wwstage: nested-array outer-index esz = sub-array size (#270-2)

elemsizeofc drilled a 2D `[N][M]T` base's OUTER-index stride down to the
scalar T (the documented elemsizeof FOOTGUN: it bottoms out at the inner
prim size, 4 for [M]u32). The `direct != 8` short-circuit then returned
that scalar size, so wwstage emitted esz=$4 where cstage emits $12 (the
sub-array size, idx_eff(bt)->sub->size = sub.size*elen, type.c:121). The
runtime stayed self-consistent (write+read the same wrong stride) so it
masked until a CROSS-CELL access — a[0][j] and a[1][j] alias.

Fix: detect a nested-array element ([M]T inside [N][M]T) before the
short-circuit and return the element-array tinfo's natural .size, the
sub-array stride. wwstage-only; aligns up to cstage. w6c unchanged.

949 rows: nest2d_u32/u8/i32 (cross-cell write+readback, byte-id).

selfhost/cmd/w6c/main.combined.ww

@@ -16569,6 +16569,24 @@ fn elemsizeof(t: *node) i32 = {
 // whose Alias resolves to a tagged union (e.g. `[N]formattable`).
 fn elemsizeofc(c: *cgen, t: *node) i32 = {
 	if (t == nil) { return 1; };
+	// #270-2: a NESTED-array element ([N][M]T) — the OUTER index stride
+	// is the WHOLE sub-array [M]T, not the scalar T that elemsizeof
+	// drills down to (the documented elemsizeof FOOTGUN). elemsizeof
+	// returns the inner prim size (4 for [M]u32), so the `direct != 8`
+	// short-circuit below would mis-emit esz=$4 where cstage emits the
+	// sub-array stride $12. Mirror cstage esz = idx_eff(bt)->sub->size
+	// (cmd/w6c/cgen.c:4923): the element-array tinfo's natural size
+	// (sub.size*elen, type.c:121) IS the outer stride.
+	let nk: nkind = t.kind;
+	let nest: *node = nil;
+	if (nk == nkind.N_TPTR)   { nest = t.lhs; };
+	if (nk == nkind.N_TSLICE) { nest = t.lhs; };
+	if (nk == nkind.N_TARRAY) { nest = t.lhs; };
+	if (nest != nil && nest.kind == nkind.N_TARRAY) {
+		let eti: *tinfo = nest.type_: *tinfo;
+		for (eti != nil && eti.kind == tykind.TY_NAMED) { eti = eti.under; };
+		if (eti != nil) { return eti.size: i32; };
+	};
 	let direct: i32 = elemsizeof(t);
 	if (direct != 8) { return direct; };
 	let k: nkind = t.kind;

selfhost/cmd/wcc/cgenutil.ww

@@ -1059,6 +1059,24 @@ fn elemsizeof(t: *node) i32 = {
 // whose Alias resolves to a tagged union (e.g. `[N]formattable`).
 fn elemsizeofc(c: *cgen, t: *node) i32 = {
 	if (t == nil) { return 1; };
+	// #270-2: a NESTED-array element ([N][M]T) — the OUTER index stride
+	// is the WHOLE sub-array [M]T, not the scalar T that elemsizeof
+	// drills down to (the documented elemsizeof FOOTGUN). elemsizeof
+	// returns the inner prim size (4 for [M]u32), so the `direct != 8`
+	// short-circuit below would mis-emit esz=$4 where cstage emits the
+	// sub-array stride $12. Mirror cstage esz = idx_eff(bt)->sub->size
+	// (cmd/w6c/cgen.c:4923): the element-array tinfo's natural size
+	// (sub.size*elen, type.c:121) IS the outer stride.
+	let nk: nkind = t.kind;
+	let nest: *node = nil;
+	if (nk == nkind.N_TPTR)   { nest = t.lhs; };
+	if (nk == nkind.N_TSLICE) { nest = t.lhs; };
+	if (nk == nkind.N_TARRAY) { nest = t.lhs; };
+	if (nest != nil && nest.kind == nkind.N_TARRAY) {
+		let eti: *tinfo = nest.type_: *tinfo;
+		for (eti != nil && eti.kind == tykind.TY_NAMED) { eti = eti.under; };
+		if (eti != nil) { return eti.size: i32; };
+	};
 	let direct: i32 = elemsizeof(t);
 	if (direct != 8) { return direct; };
 	let k: nkind = t.kind;

