Introduce o3tl::unsafe_downcast for known-good dynamic_cast downcasts

(This helps rewrite code that silences Coverity unchecked dynamic_cast warnings,
like 57b89ea1c2 "coverity#704563 Unchecked dynamic
cast", in a way that is cleaner and potentially executes faster.)

Change-Id: I3319c0eeec2aa822f686feacd499a76c8c9b9499
Reviewed-on: https://gerrit.libreoffice.org/80458
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

chart2/source/controller/main/DrawCommandDispatch.cxx

@@ -24,6 +24,7 @@
 #include <chartview/DrawModelWrapper.hxx>
 
 #include <com/sun/star/frame/CommandGroup.hpp>
+#include <o3tl/unsafe_downcast.hxx>
 #include <vcl/svapp.hxx>
 #include <svl/itempool.hxx>
 #include <editeng/adjustitem.hxx>
@@ -161,12 +162,7 @@ void DrawCommandDispatch::setAttributes( SdrObject* pObj )
                 pObj->SetMergedItem( SdrTextHorzAdjustItem( SDRTEXTHORZADJUST_BLOCK ) );
                 pObj->SetMergedItem( makeSdrTextAutoGrowHeightItem( false ) );
 
-                SdrObjCustomShape* pShape(dynamic_cast< SdrObjCustomShape* >( pObj ));
+                o3tl::unsafe_downcast< SdrObjCustomShape* >( pObj )->MergeDefaultAttributes( &m_aCustomShapeType );
-                assert(pShape);
-                if(pShape)
-                {
-                    pShape->MergeDefaultAttributes( &m_aCustomShapeType );
-                }
             }
         }
     }

include/o3tl/unsafe_downcast.hxx

@@ -0,0 +1,35 @@
+/* -*- 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/.
+ */
+
+#ifndef INCLUDED_O3TL_UNSAFE_DOWNCAST_HXX
+#define INCLUDED_O3TL_UNSAFE_DOWNCAST_HXX
+
+#include <cassert>
+#include <type_traits>
+
+namespace o3tl
+{
+// Do a downcast from polymorphic `BasePtr` to `DerivedPtr` when it is known that `p` is actually
+// pointing to an object of derived type (or is a nullptr).  This is potentially cheaper than
+// dynamic_cast and helps to avoid Coverity warnings about unchecked dynamic_cast.
+template <typename DerivedPtr, typename BasePtr>
+std::enable_if_t<
+    (std::is_pointer_v<
+         DerivedPtr> && std::is_pointer_v<BasePtr> && std::is_base_of_v<std::remove_pointer_t<BasePtr>, std::remove_pointer_t<DerivedPtr>>),
+    DerivedPtr>
+unsafe_downcast(BasePtr p)
+{
+    assert(p == nullptr || dynamic_cast<DerivedPtr>(p) != nullptr);
+    return static_cast<DerivedPtr>(p);
+}
+}
+
+#endif
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */

o3tl/CompilerTest_o3tl_unsafe_downcast.mk

@@ -0,0 +1,16 @@
+# -*- Mode: makefile-gmake; tab-width: 4; indent-tabs-mode: t; 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/.
+#
+
+$(eval $(call gb_CompilerTest_CompilerTest,o3tl_unsafe_downcast))
+
+$(eval $(call gb_CompilerTest_add_exception_objects,o3tl_unsafe_downcast, \
+    o3tl/qa/compile-unsafe_downcast \
+))
+
+# vim: set noet sw=4 ts=4:

o3tl/Module_o3tl.mk

@@ -24,6 +24,7 @@ $(eval $(call gb_Module_add_targets,o3tl,\
 $(eval $(call gb_Module_add_check_targets,o3tl,\
 	CppunitTest_o3tl_tests \
 	$(if $(COM_IS_CLANG),CompilerTest_o3tl_temporary) \
+	$(if $(COM_IS_CLANG),CompilerTest_o3tl_unsafe_downcast) \
 ))
 
 # vim: set noet sw=4:

o3tl/qa/compile-unsafe_downcast.cxx

@@ -0,0 +1,32 @@
+/* -*- 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/.
+ */
+
+#include <sal/config.h>
+
+#include <o3tl/unsafe_downcast.hxx> // expected-note@o3tl/unsafe_downcast.hxx:* 0+ {{}}
+
+struct Base
+{
+    virtual ~Base();
+};
+
+struct Derived : Base
+{
+};
+
+void f(Base* b, Derived* d)
+{
+    o3tl::unsafe_downcast<Derived*>(b);
+    o3tl::unsafe_downcast<Derived const*>(b);
+    o3tl::unsafe_downcast<Derived&>(*b); // expected-error {{}}
+    o3tl::unsafe_downcast<Derived*>(d);
+    o3tl::unsafe_downcast<Base*>(d); // expected-error {{}}
+}
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */