From 5ee938fd1cee415075ced9551199e45f3191df52 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Fri, 8 Mar 2024 17:33:51 -0500
Subject: [PATCH] Fix assignment of null into struct member (#4952).

---
 Changes                               |  3 ++-
 src/V3Cast.cpp                        |  5 ++--
 test_regress/t/t_class_null_struct.pl | 20 ++++++++++++++
 test_regress/t/t_class_null_struct.v  | 39 +++++++++++++++++++++++++++
 4 files changed, 64 insertions(+), 3 deletions(-)
 create mode 100755 test_regress/t/t_class_null_struct.pl
 create mode 100644 test_regress/t/t_class_null_struct.v

diff --git a/Changes b/Changes
index af031e8e1..a47207a0c 100644
--- a/Changes
+++ b/Changes
@@ -23,11 +23,12 @@ Verilator 5.023 devel
 * Support public packed struct / union (#860) (#4878). [Kefa Chen]
 * Change installation to be relocatable (#4927). [Geza Lore]
 * Fix __Vlip undefined error in --freloop (#4824). [Justin Yao Du]
-* Fix invalid cast on string structure creation (#4921). [esynr3z]
+* Fix invalid cast on string structure creation (#4921).
 * Fix try-lock spuriously fails (#4931) (#4938). [Kamil Rakoczy]
 * Fix V3Unknown unpacked struct x-assign (#4934). [Yan Xu]
 * Fix DFG removing forceable signals (#4942). [Geza Lore]
 * Fix null characters in shortened identifiers (#4946). [Abdul Hameed]
+* Fix assignment of null into struct member (#4952).
 
 
 Verilator 5.022 2024-02-24
diff --git a/src/V3Cast.cpp b/src/V3Cast.cpp
index b6a79353e..8a6df2876 100644
--- a/src/V3Cast.cpp
+++ b/src/V3Cast.cpp
@@ -82,7 +82,8 @@ class CastVisitor final : public VNVisitor {
     }
     void ensureCast(AstNodeExpr* nodep) {
         if (castSize(nodep->backp()) != castSize(nodep) || !nodep->user1()) {
-            insertCast(nodep, castSize(nodep->backp()));
+            const AstConst* const constp = VN_CAST(nodep, Const);
+            if (!(constp && constp->num().isNull())) insertCast(nodep, castSize(nodep->backp()));
         }
     }
     void ensureLower32Cast(AstCCast* nodep) {
@@ -201,7 +202,7 @@ class CastVisitor final : public VNVisitor {
         // Constants are of unknown size if smaller than 33 bits, because
         // we're too lazy to wrap every constant in the universe in
         // ((IData)#).
-        nodep->user1(nodep->isQuad() || nodep->isWide());
+        nodep->user1(nodep->isQuad() || nodep->isWide() || nodep->num().isNull());
     }
 
     // Null dereference protection
diff --git a/test_regress/t/t_class_null_struct.pl b/test_regress/t/t_class_null_struct.pl
new file mode 100755
index 000000000..49330a5fe
--- /dev/null
+++ b/test_regress/t/t_class_null_struct.pl
@@ -0,0 +1,20 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2020 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
+
+scenarios(simulator => 1);
+
+compile(
+    );
+
+execute(
+    );
+
+ok(1);
+1;
diff --git a/test_regress/t/t_class_null_struct.v b/test_regress/t/t_class_null_struct.v
new file mode 100644
index 000000000..c68a382f8
--- /dev/null
+++ b/test_regress/t/t_class_null_struct.v
@@ -0,0 +1,39 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2024 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+class Cls;
+endclass : Cls
+
+typedef struct {
+   Cls obj;
+   int number;
+} str_t;
+
+module t (/*AUTOARG*/);
+   function automatic str_t func_null();
+      return '{null, 42};
+   endfunction
+
+   function automatic str_t func_obj();
+      Cls c;
+      c = new;
+      return '{c, 43};
+   endfunction
+
+   initial begin
+      str_t result;
+      result = func_null();
+      if (result.obj != null) $stop;
+      if (result.number != 42) $stop;
+
+      result = func_obj();
+      if (result.obj == null) $stop;
+      if (result.number != 43) $stop;
+
+      $write("*-* All Finished *-*\n");
+      $finish;
+   end
+endmodule