Fix false unique assertions on `else ;` (#6450).

Changes

@@ -51,6 +51,7 @@ Verilator 5.041 devel
 * Fix internal error on out-of-bounds real array access.
 * Fix pre/post increments in assertions (#6434).
 * Fix elaboration displays with `%m` (#6445). [Alex Solomatnikov]
+* Fix false unique assertions on `else ;` (#6450). [Don Owen]
 
 
 Verilator 5.040 2025-08-30

src/V3ParseImp.h

@@ -248,6 +248,13 @@ public:
         return nump;
     }
 
+    // For 'unique if' cannot have empty else statements as then assertion
+    // would misfire
+    AstNode* newBlock(FileLine* fl, AstNode* nodep) {
+        if (nodep) return nodep;
+        return new AstBegin{fl, "", nullptr, false, true};
+    }
+
     // Bison sometimes needs error context without a token, so remember last token's line
     // Only use this if do not have and cannot get a token-relevant fileline
     FileLine* bisonLastFileline() const { return m_bisonLastFileline; }

src/verilog.y

@@ -3522,13 +3522,16 @@ statement_item<nodep>:          // IEEE: statement_item
         //
         //                      // IEEE: conditional_statement
         |       unique_priorityE yIF '(' expr ')' stmtBlock     %prec prLOWER_THAN_ELSE
-                        { AstIf* const newp = new AstIf{$2, $4, $6};
+                        { AstIf* const newp = new AstIf{$2, $4,
+                              PARSEP->newBlock($2, $6)};
                           $$ = newp;
                           if ($1 == uniq_UNIQUE) newp->uniquePragma(true);
                           if ($1 == uniq_UNIQUE0) newp->unique0Pragma(true);
                           if ($1 == uniq_PRIORITY) newp->priorityPragma(true); }
         |       unique_priorityE yIF '(' expr ')' stmtBlock yELSE stmtBlock
-                        { AstIf* const newp = new AstIf{$2, $4, $6, $8};
+                        { AstIf* const newp = new AstIf{$2, $4,
+                              PARSEP->newBlock($2, $6),
+                              PARSEP->newBlock($2, $8)};
                           $$ = newp;
                           if ($1 == uniq_UNIQUE) newp->uniquePragma(true);
                           if ($1 == uniq_UNIQUE0) newp->unique0Pragma(true);

test_regress/t/t_uniqueif_else.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2025 by Wilson Snyder. 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-License-Identifier: LGPL-3.0-only OR Artistic-2.0
+
+import vltest_bootstrap
+
+test.scenarios('simulator_st')
+
+test.compile(verilator_flags2=['--binary'])
+
+test.execute()
+
+test.passes()

test_regress/t/t_uniqueif_else.v

@@ -0,0 +1,65 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+// verilog_format: off
+`define stop $stop
+`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;
+  logic clk;
+  logic A, B, C;
+  logic reset;
+  logic x;
+
+  always #1 clk = ~clk;
+
+  initial begin
+    clk = 0;
+    reset = 0;
+    A = 0;
+    B = 0;
+    C = 0;
+    #10;
+    reset = 1;
+    #2;
+    A = 1;
+    #2;
+    `checkd(x, 1'b0);
+
+    #2;
+    B = 1;
+    #2;
+    `checkd(x, 1'b0);
+
+    #2;
+    B = 0;
+    C = 1;
+    #2;
+    `checkd(x, 1'b1);
+
+    #10;
+    $finish;
+  end
+
+  always_ff @(posedge clk or negedge reset) begin
+    if (!reset) begin
+      x <= '0;
+    end
+    else if (A) begin
+      unique if (B) begin
+        x <= '0;
+      end
+      else if (C) begin
+        x <= '1;
+      end
+      // This passes:
+      // else begin end
+      else;  // For unique if to not have a false positive
+    end
+  end
+
+endmodule