w6c+wcc/check: infer [_]T array length from initializer element count (fix #7)

`[_]T = [...]` (canonical Hare array-length inference) silently
miscompiled to a zero-length array: the parser already left the array
type's length child nil as the infer sentinel — distinct from an
explicit [N] — but neither checker stamped the real count, so `len(x)`
returned 0 with no diagnostic (rule-7 silent miscompile). Module-level
was worse on wwstage, where `x.len` on ANY global array (even an
explicit [N]) fell to the SB fallback and mis-emitted `MOVQ len(SB), AX`
(linker: undefined reference to len).

The length lives in the stamped TYPE and cgen already keys stride /
length / data-emission off it, so stamping the inferred count at the one
checker inference point closes it permanently (rob's #7 ruling):

  - check.c clet + module-level N_LET pass-2: count the initializer's
    elements and patch the array type's length (the Sym too, so a later
    x.len reads the inferred alen). No-init / non-array init can't infer
    -> loud error, never a silent zero-length array.
  - check.ww inferarraylen: the wwstage twin — stamp a synthesized
    N_INTLIT length child before resolvewalk caches the array tinfo;
    same loud-error rule. Idempotent for the module-level double-call.
  - cgenexpr.ww cgdot: the missing wwstage arm for a top-level [N]T
    global's .len / .ptr (cstage cgen.c:8011 already had it).
  - cgenutil.ww letslotsize: drop the now-redundant [_] slot-size
    intercept — a workaround for this very bug; the stamped length flows
    through the general slotsize path (rule 7).

Both stages converge byte-identical; new table-driven test 684 covers
[_]int/[_]str/[_]u8 local + module-level, len + element read-back,
dual-stage runtime + asm byte-id, plus three negative no-infer rows.

Makefile

@@ -241,6 +241,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_arr_elem_field_write \
 	  $(BIN)/test_arr_enum_elem \
 	  $(BIN)/test_arr_strslice_elem \
+	  $(BIN)/test_arr_infer_len \
 	  $(BIN)/test_dot_str_chained_arg \
 	  $(BIN)/test_dot_slice_arg \
 	  $(BIN)/test_dot_tagged_source \
@@ -561,6 +562,12 @@ $(BIN)/test_arr_strslice_elem: test/wcc/683_arr_strslice_elem.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_arr_infer_len: test/wcc/684_arr_infer_len.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_dot_str_chained_arg: test/wcc/692_dot_str_chained_arg.c $(BIN)/ww \
 		$(BIN)/w6c $(BIN)/w6a $(BIN)/w6l \
 		$(BIN)/ww_ww $(BIN)/w6c_ww $(BIN)/w6a_ww $(BIN)/w6l_ww \

cmd/wcc/check.c

@@ -1901,10 +1901,14 @@ clet(Checker *c, Node *n)
 	if (n->rhs) initt = cexpr(c, n->rhs);
 	c->alloc_octx = saved_octx;
 	/* `let xs: [_]T = arrlit;` — fill in the inferred length from the
-	 * initialiser. `resolve_type` left alen=0 as a sentinel. */
+	 * initialiser. `resolve_type` left alen=0 as a sentinel. A `[_]T`
-	if (declared && declared->kind == TY_ARRAY && declared->alen == 0 &&
+	 * with no array-literal initialiser (no init at all, or a non-array
-	    initt) {
+	 * init) can't infer its length — that is a loud error, never a
-		Type *iu = (initt->kind == TY_NAMED) ? initt->under : initt;
+	 * silent zero-length array (rule 7, #7). */
+	if (declared && declared->kind == TY_ARRAY && declared->alen == 0) {
+		Type *iu = initt
+		    ? ((initt->kind == TY_NAMED) ? initt->under : initt)
+		    : NULL;
 		if (iu && iu->kind == TY_ARRAY)
 			declared = type_array(c->a, declared->sub, iu->alen);
 		else
@@ -2545,12 +2549,46 @@ check_file(Checker *c, Node *file)
 		case N_LET: {
 			if (d->rhs) {
 				Type *rt = cexpr(c, d->rhs);
+				/* `let xs: [_]T = arrlit;` at module level — infer the
+				 * length from the initialiser, the same patch clet
+				 * applies for a local let (#7). pass-1.5 resolve_type
+				 * left alen=0 as the sentinel; patching d->type feeds
+				 * cgen's letvars registration (lv->type = d->type),
+				 * which both lays the full-length DATA row and reads
+				 * the right `.len`. */
+				if (d->type && d->type->kind == TY_ARRAY
+				    && d->type->alen == 0) {
+					Type *iu = rt
+					    ? ((rt->kind == TY_NAMED) ? rt->under : rt)
+					    : NULL;
+					if (iu && iu->kind == TY_ARRAY) {
+						d->type = type_array(c->a,
+						    d->type->sub, iu->alen);
+						/* The Sym installed in pass-1.5 still
+						 * carries the alen=0 sentinel; a later
+						 * `x.len` resolves `x` through the Sym
+						 * (its type stamps n->lhs->type, which
+						 * cgen reads as u->alen). Re-point it at
+						 * the inferred-length type too. */
+						Sym *s = scope_lookup_local(c->cur,
+						    d->str);
+						if (s) s->type = d->type;
+					} else
+						err(c, d->pos, "[_]T needs an "
+						    "array-literal initialiser");
+				}
 				if (d->type == NULL) d->type = type_default(rt);
 				if (d->type && rt != ty_err && d->type != ty_err
 				    && !type_assignable(d->type, rt)
 				    && !arrlit_init_fits(c, d->type, d->rhs))
 					err(c, d->pos, "let %s init not assignable",
 					    d->str);
+			} else if (d->type && d->type->kind == TY_ARRAY
+			    && d->type->alen == 0) {
+				/* `let x: [_]T;` — no initialiser, length can't be
+				 * inferred (rule 7, #7). */
+				err(c, d->pos, "[_]T needs an array-literal "
+				    "initialiser");
 			}
 			break;
 		}

