Fix upcasting class type parameters (#6344)

Signed-off-by: Krzysztof Bieganski <kbieganski@antmicro.com>

src/V3EmitCFunc.h

@@ -1337,7 +1337,8 @@ public:
     }
     void visit(AstCCast* nodep) override {
         // Extending a value of the same word width is just a NOP.
-        if (const AstClassRefDType* const classDtypep = VN_CAST(nodep->dtypep(), ClassRefDType)) {
+        if (const AstClassRefDType* const classDtypep
+            = VN_CAST(nodep->dtypep()->skipRefp(), ClassRefDType)) {
             putns(nodep, "(" + classDtypep->cType("", false, false) + ")(");
         } else if (nodep->size() <= VL_BYTESIZE) {
             putns(nodep, "(CData)(");

test_regress/t/t_class_param_upcast.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('simulator')
+
+test.compile()
+
+test.passes()

test_regress/t/t_class_param_upcast.v

@@ -0,0 +1,33 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2025 by Antmicro.
+// SPDX-License-Identifier: CC0-1.0
+
+class factory #(type T);
+  static function T create;
+    T obj = new;
+    return obj;
+  endfunction
+endclass
+
+class foo;
+endclass
+
+class bar extends foo;
+  static function bar create;
+    bar b = new;
+    return b;
+  endfunction
+endclass
+
+module t;
+  initial begin
+    foo f;
+    if (bit'($random))
+      f = bar::create;
+    else
+      f = factory#(foo)::create();
+    $finish;
+  end
+endmodule;