Fix randomize() on null object handle crashing instead of returning 0 (#7059) (#7066)

src/V3Randomize.cpp

@@ -3135,6 +3135,24 @@ class RandomizeVisitor final : public VNVisitor {
         }
     };
 
+    // Wrap obj.randomize() with null guard: (obj != null) ? obj.randomize() : 0
+    // IEEE 1800 requires randomize() on null handle to return 0.
+    // Uses user4() to prevent re-wrapping during iterateChildren.
+    void wrapRandomizeCallWithNullGuard(AstNodeFTaskRef* nodep) {
+        AstMethodCall* const callp = VN_CAST(nodep, MethodCall);
+        if (!callp) return;
+        if (callp->user4()) return;
+        callp->user4(true);
+        FileLine* const fl = callp->fileline();
+        AstNodeExpr* const checkp
+            = new AstNeq{fl, callp->fromp()->cloneTree(false), new AstConst{fl, AstConst::Null{}}};
+        VNRelinker relinker;
+        callp->unlinkFrBack(&relinker);
+        AstCond* const condp = new AstCond{fl, checkp, callp, new AstConst{fl, 0}};
+        condp->dtypeFrom(callp);
+        relinker.relink(condp);
+    }
+
     // Handle inline random variable control. After this, the randomize() call has no args
     void handleRandomizeArgs(AstNodeFTaskRef* const nodep) {
         if (!nodep->pinsp()) return;
@@ -3593,6 +3611,7 @@ class RandomizeVisitor final : public VNVisitor {
         AstWith* const withp = VN_CAST(nodep->pinsp(), With);
         if (!withp) {
             iterateChildren(nodep);
+            wrapRandomizeCallWithNullGuard(nodep);
             return;
         }
         withp->unlinkFrBack();
@@ -3730,6 +3749,7 @@ class RandomizeVisitor final : public VNVisitor {
         nodep->dtypeFrom(randomizeFuncp->dtypep());
         nodep->classOrPackagep(classp);
         UINFO(9, "Added `%s` randomization procedure");
+        wrapRandomizeCallWithNullGuard(nodep);
         VL_DO_DANGLING(withp->deleteTree(), withp);
     }
     void visit(AstConstraint* nodep) override {

test_regress/t/t_randomize_null_object.py

@@ -0,0 +1,21 @@
+#!/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')
+
+if not test.have_solver:
+    test.skip("No constraint solver installed")
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_randomize_null_object.v

@@ -0,0 +1,37 @@
+// 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 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);
+`define check_range(gotv,minv,maxv) do if ((gotv) < (minv) || (gotv) > (maxv)) begin $write("%%Error: %s:%0d:  got=%0d exp=%0d-%0d\n", `__FILE__,`__LINE__, (gotv), (minv), (maxv)); `stop; end while(0);
+// verilog_format: on
+
+class SimpleRandClass;
+  rand bit [7:0] value;
+  constraint value_con { value > 0 && value < 200; }
+  function new();
+  endfunction
+endclass
+
+module t;
+  SimpleRandClass obj;
+  int rand_result;
+
+  initial begin
+    obj = null;
+    rand_result = obj.randomize();
+    `checkd(rand_result, 0);
+
+    obj = new();
+    rand_result = obj.randomize();
+    `checkd(rand_result, 1);
+    `check_range(obj.value, 1, 199);
+
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule