From f191e6e0e28981adf377261542c6883ff8202a30 Mon Sep 17 00:00:00 2001
From: Hojun-Cho <ghwns6743@gmail.com>
Date: Sun, 9 Aug 2026 00:32:41 +0900
Subject: [PATCH] wcc: modulo is integer-only; compound ops carry their operand
 class

Hare's rule (harec check.c binarithm): % and the bitwise/shift five
are integer-only; + - * / need numeric operands. ww grouped % with
the numeric ops, and compound assigns never op-checked at all, so
`a % b` on floats compiled half-lowered (live cs!=ww divergence),
`a %= 2.0` plain-stored the rhs (op silently dropped, both stages),
and `s += "cd"` garbled str headers. Gate both at the checker, both
stages; the cgen float-compound fallbacks and the three unknown-
compound legacy defaults (deref/global/local) demote to rule-7 hard
stops. 34 compound-on-tagged/str/slice fixtures re-pin from the old
cgen "not wired" stops to the earlier checker diagnostics; 3 new
reject fixtures pin the closed shapes.
---
 cmd/w6c/cgen.c                                | 53 ++++++--------
 cmd/wcc/check.c                               | 31 ++++++++
 internal/wwfixture/types.ww                   |  8 +--
 selfhost/cmd/wcc/cgenexpr.ww                  | 70 ++++++++++++-------
 selfhost/cmd/wcc/check.ww                     | 37 ++++++++++
 test/wcc/data/dotfield_compound_slice/case.ww |  2 +-
 test/wcc/data/dotfield_compound_str/case.ww   |  2 +-
 .../wcc/data/dotfield_compound_tagged/case.ww |  2 +-
 test/wcc/data/float_mod_reject/case.ww        | 11 +++
 test/wcc/data/float_modeq_reject/case.ww      | 12 ++++
 .../wcc/data/idx_compound_str_indexed/case.ww |  2 +-
 test/wcc/data/idxfield_compound_slice/case.ww |  2 +-
 test/wcc/data/idxfield_compound_str/case.ww   |  2 +-
 .../wcc/data/idxfield_compound_tagged/case.ww |  2 +-
 .../wcc/data/r805_reject_agg_compound/case.ww |  2 +-
 .../wcc/data/r805_reject_str_compound/case.ww |  2 +-
 .../r989_tagderef_ampeq_tagged_deref/case.ww  |  2 +-
 .../case.ww                                   |  2 +-
 .../case.ww                                   |  2 +-
 .../case.ww                                   |  2 +-
 .../case.ww                                   |  2 +-
 .../r989_tagderef_pipeeq_tagged_deref/case.ww |  2 +-
 .../r989_tagderef_pluseq_tagged_deref/case.ww |  2 +-
 .../case.ww                                   |  2 +-
 .../case.ww                                   |  2 +-
 .../r989_tagderef_stareq_tagged_deref/case.ww |  2 +-
 .../data/r989_tagplace_gident_pluseq/case.ww  |  2 +-
 .../data/r989_tagplace_gidx_careteq/case.ww   |  2 +-
 .../data/r989_tagplace_gidx_pluseq/case.ww    |  2 +-
 .../data/r989_tagplace_gidx_stareq/case.ww    |  2 +-
 .../data/r989_tagplace_ident_careteq/case.ww  |  2 +-
 .../data/r989_tagplace_ident_lshifteq/case.ww |  2 +-
 .../data/r989_tagplace_ident_minuseq/case.ww  |  2 +-
 .../data/r989_tagplace_ident_pipeeq/case.ww   |  2 +-
 .../data/r989_tagplace_ident_pluseq/case.ww   |  2 +-
 .../data/r989_tagplace_ident_slasheq/case.ww  |  2 +-
 .../data/r989_tagplace_ident_stareq/case.ww   |  2 +-
 .../data/r989_tagplace_lidx_lshifteq/case.ww  |  2 +-
 .../data/r989_tagplace_lidx_minuseq/case.ww   |  2 +-
 .../data/r989_tagplace_lidx_pipeeq/case.ww    |  2 +-
 .../data/r989_tagplace_lidx_pluseq/case.ww    |  2 +-
 test/wcc/data/str_pluseq_reject/case.ww       |  9 +++
 42 files changed, 206 insertions(+), 93 deletions(-)
 create mode 100644 test/wcc/data/float_mod_reject/case.ww
 create mode 100644 test/wcc/data/float_modeq_reject/case.ww
 create mode 100644 test/wcc/data/str_pluseq_reject/case.ww

diff --git a/cmd/w6c/cgen.c b/cmd/w6c/cgen.c
index b3485c99..dbbb22d6 100644
--- a/cmd/w6c/cgen.c
+++ b/cmd/w6c/cgen.c
@@ -7041,25 +7041,20 @@ cgexpr(Cg *c, Node *n, Local *locals)
 			case TK_SLASHEQ: fop = divop; break;
 			default: break;
 			}
+			/* Non-SSE compound on a float lvalue is checker-
+			 * rejected (modulo/bitwise integer-only); a survivor
+			 * here means a checker gap — loud, never the old
+			 * plain-store of rhs that dropped the op. */
+			if (fop < 0)
+				fatal("float compound: op has no SSE "
+				    "lowering (rule 7)");
 			if (off != 0) {
-				if (fop < 0) {
-					/* Unsupported compound (e.g., %= on float):
-					 * fall back to plain store of rhs. */
-					ins2(c, mvop, areg(D_X0),
-					    amem(D_BP, off));
-					break;
-				}
 				ins2(c, mvop, amem(D_BP, off), areg(D_X1));
 				ins2(c, fop, areg(D_X0), areg(D_X1));
 				ins2(c, mvop, areg(D_X1), amem(D_BP, off));
 			} else {
 				ins2(c, A_LEAQ, masym(c, n->lhs->str),
 				    areg(D_CX));
-				if (fop < 0) {
-					ins2(c, mvop, areg(D_X0),
-					    amem(D_CX, 0));
-					break;
-				}
 				ins2(c, mvop, amem(D_CX, 0), areg(D_X1));
 				ins2(c, fop, areg(D_X0), areg(D_X1));
 				ins2(c, mvop, areg(D_X1), amem(D_CX, 0));
@@ -7921,10 +7916,12 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					break;
 				}
 				default:
-					/* unknown compound: legacy fallback —
-					 * store rhs only. */
-					ins2(c, A_MOVQ, areg(D_CX), areg(D_AX));
-					break;
+					/* all 10 compound tokens enumerated
+					 * above — an 11th means a parser/
+					 * checker gap, never a plain store
+					 * of rhs (rule 7). */
+					fatal("deref compound: unknown "
+					    "compound op (rule 7)");
 				}
 				ins2(c, store_op, areg(D_AX), amem(D_BX, 0));
 				break;
@@ -8364,7 +8361,6 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					ins2(c, glop, amem(D_CX, 0),
 					    areg(D_BX));
 				}
-				int did_compound = 1;
 				switch (n->op) {
 				case TK_PLUSEQ:	 ins2(c, A_ADDQ,  areg(D_AX), areg(D_BX)); break;
 				case TK_MINUSEQ: ins2(c, A_SUBQ,  areg(D_AX), areg(D_BX)); break;
@@ -8410,16 +8406,13 @@ cgexpr(Cg *c, Node *n, Local *locals)
 					break;
 				}
 				default:
-					/* unknown compound: legacy fallback —
-					 * store rhs only. */
-					did_compound = 0;
-					ins2(c, A_MOVQ, areg(D_AX),
-					    masym(c, n->lhs->str));
-					break;
+					/* all 10 compound tokens enumerated
+					 * above (rule 7). */
+					fatal("global compound: unknown "
+					    "compound op (rule 7)");
 				}
-				if (did_compound)
-					ins2(c, A_MOVQ, areg(D_BX),
-					    masym(c, n->lhs->str));
+				ins2(c, A_MOVQ, areg(D_BX),
+				    masym(c, n->lhs->str));
 				break;
 			}
 			cgexpr(c, n->rhs, locals);
@@ -8490,12 +8483,12 @@ cgexpr(Cg *c, Node *n, Local *locals)
 				break;
 			}
 			default:
-				/* unknown: just store rhs (legacy fallback) */
-				ins2(c, A_MOVQ, areg(D_AX), amem(D_BP, off));
-				goto skip_assign_store;
+				/* all 10 compound tokens enumerated above
+				 * (rule 7). */
+				fatal("local compound: unknown compound op "
+				    "(rule 7)");
 			}
 			ins2(c, A_MOVQ, areg(D_BX), amem(D_BP, off));
-		skip_assign_store: ;
 		}
 		/* C1 residual (task #22): a non-DOT lvalue no arm above
 		 * matched still falls out SILENT here — the known member is
diff --git a/cmd/wcc/check.c b/cmd/wcc/check.c
index 547a7c24..e28db632 100644
--- a/cmd/wcc/check.c
+++ b/cmd/wcc/check.c
@@ -1282,6 +1282,11 @@ cbinop(Checker *c, Node *n)
 			return ty_i64;
 		if (!type_isnum(l) || !type_isnum(r))