selfhost/cmd/w6c/main.combined.ww

@@ -14313,8 +14313,48 @@ fn checkassign(c: *checker, n: *node) void = {
 	n.rhs = desugararrayslice(c, ltn, rtn, n.rhs);
 };
 
+// inferarraylen — `let xs: [_]T = arrlit;` length inference (#7). The
+// parser leaves a `[_]` array's length child nil as the infer sentinel
+// (parse.ww, mirror cstage parse.c:186). Count the array-literal's
+// elements (skipping the `...` repeat marker, same walk as the N_ARRLIT
+// exprtype at L2905) and stamp a synthesized N_INTLIT length node so
+// tinfofornode / cgen / `.len` all read the real count — the wwstage
+// analogue of cstage clet's `declared = type_array(.., iu->alen)` patch.
+// A `[_]T` with no array-literal initialiser can't infer: loud error,
+// never a silent zero-length array (rule 7). Idempotent (skips once the
+// length child is set), so the module-level double-call (checkfile's
+// pre-resolvewalk call + checkletassign here) raises at most one error.
+fn inferarraylen(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) { return; };
+	if (n.lhs.kind != nkind.N_TARRAY) { return; };
+	if (n.lhs.rhs != nil) { return; };	// explicit [N] or already inferred
+	if (n.rhs == nil || n.rhs.kind != nkind.N_ARRLIT) {
+		cerr("error: [_]T needs an array-literal initialiser\n");
+		c.errs += 1;
+		let z: *node = newnode(nkind.N_INTLIT, "", 0, 0);
+		z.uval = 0u64;
+		n.lhs.rhs = z;	// sentinel: idempotent, error already raised
+		return;
+	};
+	let cnt: u64 = 0u64;
+	let it: *node = n.rhs.list;
+	for (it != nil) {
+		let skip: bool = false;
+		if (it.kind == nkind.N_FIELD) {
+			if (streq(it.str, "...")) { skip = true; };
+		};
+		if (!skip) { cnt += 1u64; };
+		it = it.next;
+	};
+	let cn: *node = newnode(nkind.N_INTLIT, "", 0, 0);
+	cn.uval = cnt;
+	n.lhs.rhs = cn;
+};
+
 fn checkletassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
+	inferarraylen(c, n);	// #7: must run before the n.rhs==nil bail
 	if (n.rhs == nil) { return; };	// no init
 	// hint = nil for A.6.0; A.6.1 will pass n.lhs once STRUCTLIT/ARRLIT
 	// arms consume it. Plumbing-only at this point.
@@ -14870,6 +14910,12 @@ export fn checkfile(c: *checker, file: *node) void = {
 		case nkind.N_TYPEDECL:
 			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
 		case nkind.N_LET:
+			// #7: a module-level `let xs: [_]T = arrlit;` must infer
+			// its length BEFORE resolvewalk stamps d.lhs's tinfo —
+			// otherwise the array tinfo caches the alen=0 sentinel and
+			// the patched length child never reaches the size/data
+			// reads. Idempotent with the checkletassign call below.
+			inferarraylen(c, d);
 			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
 			if (d.rhs != nil) { resolvewalk(c, d.rhs); };
 			// #130: top-level let assignability — the subtree
@@ -17396,50 +17442,10 @@ fn inferletcalltype(c: *cgen, rhs: *node) *node = {
 // tagged-init branch writes past the local and tramples the next
 // slot.
 export fn letslotsize(c: *cgen, n: *node) i32 = {
-	// `[_]T = arrlit;` — inferred-length array. slotsize would
+	// `[_]T = arrlit;` inferred-length arrays no longer need a slot-size
-	// return elem_size * 1 (treating missing length as 1); intercept
+	// intercept here: the checker (inferarraylen, check.ww) stamps the
-	// and compute the real count first.
+	// real element count onto the array type's length child before cgen
-	if (n.lhs != nil) {
+	// runs, so slotsize reads it like any explicit `[N]T` (#7).
-		if (n.lhs.kind == nkind.N_TARRAY) {
-			if (n.lhs.rhs == nil) {
-				if (n.rhs != nil) {
-					if (n.rhs.kind == nkind.N_ARRLIT) {
-						let elemn: *node = n.lhs.lhs;
-						let esz: i32 = 8;
-						if (elemn != nil) {
-							if (elemn.kind == nkind.N_TNAME) {
-								// Composite primitive: `str` is 16B
-								// (ptr+len) — primsize returns 0 for
-								// it, so it'd slot 8B without this.
-								if (streq(elemn.str, "str")) {
-									esz = primtypesize("str"): i32;
-								} else {
-									let ps: i32 = primsize(elemn.str);
-									if (ps > 0) { esz = ps; };
-								};
-							};
-						};
-						let cnt: i32 = 0;
-						let e: *node = n.rhs.list;
-						for (e != nil) {
-							let adv: bool = true;
-							if (e.kind == nkind.N_FIELD) {
-								if (streq(e.str, "...")) {
-									e = nil;
-									adv = false;
-								};
-							};
-							if (adv) {
-								cnt += 1;
-								e = e.next;
-							};
-						};
-						return esz * cnt;
-					};
-				};
-			};
-		};
-	};
 	if (n.lhs != nil) { return slotsize(c, n.lhs); };
 	// Annotation-less init: defer to the call's return type if we
 	// can infer it. Tagged-union returns need 24B; everything else
@@ -22165,6 +22171,41 @@ fn cgdot(c: *cgen, n: *node) void = {
 			};
 		};
 	};
+	// Top-level [N]T global pseudo-fields (#7): `.len` is the static
+	// element count (immediate from the array type node's length child);
+	// `.ptr` is the array's base address (LEAQ name(SB)). Without this a
+	// module-level array's `x.len` falls to the module-qualified SB
+	// fallback below and mis-emits `MOVQ len(SB), AX` (linker: undefined
+	// reference to len). Mirror of the local-array arm above and cstage
+	// cg_base_cap's `aimm(bu->alen)` immediate (cgen.c:1692).
+	if (lhs != nil) {
+		if (lhs.kind == nkind.N_IDENT) {
+			let gtn: *node = letvartnode(c, lhs.str);
+			if (gtn != nil) {
+				if (gtn.kind == nkind.N_TARRAY) {
+					if (streq(fld, "ptr")) {
+						emitline("\tLEAQ\t");
+						emitsymname(c, lhs.str);
+						emitline("(SB), AX\n");
+						return;
+					};
+					if (streq(fld, "len")) {
+						let lenn: *node = gtn.rhs;
+						let alen: i64 = 0i64;
+						if (lenn != nil) {
+							if (lenn.kind == nkind.N_INTLIT) {
+								alen = lenn.uval: i64;
+							};
+						};
+						emitline("\tMOVQ\t$");
+						emitint(alen);
+						emitline(", AX\n");
+						return;
+					};
+				};
+			};
+		};
+	};
 	// Top-level struct global field read — LEAQ name(SB), CX then
 	// load at fi.foff(CX). Mirrors the local "Direct struct local"
 	// branch above, swapping the BP frame slot for the global VA.

selfhost/cmd/wcc/cgenexpr.ww

@@ -2441,6 +2441,41 @@ fn cgdot(c: *cgen, n: *node) void = {
 			};
 		};
 	};
+	// Top-level [N]T global pseudo-fields (#7): `.len` is the static
+	// element count (immediate from the array type node's length child);
+	// `.ptr` is the array's base address (LEAQ name(SB)). Without this a
+	// module-level array's `x.len` falls to the module-qualified SB
+	// fallback below and mis-emits `MOVQ len(SB), AX` (linker: undefined
+	// reference to len). Mirror of the local-array arm above and cstage
+	// cg_base_cap's `aimm(bu->alen)` immediate (cgen.c:1692).
+	if (lhs != nil) {
+		if (lhs.kind == nkind.N_IDENT) {
+			let gtn: *node = letvartnode(c, lhs.str);
+			if (gtn != nil) {
+				if (gtn.kind == nkind.N_TARRAY) {
+					if (streq(fld, "ptr")) {
+						emitline("\tLEAQ\t");
+						emitsymname(c, lhs.str);
+						emitline("(SB), AX\n");
+						return;
+					};
+					if (streq(fld, "len")) {
+						let lenn: *node = gtn.rhs;
+						let alen: i64 = 0i64;
+						if (lenn != nil) {
+							if (lenn.kind == nkind.N_INTLIT) {
+								alen = lenn.uval: i64;
+							};
+						};
+						emitline("\tMOVQ\t$");
+						emitint(alen);
+						emitline(", AX\n");
+						return;
+					};
+				};
+			};
+		};
+	};
 	// Top-level struct global field read — LEAQ name(SB), CX then
 	// load at fi.foff(CX). Mirrors the local "Direct struct local"
 	// branch above, swapping the BP frame slot for the global VA.

selfhost/cmd/wcc/cgenutil.ww

@@ -1808,50 +1808,10 @@ fn inferletcalltype(c: *cgen, rhs: *node) *node = {
 // tagged-init branch writes past the local and tramples the next
 // slot.
 export fn letslotsize(c: *cgen, n: *node) i32 = {
-	// `[_]T = arrlit;` — inferred-length array. slotsize would
+	// `[_]T = arrlit;` inferred-length arrays no longer need a slot-size
-	// return elem_size * 1 (treating missing length as 1); intercept
+	// intercept here: the checker (inferarraylen, check.ww) stamps the
-	// and compute the real count first.
+	// real element count onto the array type's length child before cgen
-	if (n.lhs != nil) {
+	// runs, so slotsize reads it like any explicit `[N]T` (#7).
-		if (n.lhs.kind == nkind.N_TARRAY) {
-			if (n.lhs.rhs == nil) {
-				if (n.rhs != nil) {
-					if (n.rhs.kind == nkind.N_ARRLIT) {
-						let elemn: *node = n.lhs.lhs;
-						let esz: i32 = 8;
-						if (elemn != nil) {
-							if (elemn.kind == nkind.N_TNAME) {
-								// Composite primitive: `str` is 16B
-								// (ptr+len) — primsize returns 0 for
-								// it, so it'd slot 8B without this.
-								if (streq(elemn.str, "str")) {
-									esz = primtypesize("str"): i32;
-								} else {
-									let ps: i32 = primsize(elemn.str);
-									if (ps > 0) { esz = ps; };
-								};
-							};
-						};
-						let cnt: i32 = 0;
-						let e: *node = n.rhs.list;
-						for (e != nil) {
-							let adv: bool = true;
-							if (e.kind == nkind.N_FIELD) {
-								if (streq(e.str, "...")) {
-									e = nil;
-									adv = false;
-								};
-							};
-							if (adv) {
-								cnt += 1;
-								e = e.next;
-							};
-						};
-						return esz * cnt;
-					};
-				};
-			};
-		};
-	};
 	if (n.lhs != nil) { return slotsize(c, n.lhs); };
 	// Annotation-less init: defer to the call's return type if we
 	// can infer it. Tagged-union returns need 24B; everything else

selfhost/cmd/wcc/check.ww

@@ -3946,8 +3946,48 @@ fn checkassign(c: *checker, n: *node) void = {
 	n.rhs = desugararrayslice(c, ltn, rtn, n.rhs);
 };
 
+// inferarraylen — `let xs: [_]T = arrlit;` length inference (#7). The
+// parser leaves a `[_]` array's length child nil as the infer sentinel
+// (parse.ww, mirror cstage parse.c:186). Count the array-literal's
+// elements (skipping the `...` repeat marker, same walk as the N_ARRLIT
+// exprtype at L2905) and stamp a synthesized N_INTLIT length node so
+// tinfofornode / cgen / `.len` all read the real count — the wwstage
+// analogue of cstage clet's `declared = type_array(.., iu->alen)` patch.
+// A `[_]T` with no array-literal initialiser can't infer: loud error,
+// never a silent zero-length array (rule 7). Idempotent (skips once the
+// length child is set), so the module-level double-call (checkfile's
+// pre-resolvewalk call + checkletassign here) raises at most one error.
+fn inferarraylen(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) { return; };
+	if (n.lhs.kind != nkind.N_TARRAY) { return; };
+	if (n.lhs.rhs != nil) { return; };	// explicit [N] or already inferred
+	if (n.rhs == nil || n.rhs.kind != nkind.N_ARRLIT) {
+		cerr("error: [_]T needs an array-literal initialiser\n");
+		c.errs += 1;
+		let z: *node = newnode(nkind.N_INTLIT, "", 0, 0);
+		z.uval = 0u64;
+		n.lhs.rhs = z;	// sentinel: idempotent, error already raised
+		return;
+	};
+	let cnt: u64 = 0u64;
+	let it: *node = n.rhs.list;
+	for (it != nil) {
+		let skip: bool = false;
+		if (it.kind == nkind.N_FIELD) {
+			if (streq(it.str, "...")) { skip = true; };
+		};
+		if (!skip) { cnt += 1u64; };
+		it = it.next;
+	};
+	let cn: *node = newnode(nkind.N_INTLIT, "", 0, 0);
+	cn.uval = cnt;
+	n.lhs.rhs = cn;
+};
+
 fn checkletassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
+	inferarraylen(c, n);	// #7: must run before the n.rhs==nil bail
 	if (n.rhs == nil) { return; };	// no init
 	// hint = nil for A.6.0; A.6.1 will pass n.lhs once STRUCTLIT/ARRLIT
 	// arms consume it. Plumbing-only at this point.
@@ -4503,6 +4543,12 @@ export fn checkfile(c: *checker, file: *node) void = {
 		case nkind.N_TYPEDECL:
 			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
 		case nkind.N_LET:
+			// #7: a module-level `let xs: [_]T = arrlit;` must infer
+			// its length BEFORE resolvewalk stamps d.lhs's tinfo —
+			// otherwise the array tinfo caches the alen=0 sentinel and
+			// the patched length child never reaches the size/data
+			// reads. Idempotent with the checkletassign call below.
+			inferarraylen(c, d);
 			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
 			if (d.rhs != nil) { resolvewalk(c, d.rhs); };
 			// #130: top-level let assignability — the subtree