selfhost/cmd/wwdump/main.combined.ww

@@ -16569,6 +16569,24 @@ fn elemsizeof(t: *node) i32 = {
 // whose Alias resolves to a tagged union (e.g. `[N]formattable`).
 fn elemsizeofc(c: *cgen, t: *node) i32 = {
 	if (t == nil) { return 1; };
+	// #270-2: a NESTED-array element ([N][M]T) — the OUTER index stride
+	// is the WHOLE sub-array [M]T, not the scalar T that elemsizeof
+	// drills down to (the documented elemsizeof FOOTGUN). elemsizeof
+	// returns the inner prim size (4 for [M]u32), so the `direct != 8`
+	// short-circuit below would mis-emit esz=$4 where cstage emits the
+	// sub-array stride $12. Mirror cstage esz = idx_eff(bt)->sub->size
+	// (cmd/w6c/cgen.c:4923): the element-array tinfo's natural size
+	// (sub.size*elen, type.c:121) IS the outer stride.
+	let nk: nkind = t.kind;
+	let nest: *node = nil;
+	if (nk == nkind.N_TPTR)   { nest = t.lhs; };
+	if (nk == nkind.N_TSLICE) { nest = t.lhs; };
+	if (nk == nkind.N_TARRAY) { nest = t.lhs; };
+	if (nest != nil && nest.kind == nkind.N_TARRAY) {
+		let eti: *tinfo = nest.type_: *tinfo;
+		for (eti != nil && eti.kind == tykind.TY_NAMED) { eti = eti.under; };
+		if (eti != nil) { return eti.size: i32; };
+	};
 	let direct: i32 = elemsizeof(t);
 	if (direct != 8) { return direct; };
 	let k: nkind = t.kind;

test/wcc/949_dotbase_addr_slice_run.c

@@ -1020,6 +1020,43 @@ static const struct row rows[] = {
 	  "fn mk() T = { let s: T; s.a=1u32;s.b=2u32;s.c=3u32; return s; };\n"
 	  "export fn main() i32 = { let v = mk(); return (v.a+v.b+v.c): i32; };\n",
 	  6, 1 },
+	/* #270-2: nested-array OUTER-index stride. `a[i][j]` on a 2D
+	 * `[N][M]T` indexes the outer dim by the WHOLE sub-array `[M]T`
+	 * (stride = M*sizeof(T)), then the inner dim by sizeof(T). wwstage's
+	 * elemsizeofc drilled the outer stride down to the scalar T (the
+	 * documented elemsizeof FOOTGUN) → esz=$4 where cstage emits $12
+	 * (the sub-array size, type.c:121 sub->size*len). Runtime stayed
+	 * self-consistent (write+read the same wrong stride) → masked until a
+	 * CROSS-CELL test writes a[0][j] AND a[1][j] at distinct cells and
+	 * reads both back. byteid=1: post-fix wwstage aligns up to cstage's
+	 * $12. Distinct element widths assert the stride is the sub-array
+	 * size, not a fixed literal. */
+	{ "nest2d_u32",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let a: [2][3]u32;\n"
+	  "	a[0][1] = 11u32;\n"
+	  "	a[1][1] = 22u32;\n"
+	  "	a[1][2] = 33u32;\n"
+	  "	return (a[0][1] + a[1][1] + a[1][2]): i32;\n"
+	  "};\n", 66, 1 },
+	{ "nest2d_u8",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let a: [2][3]u8;\n"
+	  "	a[0][2] = 10u8;\n"
+	  "	a[1][0] = 20u8;\n"
+	  "	a[1][2] = 30u8;\n"
+	  "	return (a[0][2] + a[1][0] + a[1][2]): i32;\n"
+	  "};\n", 60, 1 },
+	{ "nest2d_i32",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "	let a: [2][3]i32;\n"
+	  "	a[0][1] = 40;\n"
+	  "	a[1][1] = 88;\n"
+	  "	return (a[1][1] - a[0][1]): i32;\n"
+	  "};\n", 48, 1 },
 	{ NULL, NULL, 0, 0 }
 };