cgen: B5-c3 #89 cgreturn return-position widen — rt/vu chase, trace-first

Trace at the c2 tip (rebuilt binaries, #80-c4 form): ret_widen's mk()
emitted `MOVQ -16(BP),AX / MOVQ AX,DX / MOVQ $0,CX` — the scalar
shuffle arm, payload word 1 ZEROED (s.b/e.aux dropped, cs silent
exit 1). Deciding predicate: the vu single peel left an alias struct
source TY_NAMED → isstruct false → scalar arm. The rt single peel was
the succ half: a 2-LEVEL alias return type stayed NAMED → the whole
tagged-return block was skipped → no tag synthesis at any return
(kb5_succ's three paired return-position insertions). Fix = the three
granted peels (bare-return rt, value-return rt, vu) →
type_chase_named; the route predicates (istagged/isstruct/istuple/
passthrough) key on the chased vu. Raw `->under` in cgen.c 62→59.
#89 CLOSES.

TRAIN INVARIANT holds: cs-only; _ww binaries bit-identical to the
bcd948d baseline md5s. cs movers vs the c2 tip bounded to EXACTLY the
return-route family: ret_widen, kb4_xret, kna_ret_errunion, kb5_succ.
Zero ww movers; detectors pinned (kb4_xampdef stays 139/139 #94;
def93/x93/l2_local stay cs-SEGV — c4's targets; fill/tuparg/v2
families unchanged).

Graduations (cs1/ww0 BYTE-DIVERGE → 0/0 BYTE-ID): g89_ret_widen (the
banked spelling), g89_ret_errunion (the live e.aux truncation seed),
and succ_2lvl flips K_RUN_NOID → K_RUN exactly as C1-CORR-1 predicted
(pins-follow-the-layer). g89_ret_named_ctl (bare NAMED control) held
0/0 byte-id throughout.

944_alias_cgen_b5_run 25→28 rows (75 checks); 944 family green;
sizelint 0.

cmd/w6c/cgen.c

@@ -12243,8 +12243,7 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 		 * is producing the void variant. Emit its tag; the payload is
 		 * undefined (void has size 0). */
 		if (n->lhs == NULL && cg_ret_type) {
-			Type *rt = cg_ret_type;
-			if (rt->kind == TY_NAMED) rt = rt->under;
+			Type *rt = type_chase_named(cg_ret_type);
 			if (rt && rt->kind == TY_TAGGED) {
 				/* #38b: an sret-classified tagged return (slot
 				 * > the AX/DX/CX/R8 cursor) writes the void-
@@ -12283,12 +12282,18 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 			}
 		}
 		if (n->lhs && cg_ret_type) {
-			Type *rt = cg_ret_type;
-			if (rt->kind == TY_NAMED) rt = rt->under;
+			/* #89: the single peel left a 2-level alias return
+			 * type NAMED (rt — the whole tagged block skipped, no
+			 * tag synthesis) and an alias struct source NAMED (vu
+			 * — isstruct missed, the scalar arm shuffled word0
+			 * into DX and ZEROED the rest of the payload). Both
+			 * silent; ww routes via rhsstructpayload→
+			 * structlookupchain since B2-c2 and is the runtime-
+			 * correct reference. */
+			Type *rt = type_chase_named(cg_ret_type);
 			if (rt && rt->kind == TY_TAGGED) {
 				Type *vt = n->lhs->type;
-				Type *vu = (vt && vt->kind == TY_NAMED)
-				    ? vt->under : vt;
+				Type *vu = type_chase_named(vt);
 				int istagged = vu && vu->kind == TY_TAGGED;
 				/* #263: passthrough forwards the source's AX/DX/CX
 				 * unchanged — correct ONLY when the source already

