From 94f8181cff52cd93b958a00418902cb1a457f0a0 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Thu, 19 Mar 2026 20:29:02 -0400
Subject: [PATCH] Fix null assignment to virtual interfaces (#5974) (#5990).
 [Maxim Fonarev]

Fixes #5974.

Co-authored-by: Maksim Fonarev <fonarickm@yandex.ru>
---
 Changes                              |  1 +
 docs/CONTRIBUTORS                    |  1 +
 include/verilated_types.h            |  5 ++++-
 src/V3Width.cpp                      |  4 +++-
 test_regress/t/t_interface_virtual.v | 15 +++++++++++++--
 5 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/Changes b/Changes
index 44a0ca304..927c23232 100644
--- a/Changes
+++ b/Changes
@@ -46,6 +46,7 @@ Verilator 5.047 devel
 * Fix recursive default assignment for sub-arrays (#4589) (#7202). [Julian Carrier]
 * Fix shift width mismatch in constraint solver SMT emission (#5420) (#7265). [Yilou Wang]
 * Fix randomize size+element queue constraints (#5582) (#7225). [Rahul Behl, Testorrent USA, Inc.]
+* Fix null assignment to virtual interfaces (#5974) (#5990). [Maxim Fonarev]
 * Fix lambda coroutines (#6106) (#7135). [Nick Brereton]
 * Fix super constructor calls with local variables (#6214) (#6933). [Igor Zaworski, Antmicro Ltd.]
 * Fix false recursive definition error (#6769) (#7118). [Alex Zhou]
diff --git a/docs/CONTRIBUTORS b/docs/CONTRIBUTORS
index d04f9760f..ef669d945 100644
--- a/docs/CONTRIBUTORS
+++ b/docs/CONTRIBUTORS
@@ -176,6 +176,7 @@ Mateusz Gancarz
 Matt Stroud
 Matthew Ballance
 Max Wipfli
+Maxim Fonarev
 Michael Bedford Taylor
 Michael Bikovitsky
 Michael Killough
diff --git a/include/verilated_types.h b/include/verilated_types.h
index 90e2b9b78..c50cc044a 100644
--- a/include/verilated_types.h
+++ b/include/verilated_types.h
@@ -1931,7 +1931,10 @@ public:
 
 struct VlNull final {
     operator bool() const { return false; }
-    bool operator==(const void* ptr) const { return !ptr; }
+    template <class T>
+    operator T*() const {
+        return nullptr;
+    }
 };
 inline bool operator==(const void* ptr, VlNull) { return !ptr; }
 
diff --git a/src/V3Width.cpp b/src/V3Width.cpp
index ab54eb30e..606382e36 100644
--- a/src/V3Width.cpp
+++ b/src/V3Width.cpp
@@ -8676,7 +8676,9 @@ class WidthVisitor final : public VNVisitor {
                        = VN_CAST(expDTypep->skipRefp(), IfaceRefDType)) {
                 const AstIfaceRefDType* underIfaceRefp
                     = VN_CAST(underp->dtypep()->skipRefp(), IfaceRefDType);
-                if (!underIfaceRefp) {
+                if (VN_IS(underp, Const) && VN_AS(underp, Const)->num().isNull()) {
+                    // '= null' is ok
+                } else if (!underIfaceRefp) {
                     underp->v3error(ucfirst(parentp->prettyOperatorName())
                                     << " expected " << expIfaceRefp->ifaceViaCellp()->prettyNameQ()
                                     << " interface on " << side << " but " << underp->prettyNameQ()
diff --git a/test_regress/t/t_interface_virtual.v b/test_regress/t/t_interface_virtual.v
index 266465594..0bd84dc24 100644
--- a/test_regress/t/t_interface_virtual.v
+++ b/test_regress/t/t_interface_virtual.v
@@ -34,10 +34,21 @@ module t;
   Clsgen #(virtual PBus) gen;
 
   initial begin
+    if (va != null) $stop;
+    if (null != va) $stop;
+    va = null;
+    if (va != null) $stop;
+    if (null != va) $stop;
     va = ia;
-    vb = ia;
-
     if (va == null) $stop;
+    if (null == va) $stop;
+    va = null;
+    if (va != null) $stop;
+    if (null != va) $stop;
+    va = ia;
+    if (va != ia) $stop;
+
+    vb = ia;
 
     $display("va==vb? %b", va == vb);
     $display("va!=vb? %b", va != vb);