wcc/ww: under-length array-literal tail zero-fills

The unspecified tail of a short array literal repeated the last value
instead of zeroing — wwstage only (cstage already zeroes; the review's
both-stages reading didn't survive ground truth). Zero-fill the tail
per the zero-value semantics ruling. Review item #13.

Makefile

@@ -253,6 +253,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_chainidx_run \
 	  $(BIN)/test_tupfieldsize_run \
 	  $(BIN)/test_tagtupfieldsize_run \
+	  $(BIN)/test_arrlit_tail_zero_run \
 	  $(BIN)/test_gunsigned_run \
 	  $(BIN)/test_taggedidx_run \
 	  $(BIN)/test_fnptrcollide_run \
@@ -712,6 +713,16 @@ $(BIN)/test_tagtupfieldsize_run: test/wcc/989_tagtupfieldsize_run.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+# 989_arrlit_tail_zero_run (#13): an under-length array literal zero-fills the
+# unspecified tail, not the last value. Builds+runs on BOTH driver twins
+# (rule-10), pinning the absolute value (pre-fix ww tail = last value).
+$(BIN)/test_arrlit_tail_zero_run: test/wcc/989_arrlit_tail_zero_run.c \
+		$(BIN)/ww $(BIN)/ww_ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 # 989_gunsigned_run (F7-c5, #25): a module-global unsigned ident on the
 # divide/shift/relational path must pick the unsigned opcode. Builds+runs on
 # BOTH driver twins (rule-10). CLASS-M — see the test header.

selfhost/cmd/w6c/main.combined.ww

@@ -42632,13 +42632,24 @@ fn emitarraylitbytes(c: *cgen, arrt: *tinfo, rhs: *node,
 	e = rhs.list;
 	let inrepeat: bool = false;
 	for (idx < alen) {
-		let v: u64 = last;
-		if (!inrepeat && e != nil) {
+		// #13: explicit elements fold normally; a `...` repeat replays the
+		// LAST value; the tail PAST the explicit elements (no `...`) is
+		// ZERO-filled. Pre-fix the default was `last`, so an under-length
+		// literal (`[4]u64 = [1, 2]`) repeated the last value into the tail
+		// instead of zero — cstage already zeroes (Hare: unspecified array
+		// elements are zeroed; the #16-task zero-value ruling); this aligns
+		// wwstage, a gate-blind cs!=ww divergence at the array-global path.
+		let v: u64 = 0u64;
+		if (inrepeat) {
+			v = last;
+		} else { if (e != nil) {
 			if (e.kind == nkind.N_FIELD) {
 				if (streq(e.str, "...")) {
 					inrepeat = true;
+					v = last;
 				} else {
 					e = e.next;
+					v = last;
 				};
 			} else {
 				let ev: *node = e;
@@ -42647,7 +42658,7 @@ fn emitarraylitbytes(c: *cgen, arrt: *tinfo, rhs: *node,
 				last = v;
 				e = e.next;
 			};
-		};
+		};};
 		let nb: u64 = v;
 		let bb: i32 = 0;
 		for (bb < esz) {

selfhost/cmd/wcc/cgen.ww

@@ -2101,13 +2101,24 @@ fn emitarraylitbytes(c: *cgen, arrt: *tinfo, rhs: *node,
 	e = rhs.list;
 	let inrepeat: bool = false;
 	for (idx < alen) {
-		let v: u64 = last;
-		if (!inrepeat && e != nil) {
+		// #13: explicit elements fold normally; a `...` repeat replays the
+		// LAST value; the tail PAST the explicit elements (no `...`) is
+		// ZERO-filled. Pre-fix the default was `last`, so an under-length
+		// literal (`[4]u64 = [1, 2]`) repeated the last value into the tail
+		// instead of zero — cstage already zeroes (Hare: unspecified array
+		// elements are zeroed; the #16-task zero-value ruling); this aligns
+		// wwstage, a gate-blind cs!=ww divergence at the array-global path.
+		let v: u64 = 0u64;
+		if (inrepeat) {
+			v = last;
+		} else { if (e != nil) {
 			if (e.kind == nkind.N_FIELD) {
 				if (streq(e.str, "...")) {
 					inrepeat = true;
+					v = last;
 				} else {
 					e = e.next;
+					v = last;
 				};
 			} else {
 				let ev: *node = e;
@@ -2116,7 +2127,7 @@ fn emitarraylitbytes(c: *cgen, arrt: *tinfo, rhs: *node,
 				last = v;
 				e = e.next;
 			};
-		};
+		};};
 		let nb: u64 = v;
 		let bb: i32 = 0;
 		for (bb < esz) {