 			return err(c, n->pos, "arithmetic on non-numeric type");
+		/* % is integer-only (harec check.c binarithm BIN_MODULO):
+		 * floats have no SSE modulo lowering, so an admitted float %
+		 * fell through cgen half-lowered (cs!=ww divergence). */
+		if (n->op == TK_PERCENT && (!type_isint(l) || !type_isint(r)))
+			return err(c, n->pos, "modulo on non-integer type");
 		return unify_arith(c, n->pos, l, r);
 	case TK_AMP: case TK_PIPE: case TK_CARET: case TK_LSHIFT:
 	case TK_RSHIFT:
@@ -2016,6 +2021,32 @@ cexpr(Checker *c, Node *n)
 		    !assignable_addrfn(c, l, n->rhs))
 			err(c, n->pos, "cannot assign %s to %s",
 			    type_name(c->a, r), type_name(c->a, l));
+		/* Compound ops carry their binary op's operand class (harec
+		 * check.c binarithm): += -= *= /= need numeric operands,
+		 * %= modulo-integer, the bitwise/shift five integer. The
+		 * assignability check above cannot see the op, so `f %= x`
+		 * and `s += "x"` passed and cgen's fallback plain-stored the
+		 * rhs, silently dropping the operation. */
+		if (n->op != TK_ASSIGN && l != ty_err && r != ty_err) {
+			switch (n->op) {
+			case TK_PLUSEQ: case TK_MINUSEQ: case TK_STAREQ:
+			case TK_SLASHEQ:
+				if (!type_isnum(l) || !type_isnum(r))
+					err(c, n->pos, "arithmetic on "
+					    "non-numeric type");
+				break;
+			case TK_PERCENTEQ:
+				if (!type_isint(l) || !type_isint(r))
+					err(c, n->pos, "modulo on "
+					    "non-integer type");
+				break;
+			default:
+				if (!type_isint(l) || !type_isint(r))
+					err(c, n->pos, "bitwise on "
+					    "non-integer type");
+				break;
+			}
+		}
 		/* #120: `w = 1.0` narrows the rhs literal to the lvalue's f32. */
 		coerce_floatlit(n->rhs, l);
 		/* #258: `s = arr` borrows the array as a full slice.
diff --git a/internal/wwfixture/types.ww b/internal/wwfixture/types.ww
index 1fd12037..eb713adb 100644
--- a/internal/wwfixture/types.ww
+++ b/internal/wwfixture/types.ww
@@ -1,13 +1,13 @@
 package wwfixture;
 
 def protocolversion: i32 = 1;
-def corpuscount: i32 = 1746;
-def errorcount: i32 = 346;
+def corpuscount: i32 = 1749;
+def errorcount: i32 = 349;
 def compilecount: i32 = 21;
 def runcount: i32 = 209;
 def runexitcount: i32 = 1170;
-def nativecount: i32 = 3492;
-def corpushash: str = "6afbab274e0c6d1479a2f0cb6e575b14f5e4ffd25a9cb8a3701307db97d3c489";
+def nativecount: i32 = 3498;
+def corpushash: str = "616b3e7f4fca31cb99409c58a29ac73623c997967ddd57651a1370c2c2e2be6a";
 
 type directive = enum i32 {
 	ERROR = 0,
diff --git a/selfhost/cmd/wcc/cgenexpr.ww b/selfhost/cmd/wcc/cgenexpr.ww
index 119b734f..38a041ad 100644
--- a/selfhost/cmd/wcc/cgenexpr.ww
+++ b/selfhost/cmd/wcc/cgenexpr.ww
@@ -9322,6 +9322,15 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 						else         { combineop = "SARQ"; };
 					};
 					}; }; }; }; }; }; };
+					// all 10 compound tokens enumerated above
+					// (/= %= returned earlier) — a MOVQ
+					// combineop is the old rhs-plain-store
+					// legacy fallback. Rule 7.
+					if (syntax.streq(combineop, "MOVQ")) {
+						let mdc: str = "deref compound: unknown compound op (rule 7)\n";
+						os.write(2, mdc.ptr, mdc.len: u64);
+						os.exit(1);
+					};
 					emitline("\t");
 					emitline(combineop);
 					emitline("\tCX, AX\n");
@@ -12555,12 +12564,14 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 					if (n.op == syntax.tkind.TK_STAREQ)  { fop = mulf; };
 					if (n.op == syntax.tkind.TK_SLASHEQ) { fop = divf; };
 					if (fop.len == 0) {
-						// Unsupported (e.g., %= on float):
-						// fall back to plain store of rhs.
