Avoid loplugin:faileddyncast for "unnecessary", up-casting dynamic_cast

Change-Id: Iecb943db0aff7ffc21cc2f6adb625be369255b32
Reviewed-on: https://gerrit.libreoffice.org/43606Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/faileddyncast.cxx

@@ -80,7 +80,13 @@ bool isAlwaysNull(CXXDynamicCastExpr const * expr) {
     return false;
 #endif
 
+#if 0
   return !DestRD->isDerivedFrom(SrcRD);
+#else
+  return !(DestRD->isDerivedFrom(SrcRD)
+           || SrcRD->isDerivedFrom(DestRD)
+           || SrcRD == DestRD);
+#endif
 }
 
 class FailedDynCast:

compilerplugins/clang/test/faileddyncast.cxx

+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4; fill-column: 100 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+struct S1 { virtual ~S1(); };
+struct S2 final: S1 {};
+struct S3: S1 {};
+
+void f(S1 * s1, S2 * s2) {
+    (void) dynamic_cast<S2 *>(s1);
+    (void) dynamic_cast<S1 *>(s2);
+    (void) dynamic_cast<S2 *>(s2);
+    (void) dynamic_cast<S3 *>(s2); // expected-error {{dynamic_cast from 'S2 *' to 'S3 *' always fails [loplugin:faileddyncast]}}
+}
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */

solenv/CompilerTest_compilerplugins_clang.mk

@@ -21,6 +21,7 @@ $(eval $(call gb_CompilerTest_add_exception_objects,compilerplugins_clang, \
     compilerplugins/clang/test/droplong \
     compilerplugins/clang/test/externvar \
     compilerplugins/clang/test/expressionalwayszero \
+    compilerplugins/clang/test/faileddyncast \
     compilerplugins/clang/test/finalprotected \
     compilerplugins/clang/test/flatten \
     compilerplugins/clang/test/loopvartoosmall \