant
lookup is already non-recursive (#198 sibling), and the LET-init /
return / assign arms now agree. Aligns DOWN to the leaner side
(rule-10 stage symmetry). ww-stricter than Hare; harec keeps the
drill at ref/harec/src/types.c:702-739 (#199b is the deferred
wrapped-slot layout port).

Pre-flight audit (drew mandate): zero transitive-widen sites in
lib/ + selfhost/ + cmd/ + examples/. No wrapper-tagged variant
(io.error, strconv.error, fmt.field) is used as a variant of a
wider union anywhere in bootstrap. Mechanical fix.

Escape hatch for callers: spread (...wrapper) inlines the wrapper's
flat variants into the parent set at parse time. Wwstage's gate
additionally preserves the recursive drill on op == TK_ELLIPSIS
because wwstage stays AST-keyed (cstage flattens at resolve_type).

771_widen_transitive: 5 rows (reject_transitive_widen,
spread_alt_widen, direct_flat_variant, branched_callee_widen,
wrapper_typed_widen). Row 2 is CS-only — wwstage's is / match on
spread-expanded variants is open-bug #190/#198.

2026-05-29 03:04:18 +09:00

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