w6c: variant-tag miss is loud, never tag 0 (rule 7)

Every variant-index lookup miss (-1) was silently clamped to tag 0
across both stages: tagged return (value + bare-void), widen store
(struct/slice/str/float/scalar arms + push fast path + field store),
widen tag-remap (identity scan + emit loop), match case compare,
is/as typetest, tryprop error remap, and the alloc-nomem propagation
(clamped to 1). All misses are checker-rejected upstream today, so
the clamps were dead -- but any future checker/cgen seam gap would
mis-tag silently (wrong arm, wrong error, false success). Rule 7:
each site now hard-stops with a per-construct diagnostic; nullable
arms keep their raw -1 by design (a miss encodes the void polarity
for `case null`). Corpus asm byte-unchanged; bootstrap fixed point
holds.

cmd/w6c/cgen.c

@@ -2671,7 +2671,9 @@ cg_widen_tag_remap(Cg *c, Type *du, Type *su, int slot_off)
 	int identity = 1, idx = 0;
 	for (Tparam *p = su->params; p; p = p->next, idx++) {
 		int di = cg_tag_for_variant(du, p->type);
-		if (di < 0) di = 0;
+		if (di < 0)
+			fatal("tagged widen: source variant missing "
+			    "from dst union (rule 7)");
 		if (di != idx) { identity = 0; break; }
 	}
 	if (identity) return;
@@ -2681,7 +2683,9 @@ cg_widen_tag_remap(Cg *c, Type *du, Type *su, int slot_off)
 	for (Tparam *p = su->params; p; p = p->next, idx++) {
 		const char *next = mklabel(c, "remap_next");
 		int di = cg_tag_for_variant(du, p->type);
-		if (di < 0) di = 0;
+		if (di < 0)
+			fatal("tagged widen: source variant missing "
+			    "from dst union (rule 7)");
 		ins2(c, A_CMPQ, aimm(idx), areg(D_AX));
 		ins1(c, A_JNE, abranch(next));
 		ins2(c, A_MOVQ, aimm(di), areg(D_AX));
@@ -3079,7 +3083,7 @@ cg_widen_tagged_store(Cg *c, Local **locals_p, Type *dst, Node *src,
 			}
 			foff += wide ? 24 : 8;
 		}
-		ins2(c, A_MOVQ, aimm(tag < 0 ? 0 : tag),
+		ins2(c, A_MOVQ, aimm(tag),
 		    amem(D_BP, write_off + 0));
 		if (via_outer) goto copy_out;
 		return;
@@ -3149,6 +3153,9 @@ cg_widen_tagged_store(Cg *c, Local **locals_p, Type *dst, Node *src,
 			ins2(c, A_MOVQ, areg(D_AX),
 			    amem(D_BP, write_off + k));
 		int tag = cg_tag_for_variant(du, st);
+		if (tag < 0)
+			fatal("tagged widen: no variant tag for source "
+			    "(rule 7)");
 		if (src->kind == N_IDENT) {
 			int soff = localfind(*locals_p, src->str);
 			int ssz = (int)su->size;
@@ -3198,7 +3205,7 @@ cg_widen_tagged_store(Cg *c, Local **locals_p, Type *dst, Node *src,
 			cg_structlit_fill(c, locals_p, su, src,
 			    DST_BP, 0, NULL, write_off + 8);
 		}
-		ins2(c, A_MOVQ, aimm(tag < 0 ? 0 : tag),
+		ins2(c, A_MOVQ, aimm(tag),
 		    amem(D_BP, write_off + 0));
 		if (via_outer) goto copy_out;
 		return;
@@ -3214,7 +3221,10 @@ cg_widen_tagged_store(Cg *c, Local **locals_p, Type *dst, Node *src,
 		ins2(c, A_MOVQ, areg(D_BX), amem(D_BP, write_off + 16));
 		ins2(c, A_MOVQ, areg(D_CX), amem(D_BP, write_off + 24));
 		int tag = cg_tag_for_variant(du, st);
-		ins2(c, A_MOVQ, aimm(tag < 0 ? 0 : tag),
+		if (tag < 0)
+			fatal("tagged widen: no variant tag for source "
+			    "(rule 7)");
+		ins2(c, A_MOVQ, aimm(tag),
 		    amem(D_BP, write_off + 0));
 		if (via_outer) goto copy_out;
 		return;
