Fix mis-randomizing an unpacked struct member when not rand/randc (#8277) (#8278)

src/V3Randomize.cpp

@@ -4073,7 +4073,9 @@ class RandomizeVisitor final : public VNVisitor {
                 if (structDtp->packed()) {
                     randp = newRandStmtsp(fl, stmtsp ? exprp->cloneTree(false) : exprp, nullptr,
                                           outputVarp, offset, smemberp);
-                } else {
+                } else if (smemberp->rand().isRandomizable()) {
+                    // IEEE 1800-2023 18.4: an unpacked struct member is only
+                    // randomized if its own declaration carries rand/randc.
                     AstStructSel* structSelp
                         = new AstStructSel{fl, exprp->cloneTree(false), smemberp->name()};
                     structSelp->dtypep(smemberp->childDTypep());

test_regress/t/t_randomize_method.v

@@ -41,9 +41,9 @@ typedef struct packed {
 } StructOuter;
 
 typedef struct {
-  int i;
-  StructOuter j;
-  Enum k;
+  rand int i;
+  rand StructOuter j;
+  rand Enum k;
   longint z;
 } StructUnpacked;
 
@@ -117,6 +117,7 @@ class OtherCls;
     str.i = 0;
     str.j = '{x: 1'b0, y: ONE, z: 64'd0, s: '{a: 32'd0, b: 1'b0, c: ONE}};
     str.k = ONE;
+    str.z = 0;
   endfunction
 
 endclass
@@ -189,6 +190,7 @@ module t;
       if (derived1.i.e != 0) $stop;
       if (derived1.k != 0) $stop;
       if (other.v != 0) $stop;
+      if (other.str.z != 0) $stop;
       if (cont.b != null) $stop;
       if (der_int.b != 0) $stop;
       if (der_contain.cls2.a != 0) $stop;

test_regress/t/t_randomize_unpacked_struct_no_rand_members.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()
+
+test.execute()
+
+test.passes()

test_regress/t/t_randomize_unpacked_struct_no_rand_members.v

@@ -0,0 +1,36 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Aditya Shevade
+// SPDX-License-Identifier: CC0-1.0
+
+// IEEE 1800-2023 18.4: a member of an unpacked struct is random only if
+// its own declaration carries rand/randc; the containing struct being
+// rand does not imply that. Here no member does, so randomize() on obj
+// must leave the whole struct untouched.
+typedef struct {
+  int a;
+  bit [7:0] b;
+} AllUnmarked;
+
+class C;
+  rand AllUnmarked s;
+  function new();
+    s.a = 42;
+    s.b = 8'ha5;
+  endfunction
+endclass
+
+module t;
+  initial begin
+    C obj;
+    obj = new;
+    repeat (10) begin
+      if (obj.randomize() == 0) $stop;
+      if (obj.s.a != 42) $stop;
+      if (obj.s.b != 8'ha5) $stop;
+    end
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule