Fix circular class inheritance hang (#8324) (#8545)

src/V3LinkDot.cpp

@@ -3463,6 +3463,24 @@ class LinkDotResolveVisitor final : public VNVisitor {
         }
         defParamPins.emplace(nodep->paramPath(), nodep);
     }
+    static bool extendsClass(const AstClass* classp, const AstClass* targetp) {
+        // Return true if classp inherits from targetp through already resolved extends
+        // If performance of this becomes a problem, use just a counter to check
+        // exceeds some number of iterations then perform the more expensive analysis
+        std::set<const AstClass*> visited;
+        std::vector<const AstClass*> todo{classp};
+        while (!todo.empty()) {
+            const AstClass* const currp = todo.back();
+            todo.pop_back();
+            if (currp == targetp) return true;
+            if (!visited.insert(currp).second) continue;
+            for (const AstClassExtends* cextp = currp->extendsp(); cextp;
+                 cextp = VN_AS(cextp->nextp(), ClassExtends)) {
+                if (const AstClass* const basep = cextp->classOrNullp()) todo.push_back(basep);
+            }
+        }
+        return false;
+    }
     static AstClocking* sensClockingp(AstNode* nodep) {
         // Return the clocking block referenced by nodep, either directly or through a modport
         if (AstClocking* const clockingp = VN_CAST(nodep, Clocking)) return clockingp;
@@ -5989,6 +6007,11 @@ class LinkDotResolveVisitor final : public VNVisitor {
                     if (baseClassp == nodep) {
                         cextp->v3error("Attempting to extend class " << nodep->prettyNameQ()
                                                                      << " from itself");
+                    } else if (extendsClass(baseClassp, nodep)) {
+                        cextp->v3error("Attempting to extend class "
+                                       << nodep->prettyNameQ() << " from "
+                                       << baseClassp->prettyNameQ()
+                                       << ", which circularly inherits from it");
                     } else if (cextp->isImplements() && !baseClassp->isInterfaceClass()) {
                         cextp->v3error("Attempting to implement from non-interface class "
                                        << baseClassp->prettyNameQ() << '\n'

test_regress/t/t_class_extends_cycle_bad.out

@@ -0,0 +1,23 @@
+%Error: t/t_class_extends_cycle_bad.v:10:21: Attempting to extend class 'Cls2B' from 'Cls2A', which circularly inherits from it
+   10 | class Cls2B extends Cls2A;
+      |                     ^~~~~
+        ... See the manual at https://verilator.org/verilator_doc.html?v=latest for more assistance.
+%Error: t/t_class_extends_cycle_bad.v:8:21: Attempting to extend class 'Cls2A' from 'Cls2B', which circularly inherits from it
+    8 | class Cls2A extends Cls2B;
+      |                     ^~~~~
+%Error: t/t_class_extends_cycle_bad.v:16:21: Attempting to extend class 'Cls3B' from 'Cls3A', which circularly inherits from it
+   16 | class Cls3B extends Cls3A;
+      |                     ^~~~~
+%Error: t/t_class_extends_cycle_bad.v:18:21: Attempting to extend class 'Cls3C' from 'Cls3B', which circularly inherits from it
+   18 | class Cls3C extends Cls3B;
+      |                     ^~~~~
+%Error: t/t_class_extends_cycle_bad.v:14:21: Attempting to extend class 'Cls3A' from 'Cls3C', which circularly inherits from it
+   14 | class Cls3A extends Cls3C;
+      |                     ^~~~~
+%Error: t/t_class_extends_cycle_bad.v:24:30: Attempting to extend class 'IfcB' from 'IfcA', which circularly inherits from it
+   24 | interface class IfcB extends IfcA;
+      |                              ^~~~
+%Error: t/t_class_extends_cycle_bad.v:22:30: Attempting to extend class 'IfcA' from 'IfcB', which circularly inherits from it
+   22 | interface class IfcA extends IfcB;
+      |                              ^~~~
+%Error: Exiting due to

test_regress/t/t_class_extends_cycle_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_class_extends_cycle_bad.v

@@ -0,0 +1,30 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain.
+// SPDX-FileCopyrightText: 2026 Saqib Khan
+// SPDX-License-Identifier: CC0-1.0
+
+// Two-class cycle
+class Cls2A extends Cls2B;
+endclass
+class Cls2B extends Cls2A;
+endclass
+
+// Three-class cycle
+class Cls3A extends Cls3C;
+endclass
+class Cls3B extends Cls3A;
+endclass
+class Cls3C extends Cls3B;
+endclass
+
+// Interface class cycle
+interface class IfcA extends IfcB;
+endclass
+interface class IfcB extends IfcA;
+endclass
+
+module t;
+  Cls2A c2 = new;
+  Cls3B c3 = new;
+endmodule