@@ -3246,7 +3256,10 @@ cg_widen_tagged_store(Cg *c, Local **locals_p, Type *dst, Node *src,
 				    amem(D_BP, write_off + k));
 		}
 		int tag = cg_tag_for_variant(du, st);
-		ins2(c, A_MOVQ, aimm(tag < 0 ? 0 : tag),
+		if (tag < 0)
+			fatal("tagged widen: no variant tag for source "
+			    "(rule 7)");
+		ins2(c, A_MOVQ, aimm(tag),
 		    amem(D_BP, write_off + 0));
 		if (via_outer) goto copy_out;
 		return;
@@ -3266,7 +3279,10 @@ cg_widen_tagged_store(Cg *c, Local **locals_p, Type *dst, Node *src,
 			    amem(D_BP, write_off + k));
 	}
 	int tag = cg_tag_for_variant(du, st);
-	ins2(c, A_MOVQ, aimm(tag < 0 ? 0 : tag), amem(D_BP, write_off + 0));
+	if (tag < 0)
+		fatal("tagged widen: no variant tag for source "
+		    "(rule 7)");
+	ins2(c, A_MOVQ, aimm(tag), amem(D_BP, write_off + 0));
 copy_out:
 	if (via_outer) {
 		/* cgexpr above clobbered base_reg — reload from spill, then
@@ -3318,7 +3334,9 @@ cg_widen_tagged_push(Cg *c, Local **locals_p, Type *dst, Node *src, int sz)
 		/* Direct-push fast path: str / slice / scalar / pointer. */
 		cgexpr(c, src, *locals_p);
 		int tag = cg_tag_for_variant(du, st);
-		if (tag < 0) tag = 0;
+		if (tag < 0)
+			fatal("tagged widen: no variant tag for source "
+			    "(rule 7)");
 		if (type_isstr(st) || (su && su->kind == TY_STR)) {
 			/* str IS []u8: slot 32 [+0]=tag, [+8]=ptr, [+16]=len,
 			 * [+24]=cap — same shape as the slice arm below. Push
@@ -5983,9 +6001,13 @@ cgexpr(Cg *c, Node *n, Local *locals)
 							    areg(D_AX));
 							int v58tag =
 							    cg_tag_for_variant(fu, st);
+							if (v58tag < 0)
+								fatal("tagged widen: "
+								    "no variant tag "
+								    "for source "
+								    "(rule 7)");
 							ins2(c, A_MOVQ,
-							    aimm(v58tag < 0
-							    ? 0 : v58tag),
+							    aimm(v58tag),
 							    amem(D_BX, foff + 0));
 							ins2(c, A_MOVQ,
 							    areg(D_AX),
@@ -11165,23 +11187,32 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					 * if the tag matches any of the alts,
 					 * jump to body; otherwise to the next
 					 * case. */
+					if (tag < 0)
+						fatal("tagged match: case type "
+						    "not in union (rule 7)");
 					char *body = mklabel(c, "match_body");
-					ins2(c, A_CMPQ, aimm(tag < 0 ? 0 : tag),
+					ins2(c, A_CMPQ, aimm(tag),
 					    areg(D_AX));
 					ins1(c, A_JE, abranch(body));
 					for (Node *alt = cs->list; alt;
 					    alt = alt->next) {
 						int atag = cg_tag_for_variant(
 						    su, alt->type);
+						if (atag < 0)
+							fatal("tagged match: case type "
+							    "not in union (rule 7)");
 						ins2(c, A_CMPQ,
-						    aimm(atag < 0 ? 0 : atag),
+						    aimm(atag),
 						    areg(D_AX));
 						ins1(c, A_JE, abranch(body));
 					}
 					ins1(c, A_JMP, abranch(next));
 					label(c, body);
 				} else {
-					ins2(c, A_CMPQ, aimm(tag < 0 ? 0 : tag),
+					if (tag < 0)
+						fatal("tagged match: case type "
+						    "not in union (rule 7)");
+					ins2(c, A_CMPQ, aimm(tag),
 					    areg(D_AX));
 					ins1(c, A_JNE, abranch(next));
 				}
@@ -11348,7 +11379,9 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			for (Tparam *p = u->params; p; p = p->next, i++) {
 				if (!cg_variant_is_error(u, i)) continue;
