Fix conditional expression with parameterized classes (#8039)

Signed-off-by: Pawel Klopotek <pklopotek@internships.antmicro.com>

src/V3Ast.cpp

@@ -1735,12 +1735,15 @@ AstNodeDType* AstNode::getCommonClassTypep(AstNode* node1p, AstNode* node2p) {
         if (castable == VCastable::DYNAMIC_CLASS) return node2p->dtypep();
     }
 
-    AstClassRefDType* classDtypep1 = VN_CAST(node1p->dtypep(), ClassRefDType);
+    AstClassRefDType* classDtypep1 = VN_CAST(node1p->dtypep()->skipRefp(), ClassRefDType);
     while (classDtypep1) {
         const VCastable castable = computeCastable(classDtypep1, node2p->dtypep(), node2p);
         if (castable == VCastable::COMPATIBLE) return classDtypep1;
-        const AstClassExtends* const extendsp = classDtypep1->classp()->extendsp();
-        classDtypep1 = extendsp ? VN_AS(extendsp->dtypep(), ClassRefDType) : nullptr;
+        AstClassExtends* const extendsp = classDtypep1->classp()->extendsp();
+        if (!extendsp) break;
+        AstNodeDType* const edtp
+            = extendsp->dtypep() ? extendsp->dtypep() : extendsp->childDTypep();
+        classDtypep1 = VN_AS(edtp->skipRefp(), ClassRefDType);
     }
     return nullptr;
 }

test_regress/t/t_param_class_cond.py

@@ -0,0 +1,18 @@
+#!/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('simulator')
+
+test.compile()
+
+test.execute()
+
+test.passes()

test_regress/t/t_param_class_cond.v

@@ -0,0 +1,49 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Antmicro
+// SPDX-License-Identifier: CC0-1.0
+
+class Base;
+  int value;
+endclass
+
+typedef Base Base_t;
+
+class SubA #(type T = int) extends Base_t;
+endclass
+
+class SubB #(type T = int) extends Base_t;
+endclass
+
+typedef SubA #(int) SubAInt_t;
+typedef SubB #(int) SubBInt_t;
+
+class Container;
+  local SubAInt_t a;
+  local SubBInt_t b;
+  function new();
+    a = new;
+    a.value = 1;
+    b = new;
+    b.value = 2;
+  endfunction
+  function Base test(int sel);
+    test = sel[0] ? a : b;
+  endfunction
+endclass
+
+module t;
+  Container c;
+  int cyc;
+  Base result;
+  initial begin
+    c = new;
+    for (cyc = 0; cyc < 100; ++cyc) begin
+      result = c.test(cyc);
+      if (result.value != (cyc[0] ? 1 : 2)) $stop;
+    end
+    $write("*-* All Finished *-*\n");
+    $finish;
+  end
+endmodule