c1/c2 widened flatvariantidxt (selfhost) with the chain + structural
tag-synthesis arms and a >=2 ambiguity os.exit, scoped to the cgen
WIDEN consumer. But flatvariantidxt is a choke-point: the wwstage is/as
ACCEPTANCE gate (check.ww:4677, the #198 spread fallback) reuses it, so
the widening leaked into checker acceptance — vs base 329481c:
* `let v:(void|ali)=…; v is base` (ali=base): cstage rejects, wwstage
ACCEPTED+built — new cs!=ww acceptance divergence (rule-10 break);
* `(void|tb)`, `v is ta` (unrelated same-layout): same leak via the c2
structural arm;
* `(ali|ali2)`, `v is base`: wwstage DIED with the cgen-internal fatal
"flatvariantidxt: source alias chain reaches >=2 variants" DURING
CHECK — a cgen diag surfacing in the checker (layering).
cstage is unaffected: its is/as gate (check.c:2036) is independent of
cg_tag_for_variant (cgen-phase only), so the fuse was already broken at
this site — the cgen-helper change moved wwstage's CHECKER but not
cstage's. This contradicts the #95 fold scope ("cgen-tag fold, no
acceptance change except the ambiguity hard-error [at the widen site]").
Fix (rob-ruled): the is/as gate needs only nominal variant membership =
pass 1. Add an explicit `exactonly` mode to flatvariantidxt — the
checker caller passes true (returns after the exact loop: no chain/
structural arms, no os.exit), every cgen caller passes false (full
tag-synthesis, unchanged). Two consumers, two modes — the honest
representation, not a wrapper. cstage's cg_tag_for_variant has no twin
checker caller, so it stays full-only and is UNTOUCHED by c3 (rule-10
satisfied: the param changes no asm — cgen always passes false; the
checker now MATCHES cstage's reject). casevariantin still backs the
#198 spread fallback.
Pins (test/wcc/944_variant_chain_b95_run.c, +4 rows -> 56 checks):
isas_chain_reject / isas_unrel_reject — BOTH stages reject the leaked
is/as shapes (shared experr substring "not a variant"); the c1 chain +
c2 structural arms no longer widen acceptance.
isas_amb_reject_notcrash — the (ali|ali2)/`is base` shape rejects
CLEANLY (the cgen fatal text would be absent -> red), NOT a crash.
twin_prim_alias_amb — rob's obligated mixed prim/alias TWIN:
(int | ai) ai=int, source aj=int — both share the int bottom under
all-variants counting, so the cgen WIDEN (full mode) hard-errors
("source alias chain reaches >=2 variants"), pinned LOUD both stages.
The deferred question (should is/as EVER accept cgen's richer chain/
structural shapes? = a checker-strictness feature, both stages together)
is filed as task #107, explicitly NOT folded here.
Invariants: c1/c2 cgen behavior unchanged (all cgen callers pass false =
full mode); suite byte-id rows + the dissolution corpus hold. make all
0; sizelint 0; peellint 0 (the mode param adds no peel sites); combined.ww
regen idempotent; test-unit "all 295 tests passed". c3 touches ZERO
cstage bytes — cmd/w6c/cgen.c carries only the c1/c2 additions, and
cmd/wcc/check.c is unchanged from base 329481c.
Hojun-Cho
fc50a27f3e