 				int j = cg_tag_for_variant(r, p->type);
-				if (j < 0) j = 0;
+				if (j < 0)
+					fatal("tryprop: error variant missing "
+					    "from fn return union (rule 7)");
 				if (j == i) continue;
 				char *skip = mklabel(c, "tryprop_skip");
 				ins2(c, A_CMPQ, aimm(i), areg(D_AX));
@@ -11586,7 +11619,10 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				ins1(c, A_JNE, abranch(ne));
 		} else {
 			int tag = cg_tag_for_variant(u, vt);
-			ins2(c, A_CMPQ, aimm(tag < 0 ? 0 : tag), areg(D_AX));
+			if (tag < 0)
+				fatal("tagged typetest: type not in union "
+				    "(rule 7)");
+			ins2(c, A_CMPQ, aimm(tag), areg(D_AX));
 			ins1(c, A_JNE, abranch(ne));
 		}
 		ins2(c, A_MOVQ, aimm(1), areg(D_AX));
@@ -11709,8 +11745,11 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			break;
 		}
 		int tag = cg_tag_for_variant(u, vt);
+		if (tag < 0)
+			fatal("tagged typetest: type not in union "
+			    "(rule 7)");
 		ins2(c, A_MOVQ, amem(D_BP, sl_off + 0), areg(D_AX));
-		ins2(c, A_CMPQ, aimm(tag < 0 ? 0 : tag), areg(D_AX));
+		ins2(c, A_CMPQ, aimm(tag), areg(D_AX));
 		ins1(c, A_JE, abranch(ok));
 		ins2(c, A_MOVQ, aimm(1), areg(D_DI));
 		ins2(c, A_MOVQ, aimm(60), areg(D_AX));
@@ -13531,7 +13570,9 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 					Type *r = cg_ret_type;
 					r = type_chase_named(r);
 					int nidx = cg_tag_for_variant(r, ty_nomem);
-					if (nidx < 0) nidx = 1;
+					if (nidx < 0)
+						fatal("tryprop: no nomem variant "
+						    "in fn return union (rule 7)");
 					ins2(c, A_MOVQ, aimm(nidx), areg(D_AX));
 					ins2(c, A_MOVQ, areg(D_BP), areg(D_SP));
 					ins1(c, A_POPQ, areg(D_BP));
@@ -14130,7 +14171,9 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 				 * slot and the caller reads memory. */
 				if (cg_sret_retsize(rt) > 0) {
 					int tag = cg_tag_for_variant(rt, ty_void);
-					if (tag < 0) tag = 0;
+					if (tag < 0)
+						fatal("tagged return: no void "
+						    "variant (rule 7)");
 					ins2(c, A_MOVQ,
 					    amem(D_BP, cg_sret_arg_off),
 					    areg(D_BX));
@@ -14150,7 +14193,9 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 					ins2(c, A_MOVQ, aimm(0), areg(D_AX));
 				} else {
 					int tag = cg_tag_for_variant(rt, ty_void);
-					if (tag < 0) tag = 0;
+					if (tag < 0)
+						fatal("tagged return: no void "
+						    "variant (rule 7)");
 					ins2(c, A_MOVQ, aimm(tag), areg(D_AX));
 				}
 				ins2(c, A_MOVQ, areg(D_BP), areg(D_SP));
@@ -14266,6 +14311,10 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 					 * caller (e.g. a slice-stride IMULQ) would
 					 * land in slot+16 / slot+24. (Task #18.) */
 					int tag = cg_tag_for_variant(rt, vt);
+					if (tag < 0)
+						fatal("tagged return: no variant "
+						    "tag for return value "
+						    "(rule 7)");
 					int rsz = (int)rt->size;
 					cgexpr(c, n->lhs, *locals);
 					if (type_isslice(vt)) {
@@ -14332,7 +14381,7 @@ cgstmt(Cg *c, Node *n, Local **locals, int *frame)
 							ins2(c, A_MOVQ, aimm(0),
 							    areg(D_R8));
 					}
-					ins2(c, A_MOVQ, aimm(tag < 0 ? 0 : tag),
+					ins2(c, A_MOVQ, aimm(tag),
 					    areg(D_AX));
 				} else {
 					/* Struct variant or tagged-subset:

