w6c: sret-wire the chained-leaf call store (o.a.b = wide())

The chained value-struct walker's N_CALL arm handled only <=24B
register returns; an sret-class rhs fell to the scalar tail and
stored ONE word (the sret dest pointer) into the leaf -- silent
truncation in BOTH stages, byteid-blind. Point the callee's hidden
RDI at the BP-relative leaf slot (the single-dot #234 arm verbatim);
a ptr-root/global chain joins the #234-tail loud-stop family.

cmd/w6c/cgen.c

@@ -6768,6 +6768,29 @@ cgexpr(Cg *c, Node *n, Local *locals)
 							}
 							break;
 						}
+						/* #234: over-cap sret STORE into a chained
+						 * value-struct leaf — `o.a.b = wide();`
+						 * (cg_sret_retsize > 0: a >24B struct OR an
+						 * over-cap tuple). Point the callee's hidden
+						 * RDI dest at the leaf slot so it writes the
+						 * WHOLE value there; pre-#234 this fell to
+						 * the scalar tail and stored ONE word (the
+						 * sret dest pointer). cg_sret_dest_off is
+						 * BP-relative ONLY, so a ptr-root/global
+						 * chain needs the runtime RDI-pointer dest
+						 * variant deferred to #234-tail and HARD-
+						 * STOPS loud (rule 7). */
+						if (n->rhs && n->rhs->kind == N_CALL
+						    && cg_sret_retsize(leaf_type) > 0) {
+							if (via_cx)
+								fatal("#234-tail: over-cap tuple "
+								    "sret store to chained "
+								    "ptr/global dest unsupported");
+							cg_sret_dest_off = base_disp + total_off;
+							cgexpr(c, n->rhs, locals);
+							cg_sret_dest_off = 0;
+							break;
+						}
 						/* TY_STRUCT terminal in the chained-DOT walker:
 						 * three rhs shapes — mirror of the single-dot
 						 * branch.

internal/wwfixture/types.ww

@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1745;
+def corpuscount: i32 = 1746;
 def errorcount: i32 = 346;
 def compilecount: i32 = 21;
 def runcount: i32 = 209;
-def runexitcount: i32 = 1169;
-def nativecount: i32 = 3490;
-def corpushash: str = "472974a00552c6b3f5db1fc6b0c1ed128d7554e134880196c833f11f99370f21";
+def runexitcount: i32 = 1170;
+def nativecount: i32 = 3492;
+def corpushash: str = "6afbab274e0c6d1479a2f0cb6e575b14f5e4ffd25a9cb8a3701307db97d3c489";
 
 type directive = enum i32 {
 	ERROR = 0,

selfhost/cmd/wcc/cgenexpr.ww

@@ -12052,6 +12052,31 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 					};
 					return;
 				};
+				// #234: over-cap sret STORE into a chained
+				// value-struct leaf — `o.a.b = wide();`
+				// (sretretsize > 0: a >24B struct OR an
+				// over-cap tuple). Point the callee's hidden
+				// RDI dest at the leaf slot so it writes the
+				// WHOLE value there; pre-#234 this fell to
+				// the scalar tail and stored ONE word (the
+				// sret dest pointer). c.sretdestoff is
+				// BP-relative ONLY, so a ptr-root/global
+				// chain needs the runtime RDI-pointer dest
+				// variant deferred to #234-tail and HARD-
+				// STOPS loud (rule 7).
+				if (n.rhs != nil
+				    && n.rhs.kind == syntax.nkind.N_CALL
+				    && sretretsizetn(c, leaftype) > 0) {
+					if (viacx) {
+						let m234: str = "#234-tail: over-cap tuple sret store to chained ptr/global dest unsupported\n";
+						os.write(2, m234.ptr, m234.len: u64);
+						os.exit(1);
+					};
+					c.sretdestoff = rootoff + totaloff;
+					cgexpr(c, n.rhs);
+					c.sretdestoff = 0;
+					return;
+				};
 				// TY_STRUCT terminal: three rhs shapes:
 				//   - N_IDENT: word-copy from the rhs local slot
 				//     (cgexpr is skipped — no whole-struct register

test/wcc/data/r234_chain_leaf_sret/case.ww

@@ -0,0 +1,29 @@
+//ww:run-exit 0
+// #234 chained-leaf arm: `o.a.b = f()` with a >24B sret-class return
+// routes the callee's hidden RDI at the leaf slot. Pre-fix both stages
+// fell to the scalar tail and stored ONE word (the sret dest pointer).
+package main;
+type big = struct { x: i64, y: i64, z: i64, w: i64 };
+type mid = struct { pad: i64, b: big };
+type outer = struct { a: mid };
+
+fn f() big = {
+	let r: big;
+	r.x = 11;
+	r.y = 22;
+	r.z = 33;
+	r.w = 44;
+	return r;
+};
+
+export fn main() i32 = {
+	let o: outer;
+	o.a.pad = 7;
+	o.a.b = f();
+	if (o.a.pad != 7) { return 1; };
+	if (o.a.b.x != 11) { return 2; };
+	if (o.a.b.y != 22) { return 3; };
+	if (o.a.b.z != 33) { return 4; };
+	if (o.a.b.w != 44) { return 5; };
+	return 0;
+};