selfhost/cmd/wwdump/main.combined.ww

@@ -14313,8 +14313,48 @@ fn checkassign(c: *checker, n: *node) void = {
 	n.rhs = desugararrayslice(c, ltn, rtn, n.rhs);
 };
 
+// inferarraylen — `let xs: [_]T = arrlit;` length inference (#7). The
+// parser leaves a `[_]` array's length child nil as the infer sentinel
+// (parse.ww, mirror cstage parse.c:186). Count the array-literal's
+// elements (skipping the `...` repeat marker, same walk as the N_ARRLIT
+// exprtype at L2905) and stamp a synthesized N_INTLIT length node so
+// tinfofornode / cgen / `.len` all read the real count — the wwstage
+// analogue of cstage clet's `declared = type_array(.., iu->alen)` patch.
+// A `[_]T` with no array-literal initialiser can't infer: loud error,
+// never a silent zero-length array (rule 7). Idempotent (skips once the
+// length child is set), so the module-level double-call (checkfile's
+// pre-resolvewalk call + checkletassign here) raises at most one error.
+fn inferarraylen(c: *checker, n: *node) void = {
+	if (n == nil) { return; };
+	if (n.lhs == nil) { return; };
+	if (n.lhs.kind != nkind.N_TARRAY) { return; };
+	if (n.lhs.rhs != nil) { return; };	// explicit [N] or already inferred
+	if (n.rhs == nil || n.rhs.kind != nkind.N_ARRLIT) {
+		cerr("error: [_]T needs an array-literal initialiser\n");
+		c.errs += 1;
+		let z: *node = newnode(nkind.N_INTLIT, "", 0, 0);
+		z.uval = 0u64;
+		n.lhs.rhs = z;	// sentinel: idempotent, error already raised
+		return;
+	};
+	let cnt: u64 = 0u64;
+	let it: *node = n.rhs.list;
+	for (it != nil) {
+		let skip: bool = false;
+		if (it.kind == nkind.N_FIELD) {
+			if (streq(it.str, "...")) { skip = true; };
+		};
+		if (!skip) { cnt += 1u64; };
+		it = it.next;
+	};
+	let cn: *node = newnode(nkind.N_INTLIT, "", 0, 0);
+	cn.uval = cnt;
+	n.lhs.rhs = cn;
+};
+
 fn checkletassign(c: *checker, n: *node) void = {
 	if (n == nil) { return; };
+	inferarraylen(c, n);	// #7: must run before the n.rhs==nil bail
 	if (n.rhs == nil) { return; };	// no init
 	// hint = nil for A.6.0; A.6.1 will pass n.lhs once STRUCTLIT/ARRLIT
 	// arms consume it. Plumbing-only at this point.
@@ -14870,6 +14910,12 @@ export fn checkfile(c: *checker, file: *node) void = {
 		case nkind.N_TYPEDECL:
 			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
 		case nkind.N_LET:
+			// #7: a module-level `let xs: [_]T = arrlit;` must infer
+			// its length BEFORE resolvewalk stamps d.lhs's tinfo —
+			// otherwise the array tinfo caches the alen=0 sentinel and
+			// the patched length child never reaches the size/data
+			// reads. Idempotent with the checkletassign call below.
+			inferarraylen(c, d);
 			if (d.lhs != nil) { resolvewalk(c, d.lhs); };
 			if (d.rhs != nil) { resolvewalk(c, d.rhs); };
 			// #130: top-level let assignability — the subtree
@@ -17396,50 +17442,10 @@ fn inferletcalltype(c: *cgen, rhs: *node) *node = {
 // tagged-init branch writes past the local and tramples the next
 // slot.
 export fn letslotsize(c: *cgen, n: *node) i32 = {
-	// `[_]T = arrlit;` — inferred-length array. slotsize would
+	// `[_]T = arrlit;` inferred-length arrays no longer need a slot-size
-	// return elem_size * 1 (treating missing length as 1); intercept
+	// intercept here: the checker (inferarraylen, check.ww) stamps the
-	// and compute the real count first.
+	// real element count onto the array type's length child before cgen
-	if (n.lhs != nil) {
+	// runs, so slotsize reads it like any explicit `[N]T` (#7).
-		if (n.lhs.kind == nkind.N_TARRAY) {
-			if (n.lhs.rhs == nil) {
-				if (n.rhs != nil) {
-					if (n.rhs.kind == nkind.N_ARRLIT) {
-						let elemn: *node = n.lhs.lhs;
-						let esz: i32 = 8;
-						if (elemn != nil) {
-							if (elemn.kind == nkind.N_TNAME) {
-								// Composite primitive: `str` is 16B
-								// (ptr+len) — primsize returns 0 for
-								// it, so it'd slot 8B without this.
-								if (streq(elemn.str, "str")) {
-									esz = primtypesize("str"): i32;
-								} else {
-									let ps: i32 = primsize(elemn.str);
-									if (ps > 0) { esz = ps; };
-								};
-							};
-						};
-						let cnt: i32 = 0;
-						let e: *node = n.rhs.list;
-						for (e != nil) {
-							let adv: bool = true;
-							if (e.kind == nkind.N_FIELD) {
-								if (streq(e.str, "...")) {
-									e = nil;
-									adv = false;
-								};
-							};
-							if (adv) {
-								cnt += 1;
-								e = e.next;
-							};
-						};
-						return esz * cnt;
-					};
-				};
-			};
-		};
-	};
 	if (n.lhs != nil) { return slotsize(c, n.lhs); };
 	// Annotation-less init: defer to the call's return type if we
 	// can infer it. Tagged-union returns need 24B; everything else
@@ -22165,6 +22171,41 @@ fn cgdot(c: *cgen, n: *node) void = {
 			};
 		};
 	};
+	// Top-level [N]T global pseudo-fields (#7): `.len` is the static
+	// element count (immediate from the array type node's length child);
+	// `.ptr` is the array's base address (LEAQ name(SB)). Without this a
+	// module-level array's `x.len` falls to the module-qualified SB
+	// fallback below and mis-emits `MOVQ len(SB), AX` (linker: undefined
+	// reference to len). Mirror of the local-array arm above and cstage
+	// cg_base_cap's `aimm(bu->alen)` immediate (cgen.c:1692).
+	if (lhs != nil) {
+		if (lhs.kind == nkind.N_IDENT) {
+			let gtn: *node = letvartnode(c, lhs.str);
+			if (gtn != nil) {
+				if (gtn.kind == nkind.N_TARRAY) {
+					if (streq(fld, "ptr")) {
+						emitline("\tLEAQ\t");
+						emitsymname(c, lhs.str);
+						emitline("(SB), AX\n");
+						return;
+					};
+					if (streq(fld, "len")) {
+						let lenn: *node = gtn.rhs;
+						let alen: i64 = 0i64;
+						if (lenn != nil) {
+							if (lenn.kind == nkind.N_INTLIT) {
+								alen = lenn.uval: i64;
+							};
+						};
+						emitline("\tMOVQ\t$");
+						emitint(alen);
+						emitline(", AX\n");
+						return;
+					};
+				};
+			};
+		};
+	};
 	// Top-level struct global field read — LEAQ name(SB), CX then
 	// load at fi.foff(CX). Mirrors the local "Direct struct local"
 	// branch above, swapping the BP frame slot for the global VA.