selfhost/cmd/wcc/cgenexpr.ww

@@ -544,7 +544,11 @@ fn cgtryprop(c: *cgen, n: *syntax.node) void = {
 		for (p != nil) {
 			if (variantiserror(u, i)) {
 				let j: i32 = flatvariantidxt(r, p.type_, false);
-				if (j < 0) { j = 0; };
+				if (j < 0) {
+					let mtp: str = "tryprop: error variant missing from fn return union (rule 7)\n";
+					os.write(2, mtp.ptr, mtp.len: u64);
+					os.exit(1);
+				};
 				if (j != i) {
 					let skip: str = mklabel(c, "tryprop_skip");
 					emitline("\tCMPQ\t$");
@@ -796,7 +800,11 @@ fn cgtypetest(c: *cgen, n: *syntax.node) void = {
 			emitline("\tJNE\t");
 		};
 	} else {
-		if (want < 0) { want = 0; };
+		if (want < 0) {
+			let mtt: str = "tagged typetest: type not in union (rule 7)\n";
+			os.write(2, mtt.ptr, mtt.len: u64);
+			os.exit(1);
+		};
 		emitline("\tCMPQ\t$");
 		emitint(want: i64);
 		emitline(", AX\n");
@@ -982,7 +990,11 @@ fn cgtypeassert(c: *cgen, n: *syntax.node) void = {
 		emitlabel(okl);
 		return;
 	};
-	if (want < 0) { want = 0; };
+	if (want < 0) {
+		let mtt: str = "tagged typetest: type not in union (rule 7)\n";
+		os.write(2, mtt.ptr, mtt.len: u64);
+		os.exit(1);
+	};
 	emitline("\tMOVQ\t");
 	emitoff(scrutoff: i64);
 	emitline("(BP), AX\n");
@@ -3115,7 +3127,12 @@ fn matcharmwant(c: *cgen, scrutt: *syntax.node, pat: *syntax.node) i32 = {
 					r = flatvariantidxt(scrutt.type_: *syntax.tinfo, pattype, false);
 				};
 			};
-			if (r >= 0) { want = r; };
+			if (r < 0) {
+				let mmt: str = "tagged match: case type not in union (rule 7)\n";
+				os.write(2, mmt.ptr, mmt.len: u64);
+				os.exit(1);
+			};
+			want = r;
 		};
 	};
 	return want;
@@ -10439,7 +10456,11 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 											if (viaptr) { emitline("\tMOVQ\t(BX), BX\n"); };
 											emitline("\tPOPQ\tAX\n");
 											let v58tag: i32 = taggedvariantindext(c, fi.tnode.type_: *syntax.tinfo, n.rhs);
-											if (v58tag < 0) { v58tag = 0; };
+											if (v58tag < 0) {
+												let mws: str = "tagged widen: no variant tag for source (rule 7)\n";
+												os.write(2, mws.ptr, mws.len: u64);
+												os.exit(1);
+											};
 											emitline("\tMOVQ\t$");
 											emitint(v58tag: i64);
 											emitline(", ");

selfhost/cmd/wcc/cgenstmt.ww

@@ -1211,8 +1211,12 @@ fn cgreturn(c: *cgen, n: *syntax.node) void = {
 					emitline("\tMOVQ\t$0, R8\n");
 				};
 			};};};
+			if (idx < 0) {
+				let mtag: str = "tagged return: no variant tag for return value (rule 7)\n";
+				os.write(2, mtag.ptr, mtag.len: u64);
+				os.exit(1);
+			};
 			emitline("\tMOVQ\t$");
-			if (idx < 0) { idx = 0; };
 			emitint(idx: i64);
 			emitline(", AX\n");
 			emitline("\tMOVQ\tBP, SP\n");
@@ -1847,7 +1851,11 @@ fn cgreturn(c: *cgen, n: *syntax.node) void = {
 			if (sretretsize(c, c.fnret) > 0) {
 				let sa38: i32 = localfind(c, "@sretarg");
 				let vidx38: i32 = voidvariantindex(c.fnret);
-				if (vidx38 < 0) { vidx38 = 0; };
+				if (vidx38 < 0) {
+					let mvd: str = "tagged return: no void variant (rule 7)\n";
+					os.write(2, mvd.ptr, mvd.len: u64);
+					os.exit(1);
+				};
 				emitline("\tMOVQ\t");
 				emitoff(sa38: i64);
 				emitline("(BP), BX\n");
@@ -1868,7 +1876,11 @@ fn cgreturn(c: *cgen, n: *syntax.node) void = {
 				emitline("\tMOVQ\t$0, AX\n");
 			} else {
 				let idx: i32 = voidvariantindex(c.fnret);
-				if (idx < 0) { idx = 0; };
+				if (idx < 0) {
+					let mvd: str = "tagged return: no void variant (rule 7)\n";
+					os.write(2, mvd.ptr, mvd.len: u64);
+					os.exit(1);
+				};
 				emitline("\tMOVQ\t$");
 				emitint(idx: i64);
 				emitline(", AX\n");
@@ -2301,7 +2313,11 @@ fn cgletbody(c: *cgen, n: *syntax.node, off: i32) void = {
 							nidx2 += 1;
 						};
 					}; };
-					if (nidx < 0) { nidx = 1; };
+					if (nidx < 0) {
+						let mnm: str = "tryprop: no nomem variant in fn return union (rule 7)\n";
+						os.write(2, mnm.ptr, mnm.len: u64);
+						os.exit(1);
+					};
 					emitline("\tMOVQ\t$");
 					emitint(nidx: i64);
 					emitline(", AX\n");

