Three tagged-union gaps:
1. Struct-payload widening was broken at every site (call, let,
assign, return, struct-field init). cg_widen_tagged_store now
materialises str / scalar / struct-lit / struct-ident / tagged
payloads at slot+8+field_off and writes the tag last. Call sites
route through cg_widen_tagged_push (scratch slot + push high→low).
2. Tagged → wider tagged widening forwarded the source tag verbatim.
cg_widen_tag_remap emits a CMPQ-chain switch that translates each
source variant index to the destination's, then zero-pads to the
wider slot. type_eq grew a TY_TAGGED arm (was returning 1 for any
two unions); type_assignable now accepts variant-subset and
rejects the rest.
3. `(...inner | T)` spread parses (cmd/wcc/parse.c, lib/ww/parse).
Marks Node.op = TK_ELLIPSIS; resolve_type unwraps NAMED + flattens
when the spread bit is set so aliases inline like Hare's
tagged_type unwrap flag.
Selfhost mirror: spread parser ported. Cgen widen helpers not yet
mirrored — wwstage stays byte-identical to cstage on the existing
test corpus, but will emit wrong asm if user code uses the new
patterns (probe sp2 shows the divergence).
700_e2e: 9 new rows covering call/let/assign/return × struct +
tagged subset, plus the spread-flatten case.
Hojun-Cho
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