selfhost/cmd/wwdump/main.combined.ww

@@ -42632,13 +42632,24 @@ fn emitarraylitbytes(c: *cgen, arrt: *tinfo, rhs: *node,
 	e = rhs.list;
 	let inrepeat: bool = false;
 	for (idx < alen) {
-		let v: u64 = last;
-		if (!inrepeat && e != nil) {
+		// #13: explicit elements fold normally; a `...` repeat replays the
+		// LAST value; the tail PAST the explicit elements (no `...`) is
+		// ZERO-filled. Pre-fix the default was `last`, so an under-length
+		// literal (`[4]u64 = [1, 2]`) repeated the last value into the tail
+		// instead of zero — cstage already zeroes (Hare: unspecified array
+		// elements are zeroed; the #16-task zero-value ruling); this aligns
+		// wwstage, a gate-blind cs!=ww divergence at the array-global path.
+		let v: u64 = 0u64;
+		if (inrepeat) {
+			v = last;
+		} else { if (e != nil) {
 			if (e.kind == nkind.N_FIELD) {
 				if (streq(e.str, "...")) {
 					inrepeat = true;
+					v = last;
 				} else {
 					e = e.next;
+					v = last;
 				};
 			} else {
 				let ev: *node = e;
@@ -42647,7 +42658,7 @@ fn emitarraylitbytes(c: *cgen, arrt: *tinfo, rhs: *node,
 				last = v;
 				e = e.next;
 			};
-		};
+		};};
 		let nb: u64 = v;
 		let bb: i32 = 0;
 		for (bb < esz) {

