Add error on class extending itself

src/V3LinkDot.cpp

@@ -2774,6 +2774,10 @@ private:
                             if (AstClass* classp = VN_CAST(foundp->nodep(), Class)) {
                                 UINFO(8, "Import to " << nodep << " from export class " << classp
                                                       << endl);
+                                if (classp == nodep) {
+                                    cextp->v3error("Attempting to extend class "
+                                                   << nodep->prettyNameQ() << " from itself");
+                                } else {
                                     AstClassRefDType* newp
                                         = new AstClassRefDType{nodep->fileline(), classp};
                                     cextp->childDTypep(newp);
@@ -2783,6 +2787,7 @@ private:
                                     m_curSymp->importFromClass(m_statep->symsp(), srcp);
                                     VL_DO_DANGLING(cpackagerefp->unlinkFrBack()->deleteTree(),
                                                    cpackagerefp);
+                                }
                                 ok = true;
                             }
                         }

test_regress/t/t_class_extends_rec_bad.out

@@ -0,0 +1,4 @@
+%Error: t/t_class_extends_rec_bad.v:7:31: Attempting to extend class 'RecursiveExtCls' from itself
+    7 | class RecursiveExtCls extends RecursiveExtCls;
+      |                               ^~~~~~~~~~~~~~~
+%Error: Exiting due to

test_regress/t/t_class_extends_rec_bad.pl

@@ -0,0 +1,19 @@
+#!/usr/bin/env perl
+if (!$::Driver) { use FindBin; exec("$FindBin::Bin/bootstrap.pl", @ARGV, $0); die; }
+# DESCRIPTION: Verilator: Verilog Test driver/expect definition
+#
+# Copyright 2020 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
+
+scenarios(linter => 1);
+
+lint(
+    fails => 1,
+    expect_filename => $Self->{golden_filename},
+    );
+
+ok(1);
+1;

test_regress/t/t_class_extends_rec_bad.v

@@ -0,0 +1,13 @@
+// DESCRIPTION: Verilator: Verilog Test module
+//
+// This file ONLY is placed under the Creative Commons Public Domain, for
+// any use, without warranty, 2020 by Wilson Snyder.
+// SPDX-License-Identifier: CC0-1.0
+
+class RecursiveExtCls extends RecursiveExtCls;
+   int i;
+endclass
+
+module t (/*AUTOARG*/);
+   RecursiveExtCls cls = new;
+endmodule