From 0d1f036f17449bb0857f2707551a9731d6c8eda4 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Sat, 6 Sep 2025 08:24:19 -0400
Subject: [PATCH] Add error on non-packed struct randc (#5999).

---
 Changes                                     |  1 +
 src/V3Randomize.cpp                         | 11 ++++++++---
 test_regress/t/t_randomize_unpacked_bad.out |  5 +++++
 test_regress/t/t_randomize_unpacked_bad.py  | 16 ++++++++++++++++
 test_regress/t/t_randomize_unpacked_bad.v   | 18 ++++++++++++++++++
 5 files changed, 48 insertions(+), 3 deletions(-)
 create mode 100644 test_regress/t/t_randomize_unpacked_bad.out
 create mode 100755 test_regress/t/t_randomize_unpacked_bad.py
 create mode 100644 test_regress/t/t_randomize_unpacked_bad.v

diff --git a/Changes b/Changes
index a0ab32e4d..e998f6257 100644
--- a/Changes
+++ b/Changes
@@ -13,6 +13,7 @@ Verilator 5.041 devel
 
 **Other:**
 
+* Add error on non-packed struct randc (#5999). [Seth Pellegrino]
 * Improve automatic selection of logic for DFG synthesis (#6370). [Geza Lore]
 * Improve `covergroup with function sample` handling  (#6387). [Jakub Wasilewski]
 * Optimize dead functions without references (#6380). [Artur Bieniek, Antmicro Ltd.]
diff --git a/src/V3Randomize.cpp b/src/V3Randomize.cpp
index 87d610dec..bb5e1aca2 100644
--- a/src/V3Randomize.cpp
+++ b/src/V3Randomize.cpp
@@ -1602,9 +1602,7 @@ class RandomizeVisitor final : public VNVisitor {
 
         if (AstEnumDType* const enumDtp = VN_CAST(varp->dtypep()->skipRefToEnump(), EnumDType)) {
             items = static_cast<uint64_t>(enumDtp->itemCount());
-        } else {
-            AstBasicDType* const basicp = varp->dtypep()->skipRefp()->basicp();
-            UASSERT_OBJ(basicp, varp, "Unexpected randc variable dtype");
+        } else if (AstBasicDType* const basicp = varp->dtypep()->skipRefp()->basicp()) {
             if (basicp->width() > 32) {
                 varp->v3error("Maximum implemented width for randc is 32 bits, "
                               << varp->prettyNameQ() << " is " << basicp->width() << " bits");
@@ -1612,6 +1610,13 @@ class RandomizeVisitor final : public VNVisitor {
                 return nullptr;
             }
             items = 1ULL << basicp->width();
+        } else if (AstStructDType* const dtp = VN_CAST(varp->dtypep()->skipRefp(), StructDType)) {
+            UASSERT_OBJ(!dtp->packed(), dtp, "skipRef should have hidden packed before got here");
+            dtp->v3error("Unpacked structs shall not be declared as randc"
+                         " (IEEE 1800-2023 18.4)");
+            return nullptr;
+        } else {
+            varp->v3fatalSrc("Unexpected randc variable dtype");
         }
         AstCDType* newdtp = findVlRandCDType(varp->fileline(), items);
         AstVar* newp
diff --git a/test_regress/t/t_randomize_unpacked_bad.out b/test_regress/t/t_randomize_unpacked_bad.out
new file mode 100644
index 000000000..aca911dad
--- /dev/null
+++ b/test_regress/t/t_randomize_unpacked_bad.out
@@ -0,0 +1,5 @@
+%Error: t/t_randomize_unpacked_bad.v:8:9: Unpacked structs shall not be declared as randc (IEEE 1800-2023 18.4)
+    8 |   randc struct {logic m_x;} s;   
+      |         ^~~~~~
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: Exiting due to
diff --git a/test_regress/t/t_randomize_unpacked_bad.py b/test_regress/t/t_randomize_unpacked_bad.py
new file mode 100755
index 000000000..55203b6c9
--- /dev/null
+++ b/test_regress/t/t_randomize_unpacked_bad.py
@@ -0,0 +1,16 @@
+#!/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('linter')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()
diff --git a/test_regress/t/t_randomize_unpacked_bad.v b/test_regress/t/t_randomize_unpacked_bad.v
new file mode 100644
index 000000000..d6a85358a
--- /dev/null
+++ b/test_regress/t/t_randomize_unpacked_bad.v
@@ -0,0 +1,18 @@
+// 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
+
+class ex;
+  randc struct {logic m_x;} s;  // <--- Bad: randc illegal on unpacked struct
+
+  randc struct packed {logic m_x;} p_s;  // Ok
+endclass : ex
+
+module foo;
+  initial begin
+    ex e;
+    void'(e.randomize());
+  end
+endmodule