-						emitline("\t");
-						emitline(mov);
-						emitline("\tX0, (CX)\n");
-						return;
+						// Non-SSE compound on a float lvalue is
+						// checker-rejected (modulo/bitwise
+						// integer-only); a survivor here means a
+						// checker gap — loud, never the old
+						// plain-store of rhs that dropped the op.
+						let mfc: str = "float compound: op has no SSE lowering (rule 7)\n";
+						os.write(2, mfc.ptr, mfc.len: u64);
+						os.exit(1);
 					};
 					emitline("\t");
 					emitline(mov);
@@ -12722,7 +12733,6 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 					emitline(glop);
 					emitline("\t(CX), BX\n");
 				};
-				let didcompound: bool = true;
 				if (n.op == syntax.tkind.TK_PLUSEQ)   { emitline("\tADDQ\tAX, BX\n"); }
 				else { if (n.op == syntax.tkind.TK_MINUSEQ)  { emitline("\tSUBQ\tAX, BX\n"); }
 				else { if (n.op == syntax.tkind.TK_STAREQ)   { emitline("\tIMULQ\tAX, BX\n"); }
@@ -12778,19 +12788,15 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 					};
 				}
 				else {
-					// Unsupported compound: store rhs
-					// directly. Mirrors the local path's
-					// legacy fallback for unknown ops.
-					didcompound = false;
-					emitline("\tMOVQ\tAX, ");
-					emitsymname(c, nm);
-					emitline("(SB)\n");
+					// all 10 compound tokens enumerated
+					// above (rule 7).
+					let mgc: str = "global compound: unknown compound op (rule 7)\n";
+					os.write(2, mgc.ptr, mgc.len: u64);
+					os.exit(1);
 				};};};};};};};};};
-				if (didcompound) {
-					emitline("\tMOVQ\tBX, ");
-					emitsymname(c, nm);
-					emitline("(SB)\n");
-				};
+				emitline("\tMOVQ\tBX, ");
+				emitsymname(c, nm);
+				emitline("(SB)\n");
 				return;
 			};
 			// #10 Fold B: over-cap tuple reassign `t = f();`. t's
@@ -12978,12 +12984,14 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 				if (n.op == syntax.tkind.TK_STAREQ)  { fop = mulf; };
 				if (n.op == syntax.tkind.TK_SLASHEQ) { fop = divf; };
 				if (fop.len == 0) {
-					emitline("\t");
-					emitline(mov);
-					emitline("\tX0, ");
-					emitoff(off: i64);
-					emitline("(BP)\n");
-					return;
+					// Non-SSE compound on a float lvalue is
+					// checker-rejected (modulo/bitwise
+					// integer-only); a survivor here means a
+					// checker gap — loud, never the old
+					// plain-store of rhs that dropped the op.
+					let mfc: str = "float compound: op has no SSE lowering (rule 7)\n";
+					os.write(2, mfc.ptr, mfc.len: u64);
+					os.exit(1);
 				};
 				emitline("\t");
 				emitline(mov);
@@ -13156,6 +13164,18 @@ fn cgassign(c: *cgen, n: *syntax.node) void = {
 					emitline("\tMOVQ\tDX, BX\n");
 				};
 			};
+			// all 10 compound tokens enumerated above — an 11th
+			// stored the untouched loaded value back (silent
+			// no-op). Rule 7.
+			if (n.op != syntax.tkind.TK_PLUSEQ   && n.op != syntax.tkind.TK_MINUSEQ
+			    && n.op != syntax.tkind.TK_STAREQ    && n.op != syntax.tkind.TK_AMPEQ
+			    && n.op != syntax.tkind.TK_PIPEEQ    && n.op != syntax.tkind.TK_CARETEQ
+			    && n.op != syntax.tkind.TK_LSHIFTEQ  && n.op != syntax.tkind.TK_RSHIFTEQ
+			    && n.op != syntax.tkind.TK_SLASHEQ   && n.op != syntax.tkind.TK_PERCENTEQ) {
+				let mlc: str = "local compound: unknown compound op (rule 7)\n";
+				os.write(2, mlc.ptr, mlc.len: u64);
+				os.exit(1);
+			};
 			emitline("\tMOVQ\tBX, ");
 			emitoff(off: i64);
 			emitline("(BP)\n");
diff --git a/selfhost/cmd/wcc/check.ww b/selfhost/cmd/wcc/check.ww
index 0a4c746f..36924435 100644
--- a/selfhost/cmd/wcc/check.ww
+++ b/selfhost/cmd/wcc/check.ww
@@ -3069,6 +3069,15 @@ fn binoptype(c: *checker, e: *syntax.node) *syntax.node = {
 		    (rtn != nil && !numkindast(c, rtn))) {
 			deffolderr(c, e, "arithmetic on non-numeric type");
 		};
+		// % is integer-only (harec check.c binarithm BIN_MODULO):
+		// floats have no SSE modulo lowering, so an admitted float %
+		// fell through cgen half-lowered (cs!=ww divergence).