test/wcc/684_arr_infer_len.c

@@ -0,0 +1,349 @@
+/*
+ * 684_arr_infer_len — cstage and wwstage agree, byte-for-byte and at
+ * runtime, that a `[_]T = [...]` array infers its length from the
+ * initialiser's element count (task #7, a canonical Hare form).
+ *
+ * The bug: `[_]T` silently miscompiled to a zero-length array. The
+ * parser already left the array type's length child nil as the infer
+ * sentinel (distinct from an explicit `[N]`), but neither checker
+ * stamped the real count — so `len(x)` / `x.len` returned 0 with no
+ * diagnostic (rule-7 silent miscompile). Module-level was worse on
+ * wwstage: `x.len` on ANY global array (even explicit `[N]`) fell to
+ * the SB fallback and mis-emitted `MOVQ len(SB), AX` (linker:
+ * undefined reference to len).
+ *
+ * The fix (BOTH stages, converged byte-identical):
+ *   - checker (cmd/wcc/check.c clet + module-level N_LET;
+ *     selfhost/cmd/wcc/check.ww inferarraylen): count the array-literal
+ *     elements and stamp the length onto the array TYPE. cgen already
+ *     keys stride/length/data off the stamped length, so it "just
+ *     works" (rob's #7 ruling). A `[_]T` with no array-literal init
+ *     can't infer → LOUD error, never a silent zero-length array.
+ *   - cgen (selfhost/cmd/wcc/cgenexpr.ww cgdot): a top-level `[N]T`
+ *     global's `.len` / `.ptr` pseudo-fields, the wwstage arm that was
+ *     missing (cstage cgen.c:8011 already handled it).
+ *
+ * Coverage: `[_]int` / `[_]str` / `[_]u8`, both local and module-level,
+ * `len` read-back and element read-back, dual-stage runtime + asm
+ * byte-id. Mutation-sanity: the old collapse-to-0 fails every `*_len`
+ * row (len would be 0, not the count). Plus three negative rows where
+ * `[_]T` can't infer (no init / non-array init) — both stages must
+ * FAIL the build.
+ *
+ *   row             | shape                                | want
+ *   ----------------+--------------------------------------+------
+ *   local_int_len   | local [_]int=[10,20,30,40], x.len    | 4
+ *   local_int_elem  | local, x[2]                           | 30
+ *   mod_int_len     | global [_]int=[..], x.len            | 4
+ *   mod_int_elem    | global, x[2]                          | 30
+ *   local_str_len   | local [_]str=["a","b","c"], x.len    | 3
+ *   local_str_elem  | local [_]str=["ab","cde"], x[0].len  | 2
+ *   mod_str_len     | global [_]str=["a","b","c"], x.len   | 3
+ *   local_u8_len    | local [_]u8=[1..5], x.len            | 5
+ *   local_u8_elem   | local, x[4]                           | 5
+ */
+#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; };
+
+static const struct row rows[] = {
+	{ "local_int_len",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]int = [10, 20, 30, 40];\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  4 },
+
+	{ "local_int_elem",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]int = [10, 20, 30, 40];\n"
+	  "\treturn x[2]: i32;\n"
+	  "};\n",
+	  30 },
+
+	{ "mod_int_len",
+	  "package main;\n"
+	  "let x: [_]int = [10, 20, 30, 40];\n"
+	  "export fn main() i32 = {\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  4 },
+
+	{ "mod_int_elem",
+	  "package main;\n"
+	  "let x: [_]int = [10, 20, 30, 40];\n"
+	  "export fn main() i32 = {\n"
+	  "\treturn x[2]: i32;\n"
+	  "};\n",
+	  30 },
+
+	{ "local_str_len",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]str = [\"a\", \"b\", \"c\"];\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  3 },
+
+	{ "local_str_elem",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]str = [\"ab\", \"cde\"];\n"
+	  "\treturn x[0].len: i32;\n"
+	  "};\n",
+	  2 },
+
+	{ "mod_str_len",
+	  "package main;\n"
+	  "let x: [_]str = [\"a\", \"b\", \"c\"];\n"
+	  "export fn main() i32 = {\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  3 },
+
+	{ "local_u8_len",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]u8 = [1u8, 2u8, 3u8, 4u8, 5u8];\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  5 },
+
+	{ "local_u8_elem",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]u8 = [1u8, 2u8, 3u8, 4u8, 5u8];\n"
+	  "\treturn x[4]: i32;\n"
+	  "};\n",
+	  5 },
+};
+
+/* `[_]T` that can't infer its length — both stages must FAIL the build
+ * (loud diagnostic, not a silent zero-length array). */
+static const char *neg[] = {
+	/* no init, local */
+	"package main;\n"
+	"export fn main() i32 = {\n"
+	"\tlet x: [_]str;\n"
+	"\treturn 0;\n"
+	"};\n",
+	/* no init, module-level */
+	"package main;\n"
+	"let x: [_]int;\n"
+	"export fn main() i32 = { return 0; };\n",
+	/* non-array initialiser */
