Fix crash on sub-interface method call via a virtual interface

src/V3Width.cpp

@@ -4782,6 +4782,15 @@ class WidthVisitor final : public VNVisitor {
     }
     void methodCallIfaceRef(AstMethodCall* nodep, AstIfaceRefDType* adtypep) {
         AstIface* const ifacep = adtypep->ifacep();
+        if (!ifacep) {
+            // A sub-interface reached through a virtual interface handle has a
+            // ref dtype with cellp set but ifacep null; the per-instance binding
+            // is not modelled, so the call cannot be resolved to one instance.
+            nodep->v3warn(E_UNSUPPORTED, "Unsupported: method call on a sub-interface "
+                                         "accessed through a virtual interface");
+            nodep->dtypeSetVoid();
+            return;
+        }
         UINFO(5, __FUNCTION__ << ":" << nodep);
         if (AstNodeFTask* const ftaskp
             = VN_CAST(m_memberMap.findMember(ifacep, nodep->name()), NodeFTask)) {

test_regress/t/t_interface_virtual_sub_iface_method_bad.out

@@ -0,0 +1,6 @@
+%Error-UNSUPPORTED: t/t_interface_virtual_sub_iface_method_bad.v:21:16: Unsupported: method call on a sub-interface accessed through a virtual interface
+                                                                      : ... note: In instance 't'
+   21 |     vif.sub_if.set_active();
+      |                ^~~~~~~~~~
+                    ... For error description see https://verilator.org/warn/UNSUPPORTED?v=latest
+%Error: Exiting due to

test_regress/t/t_interface_virtual_sub_iface_method_bad.py

@@ -0,0 +1,16 @@
+#!/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('linter')
+
+test.lint(fails=True, expect_filename=test.golden_filename)
+
+test.passes()

test_regress/t/t_interface_virtual_sub_iface_method_bad.v

@@ -0,0 +1,33 @@
+// 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
+
+interface inner_if;
+  logic active = 1'b0;
+  function automatic void set_active();
+    active = 1'b1;
+  endfunction
+endinterface
+
+interface outer_if;
+  inner_if sub_if ();
+endinterface
+
+class cfg_c;
+  virtual outer_if vif;
+  function void doit();
+    vif.sub_if.set_active();
+  endfunction
+endclass
+
+module t;
+  outer_if oif ();
+  cfg_c cfg;
+  initial begin
+    cfg = new();
+    cfg.vif = oif;
+    cfg.doit();
+  end
+endmodule