test/wcc/989_arrlit_tail_zero_run.c

@@ -0,0 +1,161 @@
+/*
+ * 989_arrlit_tail_zero_run (#13) — an UNDER-LENGTH array literal zero-fills
+ * the unspecified tail (Hare: unspecified array elements are zeroed; the
+ * #16-task zero-value ruling), not the last value.
+ *
+ * THE BUG (wwstage only, gate-blind cs!=ww): emitarraylitbytes' int-element
+ * tail loop defaulted each unfilled slot to `last` (the previously emitted
+ * value), so `let g: [4]u64 = [1, 2]` emitted [1,2,2,2] — the DATAW tail
+ * repeated \x02. cstage already zeroes the tail (cg_array_data). THE FIX:
+ * default the tail to 0; replay `last` only inside a `...` repeat.
+ *
+ * The float / struct / nested-array element paths already zero-filled their
+ * tails; only the int path defaulted to `last`. The str/slice element paths
+ * ride a different helper (emitstrarraydata / emitslicedata) and were also
+ * already correct — the str_tail row documents the family is uniformly
+ * zeroed, it is coverage, not a teeth on this fix.
+ *
+ *   row           | shape                          | exit (cs==ww)
+ *   --------------+--------------------------------+--------------
+ *   scalar_tail   | [4]u64=[1,2]; sum              | 3   (1+2+0+0)
+ *   repeat_ctl    | [4]u64=[7,9...]; sum (control) | 34  (7+9+9+9)
+ *   str_tail      | [4]str=["ab","cde"]; tail.len  | 5   (2+3, tail len 0)
+ * Pre-fix (int tail = last): scalar_tail ww=7 (1+2+2+2) != cs=3.
+ */
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/stat.h>
+#include <sys/wait.h>
+
+static int
+runwait(const char *cmd)
+{
+	int rc = system(cmd);
+	if (rc == -1) return -1;
+	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
+	return -1;
+}
+
+struct row { const char *label; const char *src; int want_exit; };
+
+static const struct row rows[] = {
+	{ "scalar_tail",
+	  "package main;\n"
+	  "let g: [4]u64 = [1, 2];\n"
+	  "export fn main() i32 = {\n"
+	  "	return (g[0] + g[1] + g[2] + g[3]): i32;\n"
+	  "};\n",
+	  3 },
+
+	{ "repeat_ctl",
+	  "package main;\n"
+	  "let g: [4]u64 = [7, 9...];\n"
+	  "export fn main() i32 = {\n"
+	  "	return (g[0] + g[1] + g[2] + g[3]): i32;\n"
+	  "};\n",
+	  34 },
+
+	{ "str_tail",
+	  "package main;\n"
+	  "let g: [4]str = [\"ab\", \"cde\"];\n"
+	  "export fn main() i32 = {\n"
+	  "	if (g[2].len != 0) { return 21; };\n"
+	  "	if (g[3].len != 0) { return 22; };\n"
+	  "	return (g[0].len + g[1].len): i32;\n"
+	  "};\n",
+	  5 },
+};
+
+static int
+run_build(const char *driver, const struct row *r, int i)
+{
+	char src[64], tmpdir[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/atz_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/atz_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -2;
+	fputs(r->src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, src);
+	int brc = runwait(cmd);
+
+	const char *base = strrchr(src, '/');
+	base = base ? base + 1 : src;
+	char outbin[128];
+	snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+	char *dot = strrchr(outbin, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+
+	int got = -1;
+	if (brc == 0) got = runwait(outbin);
+
+	unlink(src); unlink(outbin); rmdir(tmpdir);
+	return brc == 0 ? got : -1;
+}
+
+int
+main(void)
+{
+	const char *bin = getenv("BIN");
+	if (!bin) bin = "out/bin";
+	char absbin[1024];
+	if (bin[0] != '/') {
+		char cwd[1024];
+		if (getcwd(cwd, sizeof cwd) == NULL) return 1;
+		snprintf(absbin, sizeof absbin, "%s/%s", cwd, bin);
+		bin = absbin;
+	}
+
+	char cdrv[1024], wdrv[1024];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+	int have_ww = (access(wdrv, X_OK) == 0);
+
+	int n = (int)(sizeof rows / sizeof rows[0]);
+	int total = 0, fail = 0;
+
+	for (int i = 0; i < n; i++) {
+		total++;
+		int gc = run_build(cdrv, &rows[i], i);
+		if (gc < 0) {
+			fprintf(stderr, "arrlit_tail_zero[cstage][%s]: build/run "
+			    "failed (got %d)\n", rows[i].label, gc);
+			fail++;
+			continue;
+		}
+		if (gc != rows[i].want_exit) {
+			fprintf(stderr, "arrlit_tail_zero[cstage][%s]: exit=%d "
+			    "want=%d\n", rows[i].label, gc, rows[i].want_exit);
+			fail++;
+		}
+		if (!have_ww) {
+			fprintf(stderr, "arrlit_tail_zero: skip wwstage (no %s)\n", wdrv);
+			continue;
+		}
+		int gw = run_build(wdrv, &rows[i], i);
+		if (gw != gc) {
+			fprintf(stderr, "arrlit_tail_zero[%s]: cs=%d != ww=%d "
+			    "(tail fill divergence — #13)\n", rows[i].label, gc, gw);
+			fail++;
+		}
+		if (gw != rows[i].want_exit) {
+			fprintf(stderr, "arrlit_tail_zero[wwstage][%s]: exit=%d "
+			    "want=%d\n", rows[i].label, gw, rows[i].want_exit);
+			fail++;
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr, "arrlit_tail_zero_run: %d/%d checks failed\n",
+		    fail, total);
+		return 1;
+	}
+	printf("arrlit_tail_zero_run: %d/%d ok\n", total, total);
+	return 0;
+}