Fix regression rejecting boolean `!x` inside sequence expressions (#7549) (#7551)

Fixes #7549.

src/V3AstNodeExpr.h

@@ -5595,9 +5595,13 @@ public:
 };
 class AstLogNot final : public AstNodeUniop {
     // @astgen makeDfgVertex
+private:
+    const bool m_fromProperty;  // True if from property 'not' keyword (IEEE 1800-2023 16.12.3),
+                                // false for boolean '!' (IEEE 1800-2023 11.4.7)
 public:
-    AstLogNot(FileLine* fl, AstNodeExpr* lhsp)
+    AstLogNot(FileLine* fl, AstNodeExpr* lhsp, bool fromProperty = false)
-        : ASTGEN_SUPER_LogNot(fl, lhsp) {
+        : ASTGEN_SUPER_LogNot(fl, lhsp)
+        , m_fromProperty{fromProperty} {
         dtypeSetBit();
     }
     ASTGEN_MEMBERS_AstLogNot;
@@ -5609,6 +5613,9 @@ public:
     bool cleanOut() const override { return true; }
     bool cleanLhs() const override { return true; }
     bool sizeMattersLhs() const override { return false; }
+    bool fromProperty() const { return m_fromProperty; }
+    void dump(std::ostream& str) const override;
+    void dumpJson(std::ostream& str) const override;
 };
 class AstNToI final : public AstNodeUniop {
     // String to any-size integral

src/V3AstNodes.cpp

@@ -1912,6 +1912,14 @@ void AstCCast::dumpJson(std::ostream& str) const {
     dumpJsonNumFunc(str, size);
     dumpJsonGen(str);
 }
+void AstLogNot::dump(std::ostream& str) const {
+    this->AstNodeUniop::dump(str);
+    if (fromProperty()) str << " [fromProperty]";
+}
+void AstLogNot::dumpJson(std::ostream& str) const {
+    dumpJsonBoolFuncIf(str, fromProperty);
+    dumpJsonGen(str);
+}
 void AstCvtArrayToArray::dump(std::ostream& str) const {
     this->AstNodeExpr::dump(str);
     str << " reverse=" << reverse();

src/V3Width.cpp

@@ -7658,7 +7658,7 @@ class WidthVisitor final : public VNVisitor {
         }
     }
 
-    void visit_log_not(AstNode* nodep) {
+    void visit_log_not(AstLogNot* nodep) {
         // CALLER: LogNot
         // Width-check: lhs 1 bit
         // Real: Allowed; implicitly compares with zero
@@ -7674,7 +7674,10 @@ class WidthVisitor final : public VNVisitor {
         if (m_vup->prelim()) {
             iterateCheckBool(nodep, "LHS", nodep->op1p(), BOTH);
             nodep->dtypeSetBit();
-            if (m_underSExpr) {
+            // IEEE 1800-2023 16.12.3: property 'not' is not a sequence operator.
+            // Boolean '!' is allowed in sequences (16.7 expression_or_dist).
+            // The parser distinguishes the two via AstLogNot::fromProperty().
+            if (m_underSExpr && nodep->fromProperty()) {
                 nodep->v3error("Unexpected 'not' in sequence expression context");
                 AstConst* const newp = new AstConst{nodep->fileline(), 0};
                 newp->dtypeFrom(nodep);

src/verilog.y

@@ -6737,7 +6737,7 @@ pexpr<nodeExprp>:  // IEEE: property_expr  (The name pexpr is important as regex
         //                      // Expanded below
         //
                 yNOT pexpr
-                        { $$ = new AstLogNot{$1, $2}; }
+                        { $$ = new AstLogNot{$1, $2, /*fromProperty=*/true}; }
         |       ySTRONG '(' sexpr ')'
                         { $$ = $3; BBUNSUP($2, "Unsupported: strong (in property expression)"); }
         |       yWEAK '(' sexpr ')'