+		if (op == syntax.tkind.TK_PERCENT) {
+			if ((ltn != nil && !intkindast(c, ltn)) ||
+			    (rtn != nil && !intkindast(c, rtn))) {
+				deffolderr(c, e, "modulo on non-integer type");
+			};
+		};
 		return unifyarith(c, e, ltn, rtn);
 	};
 	if (op == syntax.tkind.TK_AMP    || op == syntax.tkind.TK_PIPE   ||
@@ -6137,6 +6146,34 @@ fn checkassign(c: *checker, n: *syntax.node) void = {
 	};
 	let ltn: *syntax.node = exprtype(c, n.lhs, nil);
 	let rtn: *syntax.node = exprtype(c, n.rhs, nil);
+	// Compound ops carry their binary op's operand class (harec
+	// check.c binarithm): += -= *= /= need numeric operands,
+	// %= modulo-integer, the bitwise/shift five integer. Without
+	// this `f %= x` and `s += "x"` passed and cgen's fallback
+	// plain-stored the rhs, silently dropping the operation.
+	// Mirror cstage cmd/wcc/check.c N_ASSIGN compound gate.
+	if (n.op != syntax.tkind.TK_ASSIGN) {
+		if (n.op == syntax.tkind.TK_PLUSEQ  || n.op == syntax.tkind.TK_MINUSEQ ||
+		    n.op == syntax.tkind.TK_STAREQ  || n.op == syntax.tkind.TK_SLASHEQ) {
+			if ((ltn != nil && !numkindast(c, ltn)) ||
+			    (rtn != nil && !numkindast(c, rtn))) {
+				cerr("error: arithmetic on non-numeric type\n");
+				c.errs += 1;
+			};
+		} else { if (n.op == syntax.tkind.TK_PERCENTEQ) {
+			if ((ltn != nil && !intkindast(c, ltn)) ||
+			    (rtn != nil && !intkindast(c, rtn))) {
+				cerr("error: modulo on non-integer type\n");
+				c.errs += 1;
+			};
+		} else {
+			if ((ltn != nil && !intkindast(c, ltn)) ||
+			    (rtn != nil && !intkindast(c, rtn))) {
+				cerr("error: bitwise on non-integer type\n");
+				c.errs += 1;
+			};
+		}; };
+	};
 	// #120: `w = 1.0` narrows the rhs literal to the lvalue's f32. Mirror
 	// cstage cmd/wcc/check.c N_ASSIGN coerce_floatlit.
 	coercefloatlit(c, n.rhs, ltn);
diff --git a/test/wcc/data/dotfield_compound_slice/case.ww b/test/wcc/data/dotfield_compound_slice/case.ww
index 83c20396..2b331fb6 100644
--- a/test/wcc/data/dotfield_compound_slice/case.ww
+++ b/test/wcc/data/dotfield_compound_slice/case.ww
@@ -1,4 +1,4 @@
-//ww:error "single-dot field compound on slice field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_slice"
 // (#34/rule-7): a compound assign on a single-dot SLICE field is unwired in
 // both stages and MUST loud-fail. The substring is the shared diagnostic body
diff --git a/test/wcc/data/dotfield_compound_str/case.ww b/test/wcc/data/dotfield_compound_str/case.ww
index dd1e255e..f91dcef6 100644
--- a/test/wcc/data/dotfield_compound_str/case.ww
+++ b/test/wcc/data/dotfield_compound_str/case.ww
@@ -1,4 +1,4 @@
-//ww:error "single-dot field compound on str field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_str"
 // (#34/rule-7): a compound assign on a single-dot STR field is unwired in both
 // stages and MUST loud-fail. The substring is the shared diagnostic body —
diff --git a/test/wcc/data/dotfield_compound_tagged/case.ww b/test/wcc/data/dotfield_compound_tagged/case.ww
index a9fcfc2a..07a00ae9 100644
--- a/test/wcc/data/dotfield_compound_tagged/case.ww
+++ b/test/wcc/data/dotfield_compound_tagged/case.ww
@@ -1,4 +1,4 @@
-//ww:error "single-dot field compound on tagged field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_dotfield_compound_run.c reject row "dotfld_he_tagged"
 // (#34/rule-7): a compound assign on a single-dot TAGGED-union field is unwired
 // in both stages and MUST loud-fail. The substring is the shared diagnostic
diff --git a/test/wcc/data/float_mod_reject/case.ww b/test/wcc/data/float_mod_reject/case.ww
new file mode 100644
index 00000000..09e3951c
--- /dev/null
+++ b/test/wcc/data/float_mod_reject/case.ww
@@ -0,0 +1,11 @@
+//ww:error "modulo on non-integer type"
+// % is integer-only (harec check.c binarithm BIN_MODULO). Pre-gate a
+// float % half-lowered through cgen as a live cs!=ww divergence.