test/wcc/944_alias_cgen_b5_run.c

@@ -32,12 +32,10 @@
  *                       | cgexpr INLINE set (B6), NOT the     |
  *                       | :2456 funnel (C1-CORR-2, corrects   |
  *                       | rob R2) — byte-id waived until B6   | 0/0
- *   succ_bound_c3       | kb5_succ: `?`-success remap over    |
- *                       | 2-lvl alias result — both-correct   |
- *                       | divergent at RETURN positions only  |
- *                       | (:12278/:12318/:12322 family,       |
- *                       | C1-CORR-1) — joins c3's graduation  |
- *                       | set; byte-id waived until c3        | 0/0
+ *   succ_2lvl           | kb5_succ: `?`-success remap over    |
+ *                       | 2-lvl alias result — was divergent  |
+ *                       | at RETURN positions only (C1-CORR-1)|
+ *                       | — GRADUATED at c3 with the rt chase | 0/0
  *   null_2lvl           | kb5_null: nullable fold + deref     |
  *                       | (type_isnullable/nullable_ptr_tag)  |
  *                       | — divergence flip                   | 0/0
@@ -92,6 +90,15 @@
  *                       | fatal gave way to the #129 A.2      |
  *                       | foldability loud — now fill0-class  |
  *                       | both-loud (twin texts)              | err/err
+ *   ---- c3 (#89: cgreturn return-position widen rt/vu chase) ------
+ *   g89_ret_widen       | alias-struct return into union —    |
+ *                       | was cs word0-to-DX + CX zeroed      |
+ *                       | (payload dropped, silent exit 1)    | 0/0
+ *   g89_ret_errunion    | 1-LVL alias error-union return —    |
+ *                       | cs MOVQ AX,DX dropped e.aux (the    |
+ *                       | live-truncation seed)               | 0/0
+ *   g89_ret_named_ctl   | bare NAMED struct return control —  |
+ *                       | held 0/0 byte-id throughout         | 0/0
  *
  * K_RUN rows build+run BOTH drivers (cs exit==cswant, ww exit==wwwant)
  * and assert cstage/wwstage asm byte-id. K_RUN_NOID asserts both run
@@ -270,11 +277,11 @@ static const struct row rows[] = {
 	  "	};\n"
 	  "	return 0;\n"
 	  "};\n", 0, 0, K_RUN_NOID, NULL },	/* byte-id: B6 */
-	/* C1-CORR-1: the residual divergence is exactly three paired
-	 * return-position tag syntheses — the cgreturn return-route
-	 * family (:12278/:12318/:12322). Joins c3's graduation set
-	 * (pins-follow-the-layer); flips to K_RUN there. */
-	{ "succ_bound_c3",
+	/* C1-CORR-1 → GRADUATED at c3: the residual divergence was
+	 * exactly three paired return-position tag syntheses — the
+	 * cgreturn return-route family; the c3 rt chase closed it
+	 * (pins-follow-the-layer, ken c1-boundary addendum). */
+	{ "succ_2lvl",
 	  "package main;\n"
 	  "type e0 = !i64;\n"
 	  "type r0 = (i64 | e0);\n"
@@ -293,7 +300,7 @@ static const struct row rows[] = {
 	  "	case e0 => { return 2; };\n"
 	  "	};\n"
 	  "	return 0;\n"
-	  "};\n", 0, 0, K_RUN_NOID, NULL },	/* byte-id: c3 */
+	  "};\n", 0, 0, K_RUN, NULL },	/* graduated: #89 (B5-c3) */
 	{ "null_2lvl",
 	  "package main;\n"
 	  "type np0 = (*i64 | void);\n"
@@ -555,6 +562,62 @@ static const struct row rows[] = {
 	  "	return 0;\n"
 	  "};\n", 0, 0, K_BUILDERR,
 	  "#129 A.2 scope" },
+	/* ---- c3: #89 — the cgreturn rt/vu single peels left a 2-level
+	 * alias RETURN TYPE un-tagged (no tag synthesis at all, the succ
+	 * shape) and an alias-struct SOURCE un-structed (the scalar arm
+	 * shuffled word0 into DX and ZEROED the rest — s.b/e.aux
+	 * dropped, cs silent exit 1, ww runtime-correct since B2-c2). */
+	{ "g89_ret_widen",
+	  "package main;\n"
+	  "type base = struct { a: size, b: size };\n"
+	  "type ali = base;\n"
+	  "fn mk() (void | ali) = {\n"
+	  "	let x: ali;\n"
+	  "	x.a = 4; x.b = 9;\n"
+	  "	return x;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "	match (mk()) {\n"
+	  "	case let s: ali => { if (s.b != 9) { return 1; }; };\n"
+	  "	case void => { return 2; };\n"
+	  "	};\n"
+	  "	return 0;\n"
+	  "};\n", 0, 0, K_RUN, NULL },
+	{ "g89_ret_errunion",
+	  "package main;\n"
+	  "type failure = struct { code: i32, aux: size };\n"
+	  "type myerr = failure;\n"
+	  "fn work(bad: bool) (i32 | myerr) = {\n"
+	  "	if (bad) {\n"
+	  "		let e: myerr;\n"
+	  "		e.code = 7; e.aux = 99;\n"
+	  "		return e;\n"
+	  "	};\n"
+	  "	return 42;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "	match (work(true)) {\n"
+	  "	case let e: myerr => { if (e.aux != 99) { return 1; }; };\n"
+	  "	case i32 => { return 2; };\n"
+	  "	};\n"
+	  "	return 0;\n"
+	  "};\n", 0, 0, K_RUN, NULL },
+	{ "g89_ret_named_ctl",
+	  "package main;\n"
+	  "type base = struct { a: size, b: size };\n"
+	  "fn make() (void | base) = {\n"
+	  "	let x: base;\n"
+	  "	x.a = 4; x.b = 9;\n"
+	  "	return x;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "	let v = make();\n"
+	  "	match (v) {\n"
+	  "	case let s: base => { if (s.b != 9) { return 1; }; };\n"
+	  "	case void => { return 2; };\n"
+	  "	};\n"
+	  "	return 0;\n"
+	  "};\n", 0, 0, K_RUN, NULL },
 };
 
 static int