selfhost/cmd/wcc/cgenutil.ww

@@ -4016,7 +4016,11 @@ fn cgwidentagremap(c: *cgen, du: *syntax.tinfo, su: *syntax.tinfo, slot_off: i32
 	let idx: i32 = 0;
 	for (p != nil) {
 		let di: i32 = flatvariantidxt(dt, p.type_, false);
-		if (di < 0) { di = 0; };
+		if (di < 0) {
+			let mrm: str = "tagged widen: source variant missing from dst union (rule 7)\n";
+			os.write(2, mrm.ptr, mrm.len: u64);
+			os.exit(1);
+		};
 		if (di != idx) { identity = false; p = nil; }
 		else           { p = p.tnext; idx += 1; };
 	};
@@ -4030,7 +4034,11 @@ fn cgwidentagremap(c: *cgen, du: *syntax.tinfo, su: *syntax.tinfo, slot_off: i32
 	for (p != nil) {
 		let next: str = mklabel(c, "remap_next");
 		let di: i32 = flatvariantidxt(dt, p.type_, false);
-		if (di < 0) { di = 0; };
+		if (di < 0) {
+			let mrm: str = "tagged widen: source variant missing from dst union (rule 7)\n";
+			os.write(2, mrm.ptr, mrm.len: u64);
+			os.exit(1);
+		};
 		emitline("\tCMPQ\t$");
 		emitint(idx: i64);
 		emitline(", AX\n");
@@ -5126,7 +5134,11 @@ fn cgwidentaggedstorebp(c: *cgen, dst: *syntax.tinfo, src: *syntax.node, slot_of
 				zoff += 8;
 			};
 			let tag: i32 = taggedvariantindext(c, dt, src);
-			if (tag < 0) { tag = 0; };
+			if (tag < 0) {
+				let mws: str = "tagged widen: no variant tag for source (rule 7)\n";
+				os.write(2, mws.ptr, mws.len: u64);
+				os.exit(1);
+			};
 			if (src.kind == syntax.nkind.N_STRUCTLIT) {
 				// #23: delegate to the single fill path. The
 				// inline field loop this replaces was a
@@ -5206,7 +5218,11 @@ fn cgwidentaggedstorebp(c: *cgen, dst: *syntax.tinfo, src: *syntax.node, slot_of
 		emitoff((slot_off + 24): i64);
 		emitline("(BP)\n");
 		let tag: i32 = taggedvariantindext(c, dt, src);
-		if (tag < 0) { tag = 0; };
+		if (tag < 0) {
+			let mws: str = "tagged widen: no variant tag for source (rule 7)\n";
+			os.write(2, mws.ptr, mws.len: u64);
+			os.exit(1);
+		};
 		emitline("\tMOVQ\t$");
 		emitint(tag: i64);
 		emitline(", ");
@@ -5309,7 +5325,11 @@ fn cgwidentaggedstorebp(c: *cgen, dst: *syntax.tinfo, src: *syntax.node, slot_of
 		};
 	};
 	let tag: i32 = taggedvariantindext(c, dt, src);
-	if (tag < 0) { tag = 0; };
+	if (tag < 0) {
+		let mws: str = "tagged widen: no variant tag for source (rule 7)\n";
+		os.write(2, mws.ptr, mws.len: u64);
+		os.exit(1);
+	};
 	emitline("\tMOVQ\t$");
 	emitint(tag: i64);
 	emitline(", ");