+package main;
+export fn main() i32 = {
+	let a: f64 = 7.0;
+	let b: f64 = 2.0;
+	let m: f64 = a % b;
+	if (m == 1.0) { return 1; };
+	return 0;
+};
diff --git a/test/wcc/data/float_modeq_reject/case.ww b/test/wcc/data/float_modeq_reject/case.ww
new file mode 100644
index 00000000..1e682610
--- /dev/null
+++ b/test/wcc/data/float_modeq_reject/case.ww
@@ -0,0 +1,12 @@
+//ww:error "modulo on non-integer type"
+// Compound ops carry their binary op's operand class: %= on a float
+// lvalue plain-stored the rhs pre-gate (`a %= 2.0` became `a = 2.0`,
+// silently, both stages).
+package main;
+let g: f64 = 9.0;
+export fn main() i32 = {
+	let a: f64 = 7.0;
+	a %= 2.0;
+	g %= 2.0;
+	return 0;
+};
diff --git a/test/wcc/data/idx_compound_str_indexed/case.ww b/test/wcc/data/idx_compound_str_indexed/case.ww
index f984210f..4f670952 100644
--- a/test/wcc/data/idx_compound_str_indexed/case.ww
+++ b/test/wcc/data/idx_compound_str_indexed/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on str element not wired (#133/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/948_idx_compound_run.c reject row "he_str_indexed"
 // (#133-expanded rule-7): a compound assign on an indexed STR element is
 // unwired in both stages and MUST loud-fail. The substring is the FULL shared
diff --git a/test/wcc/data/idxfield_compound_slice/case.ww b/test/wcc/data/idxfield_compound_slice/case.ww
index 7654b61f..aa7940fe 100644
--- a/test/wcc/data/idxfield_compound_slice/case.ww
+++ b/test/wcc/data/idxfield_compound_slice/case.ww
@@ -1,4 +1,4 @@
-//ww:error "arr[i].field compound on slice field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_slice"
 // (#33/rule-7): a compound assign on an indexed-element SLICE field is unwired
 // in both stages and MUST loud-fail. The substring is the shared diagnostic
diff --git a/test/wcc/data/idxfield_compound_str/case.ww b/test/wcc/data/idxfield_compound_str/case.ww
index edcfed34..5ae339ea 100644
--- a/test/wcc/data/idxfield_compound_str/case.ww
+++ b/test/wcc/data/idxfield_compound_str/case.ww
@@ -1,4 +1,4 @@
-//ww:error "arr[i].field compound on str field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_str"
 // (#33/rule-7): a compound assign on an indexed-element STR field is unwired in
 // both stages and MUST loud-fail. The substring is the shared diagnostic body
diff --git a/test/wcc/data/idxfield_compound_tagged/case.ww b/test/wcc/data/idxfield_compound_tagged/case.ww
index deacb0b5..24e9ed26 100644
--- a/test/wcc/data/idxfield_compound_tagged/case.ww
+++ b/test/wcc/data/idxfield_compound_tagged/case.ww
@@ -1,4 +1,4 @@
-//ww:error "arr[i].field compound on tagged field"
+//ww:error "arithmetic on non-numeric type"
 // Lifted from test/wcc/949_idxfield_compound_run.c reject row "fld_he_tagged"
 // (#33/rule-7): a compound assign on an indexed-element TAGGED-union field is
 // unwired in both stages and MUST loud-fail. The substring is the shared
diff --git a/test/wcc/data/r805_reject_agg_compound/case.ww b/test/wcc/data/r805_reject_agg_compound/case.ww
index 2e4c3630..58e30f29 100644
--- a/test/wcc/data/r805_reject_agg_compound/case.ww
+++ b/test/wcc/data/r805_reject_agg_compound/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: compound on aggregate field not wired (rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 type capture = struct { content: str, start: size, end: size };
 type t = struct { pc: size, cap: capture };
diff --git a/test/wcc/data/r805_reject_str_compound/case.ww b/test/wcc/data/r805_reject_str_compound/case.ww
index 7d17daaa..a25d6868 100644
--- a/test/wcc/data/r805_reject_str_compound/case.ww
+++ b/test/wcc/data/r805_reject_str_compound/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: compound on str/slice field not wired (rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 type t = struct { pc: size, name: str };
 fn addname(ts: *[]t, i: size) void = { (*ts)[i].name += "x"; };
diff --git a/test/wcc/data/r989_tagderef_ampeq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_ampeq_tagged_deref/case.ww
index b90baa0c..1309b601 100644
--- a/test/wcc/data/r989_tagderef_ampeq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_ampeq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_careteq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_careteq_tagged_deref/case.ww