+	"package main;\n"
+	"export fn main() i32 = {\n"
+	"\tlet x: [_]int = 5;\n"
+	"\treturn 0;\n"
+	"};\n",
+};
+
+static int
+run_driver(const char *driver, const struct row *r, int i)
+{
+	char src[64], tmpdir[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/ail_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/ail_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: build via %s failed\n",
+		    r->label, driver);
+		unlink(src); rmdir(tmpdir);
+		return -1;
+	}
+
+	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 = runwait(outbin);
+
+	unlink(src); unlink(outbin); rmdir(tmpdir);
+	return got;
+}
+
+/* build_should_fail — a `[_]T` that can't infer must error on `driver`;
+ * returns 0 when the build correctly FAILS, non-zero when it wrongly
+ * succeeded. */
+static int
+build_should_fail(const char *driver, const char *src, int i)
+{
+	char s[64], tmpdir[64], cmd[1024];
+	snprintf(s, sizeof s, "/tmp/ailn_%d_%d.ww", getpid(), i);
+	snprintf(tmpdir, sizeof tmpdir, "/tmp/ailn_%d_d_%d", getpid(), i);
+
+	FILE *f = fopen(s, "wb");
+	if (!f) return -1;
+	fputs(src, f);
+	fclose(f);
+
+	mkdir(tmpdir, 0755);
+	snprintf(cmd, sizeof cmd, "cd %s && %s build %s 2>/dev/null",
+	    tmpdir, driver, s);
+	int rc = runwait(cmd);
+	unlink(s);
+	/* clean any emitted binary */
+	const char *base = strrchr(s, '/');
+	base = base ? base + 1 : s;
+	char outbin[128];
+	snprintf(outbin, sizeof outbin, "%s/%s", tmpdir, base);
+	char *dot = strrchr(outbin, '.');
+	if (dot && strcmp(dot, ".ww") == 0) *dot = '\0';
+	unlink(outbin);
+	rmdir(tmpdir);
+	return rc == 0 ? -1 : 0;	/* build must NOT succeed */
+}
+
+/* asm_byte_identical — w6c vs w6c_ww .s for the same source must match. */
+static int
+asm_byte_identical(const char *bin, const struct row *r, int i)
+{
+	char src[64], cs[64], ws[64], cmd[1024];
+	snprintf(src, sizeof src, "/tmp/ail_asm_%d_%d.ww", getpid(), i);
+	snprintf(cs,  sizeof cs,  "/tmp/ail_asm_%d_%d_c.s", getpid(), i);
+	snprintf(ws,  sizeof ws,  "/tmp/ail_asm_%d_%d_w.s", getpid(), i);
+
+	FILE *f = fopen(src, "wb");
+	if (!f) return -1;
+	fputs(r->src, f);
+	fclose(f);
+
+	snprintf(cmd, sizeof cmd, "%s/w6c -o %s %s 2>/dev/null", bin, cs, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: w6c errored\n", r->label);
+		unlink(src);
+		return -1;
+	}
+	snprintf(cmd, sizeof cmd, "%s/w6c_ww -o %s %s 2>/dev/null",
+	    bin, ws, src);
+	if (runwait(cmd) != 0) {
+		fprintf(stderr, "row[%s]: w6c_ww errored\n", r->label);
+		unlink(src); unlink(cs);
+		return -1;
+	}
+
+	FILE *fc = fopen(cs, "rb");
+	FILE *fw = fopen(ws, "rb");
+	int rc = 0;
+	if (!fc || !fw) {
+		rc = -1;
+	} else {
+		for (;;) {
+			int a = fgetc(fc);
+			int b = fgetc(fw);
+			if (a != b) { rc = -1; break; }
+			if (a == EOF) break;
+		}
+	}
+	if (fc) fclose(fc);
+	if (fw) fclose(fw);
+	if (rc != 0)
+		fprintf(stderr, "row[%s]: cstage vs wwstage asm differs\n",
+		    r->label);
+	unlink(src); unlink(cs); unlink(ws);
+	return rc;
+}
+
+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];
+	snprintf(cdrv, sizeof cdrv, "%s/ww", bin);
+	char wdrv[1024];
+	snprintf(wdrv, sizeof wdrv, "%s/ww_ww", bin);
+
+	struct { const char *name; const char *path; int gated_on_existence; }
+	drivers[] = {
+		{ "cstage",  cdrv, 0 },
+		{ "wwstage", wdrv, 1 },
+		{ NULL, NULL, 0 },
+	};
+
+	int n = (int)(sizeof rows / sizeof rows[0]);
+	int nn = (int)(sizeof neg / sizeof neg[0]);
+	int total = 0, fail = 0;
+
+	for (int d = 0; drivers[d].name; d++) {
+		if (drivers[d].gated_on_existence
+		    && access(drivers[d].path, X_OK) != 0) {
+			fprintf(stderr, "arr_infer_len: skip %s (no %s)\n",
+			    drivers[d].name, drivers[d].path);
+			continue;
+		}
+		for (int i = 0; i < n; i++) {
+			int got = run_driver(drivers[d].path, &rows[i], i);
+			total++;
+			if (got != rows[i].want) {
+				fprintf(stderr,
+				    "arr_infer_len[%s][%s]: exit=%d want=%d\n",
+				    drivers[d].name, rows[i].label,
+				    got, rows[i].want);
+				fail++;
+			}
+		}
+		for (int i = 0; i < nn; i++) {
+			total++;
+			if (build_should_fail(drivers[d].path, neg[i],
+			    100 + i) != 0) {
+				fprintf(stderr,
+				    "arr_infer_len[%s][neg%d]: built ok, "
+				    "expected a loud error\n",
+				    drivers[d].name, i);
+				fail++;
+			}
+		}
+	}
+
+	if (access(wdrv, X_OK) == 0) {
+		for (int i = 0; i < n; i++) {
+			total++;
+			if (asm_byte_identical(bin, &rows[i], i) != 0)
+				fail++;
+		}
+	}
+
+	if (fail) {
+		fprintf(stderr,
+		    "arr_infer_len: %d/%d fixtures failed\n", fail, total);
+		return 1;
+	}
+	printf("arr_infer_len: %d/%d ok\n", total, total);
+	return 0;
+}