test_regress/t/t_assert_bang_in_seq.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# This program is free software; you can redistribute it and/or modify it
+# under the terms of either the GNU Lesser General Public License Version 3
+# or the Perl Artistic License Version 2.0.
+# SPDX-FileCopyrightText: 2026 Wilson Snyder
+# SPDX-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator')
+
+test.compile(verilator_flags2=['--assert', '--timing'])
+
+test.execute()
+
+test.passes()

test_regress/t/t_assert_bang_in_seq.v

@@ -0,0 +1,79 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 PlanV GmbH
+// SPDX-License-Identifier: CC0-1.0
+
+// verilog_format: off
+`define stop $stop
+`define checkh(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0x exp=%0x (%s !== %s)\n", `__FILE__,`__LINE__, (gotv), (expv), `"gotv`", `"expv`"); `stop; end while(0);
+`define checkd(gotv,expv) do if ((gotv) !== (expv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d\n", `__FILE__,`__LINE__, (gotv), (expv)); `stop; end while(0);
+// verilog_format: on
+
+module t (
+    input clk
+);
+
+  int cyc;
+  reg [63:0] crc;
+
+  // Non-adjacent CRC bits avoid LFSR shift correlation.
+  wire a = crc[0];
+  wire b = crc[4];
+  wire c = crc[8];
+  wire d = crc[12];
+
+  int count_fail1 = 0;
+  int count_fail2 = 0;
+  int count_fail3 = 0;
+  int count_fail4 = 0;
+  int count_fail5 = 0;
+  int count_fail6 = 0;
+
+  // Test 1: a ##1 !b -- boolean ! as consequent of cycle delay.
+  assert property (@(posedge clk) a ##1 !b)
+  else count_fail1 <= count_fail1 + 1;
+
+  // Test 2: a ##1 (!b && !c) -- conjunction of two bangs in seq.
+  assert property (@(posedge clk) a ##1 (!b && !c))
+  else count_fail2 <= count_fail2 + 1;
+
+  // Test 3: a |-> ##1 !b -- bang on consequent of overlapping implication
+  // (mirrors core-v-verif `|-> ##1 !irq_enabled`).
+  assert property (@(posedge clk) a |-> ##1 !b)
+  else count_fail3 <= count_fail3 + 1;
+
+  // Test 4: a ##1 !$past(b) -- bang applied to sampled value function.
+  assert property (@(posedge clk) a ##1 !$past(b))
+  else count_fail4 <= count_fail4 + 1;
+
+  // Test 5: a ##0 !b ##1 c -- bang interleaved between cycle delays.
+  assert property (@(posedge clk) a ##0 !b ##1 c)
+  else count_fail5 <= count_fail5 + 1;
+
+  // Test 6: a |=> !d -- bang on consequent of non-overlapping implication.
+  assert property (@(posedge clk) a |=> !d)
+  else count_fail6 <= count_fail6 + 1;
+
+  always @(posedge clk) begin
+`ifdef TEST_VERBOSE
+    $write("[%0t] cyc==%0d crc=%x a=%b b=%b c=%b d=%b\n", $time, cyc, crc, a, b, c, d);
+`endif
+    cyc <= cyc + 1;
+    crc <= {crc[62:0], crc[63] ^ crc[2] ^ crc[0]};
+    if (cyc == 0) begin
+      crc <= 64'h5aef0c8d_d70a4497;
+    end
+    else if (cyc == 99) begin
+      `checkh(crc, 64'hc77bb9b3784ea091);
+      `checkd(count_fail1, 66);  // Questa: 66
+      `checkd(count_fail2, 69);  // Questa: 69
+      `checkd(count_fail3, 26);  // Questa: 26
+      `checkd(count_fail4, 66);  // Questa: 66
+      `checkd(count_fail5, 80);  // Questa: 80
+      `checkd(count_fail6, 27);  // Questa: 27
+      $write("*-* All Finished *-*\n");
+      $finish;
+    end
+  end
+endmodule