index 70d5c69a..4aacfc48 100644
--- a/test/wcc/data/r989_tagderef_careteq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_careteq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_lshifteq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_lshifteq_tagged_deref/case.ww
index 0db39392..b757237a 100644
--- a/test/wcc/data/r989_tagderef_lshifteq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_lshifteq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_minuseq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_minuseq_tagged_deref/case.ww
index 46057ce4..1ab7bd0d 100644
--- a/test/wcc/data/r989_tagderef_minuseq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_minuseq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_percenteq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_percenteq_tagged_deref/case.ww
index e6df8d9c..b71e3a33 100644
--- a/test/wcc/data/r989_tagderef_percenteq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_percenteq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "modulo on non-integer type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_pipeeq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_pipeeq_tagged_deref/case.ww
index 26015cec..306d487f 100644
--- a/test/wcc/data/r989_tagderef_pipeeq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_pipeeq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_pluseq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_pluseq_tagged_deref/case.ww
index 1674bb75..b0f6c5c5 100644
--- a/test/wcc/data/r989_tagderef_pluseq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_pluseq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_rshifteq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_rshifteq_tagged_deref/case.ww
index 0063820b..7cd591a4 100644
--- a/test/wcc/data/r989_tagderef_rshifteq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_rshifteq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_slasheq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_slasheq_tagged_deref/case.ww
index 688c5517..18a25520 100644
--- a/test/wcc/data/r989_tagderef_slasheq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_slasheq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagderef_stareq_tagged_deref/case.ww b/test/wcc/data/r989_tagderef_stareq_tagged_deref/case.ww
index 8ee05b00..d5a14bd9 100644
--- a/test/wcc/data/r989_tagderef_stareq_tagged_deref/case.ww
+++ b/test/wcc/data/r989_tagderef_stareq_tagged_deref/case.ww
@@ -1,4 +1,4 @@
-//ww:error "assign-resolver: tagged field not wired (rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = {
 	let v: (int | bool) = 7;
diff --git a/test/wcc/data/r989_tagplace_gident_pluseq/case.ww b/test/wcc/data/r989_tagplace_gident_pluseq/case.ww
index 1ca21ff6..ee91c79e 100644
--- a/test/wcc/data/r989_tagplace_gident_pluseq/case.ww
+++ b/test/wcc/data/r989_tagplace_gident_pluseq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 let g: (int | bool) = 7;
 export fn main() int = { g += 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_gidx_careteq/case.ww b/test/wcc/data/r989_tagplace_gidx_careteq/case.ww
index 89ad1514..86358835 100644
--- a/test/wcc/data/r989_tagplace_gidx_careteq/case.ww
+++ b/test/wcc/data/r989_tagplace_gidx_careteq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 let gs: [3](int | bool) = [1, 2, 3];
 export fn main() int = { gs[0] ^= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_gidx_pluseq/case.ww b/test/wcc/data/r989_tagplace_gidx_pluseq/case.ww
index 5d6b34c3..cdfeba66 100644
--- a/test/wcc/data/r989_tagplace_gidx_pluseq/case.ww
+++ b/test/wcc/data/r989_tagplace_gidx_pluseq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 let gs: [3](int | bool) = [1, 2, 3];
 export fn main() int = { gs[0] += 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_gidx_stareq/case.ww b/test/wcc/data/r989_tagplace_gidx_stareq/case.ww
index ffaf9aaf..2a453277 100644
--- a/test/wcc/data/r989_tagplace_gidx_stareq/case.ww
+++ b/test/wcc/data/r989_tagplace_gidx_stareq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 let gs: [3](int | bool) = [1, 2, 3];
 export fn main() int = { gs[0] *= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_careteq/case.ww b/test/wcc/data/r989_tagplace_ident_careteq/case.ww
index 5a17e05e..8f5f185d 100644
--- a/test/wcc/data/r989_tagplace_ident_careteq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_careteq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g ^= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_lshifteq/case.ww b/test/wcc/data/r989_tagplace_ident_lshifteq/case.ww
index b61f0e11..805713f0 100644
--- a/test/wcc/data/r989_tagplace_ident_lshifteq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_lshifteq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g <<= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_minuseq/case.ww b/test/wcc/data/r989_tagplace_ident_minuseq/case.ww
index ff7a8128..17190365 100644
--- a/test/wcc/data/r989_tagplace_ident_minuseq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_minuseq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g -= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_pipeeq/case.ww b/test/wcc/data/r989_tagplace_ident_pipeeq/case.ww
index c6b166ec..49d04c86 100644
--- a/test/wcc/data/r989_tagplace_ident_pipeeq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_pipeeq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g |= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_pluseq/case.ww b/test/wcc/data/r989_tagplace_ident_pluseq/case.ww
index f808e290..a36ad338 100644
--- a/test/wcc/data/r989_tagplace_ident_pluseq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_pluseq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g += 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_slasheq/case.ww b/test/wcc/data/r989_tagplace_ident_slasheq/case.ww
index 296776e5..6a34de19 100644
--- a/test/wcc/data/r989_tagplace_ident_slasheq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_slasheq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g /= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_ident_stareq/case.ww b/test/wcc/data/r989_tagplace_ident_stareq/case.ww
index b24c55b8..57ccc7ae 100644
--- a/test/wcc/data/r989_tagplace_ident_stareq/case.ww
+++ b/test/wcc/data/r989_tagplace_ident_stareq/case.ww
@@ -1,3 +1,3 @@
-//ww:error "ident compound on tagged not wired (#21/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = { let g: (int | bool) = 7; g *= 1; return 0; };
diff --git a/test/wcc/data/r989_tagplace_lidx_lshifteq/case.ww b/test/wcc/data/r989_tagplace_lidx_lshifteq/case.ww
index 8bd0502b..1f5be4f5 100644
--- a/test/wcc/data/r989_tagplace_lidx_lshifteq/case.ww
+++ b/test/wcc/data/r989_tagplace_lidx_lshifteq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let a: [3](int | bool) = [1, 2, 3];
diff --git a/test/wcc/data/r989_tagplace_lidx_minuseq/case.ww b/test/wcc/data/r989_tagplace_lidx_minuseq/case.ww
index fb1a37d6..e25aeb32 100644
--- a/test/wcc/data/r989_tagplace_lidx_minuseq/case.ww
+++ b/test/wcc/data/r989_tagplace_lidx_minuseq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = {
 	let a: [3](int | bool) = [1, 2, 3];
diff --git a/test/wcc/data/r989_tagplace_lidx_pipeeq/case.ww b/test/wcc/data/r989_tagplace_lidx_pipeeq/case.ww
index bfe41f28..0c5cbc6e 100644
--- a/test/wcc/data/r989_tagplace_lidx_pipeeq/case.ww
+++ b/test/wcc/data/r989_tagplace_lidx_pipeeq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "bitwise on non-integer type"
 package main;
 export fn main() int = {
 	let a: [3](int | bool) = [1, 2, 3];
diff --git a/test/wcc/data/r989_tagplace_lidx_pluseq/case.ww b/test/wcc/data/r989_tagplace_lidx_pluseq/case.ww
index d8740503..d6ae0b02 100644
--- a/test/wcc/data/r989_tagplace_lidx_pluseq/case.ww
+++ b/test/wcc/data/r989_tagplace_lidx_pluseq/case.ww
@@ -1,4 +1,4 @@
-//ww:error "indexed-lvalue compound on tagged element not wired (#133/rule-7)"
+//ww:error "arithmetic on non-numeric type"
 package main;
 export fn main() int = {
 	let a: [3](int | bool) = [1, 2, 3];
diff --git a/test/wcc/data/str_pluseq_reject/case.ww b/test/wcc/data/str_pluseq_reject/case.ww
new file mode 100644
index 00000000..0a172624
--- /dev/null
+++ b/test/wcc/data/str_pluseq_reject/case.ww
@@ -0,0 +1,9 @@
+//ww:error "arithmetic on non-numeric type"
+// += needs numeric operands; `s += "cd"` passed the op-blind
+// assignability check and cgen garbled the header words silently.
+package main;
+export fn main() i32 = {
+	let s: str = "ab";
+	s += "cd";
+	return 0;
+};