Improve loplugin:unreffun

...to avoid a false positive in --enable-qt5

> [build CXX] vcl/qt5/Qt5Widget.cxx
> /home/sbergman/lo/core/vcl/qt5/Qt5Widget.cxx:466:21: error: Unreferenced function declaration [loplugin:unreffun]
>     friend QWidget* createQt5Widget(Qt5Frame& rFrame, QWidget* parent, Qt::WindowFlags f);
>     ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 1 error generated.

Change-Id: I01e47bc5f5147916737d62fe18ce5312a0221a3f
Reviewed-on: https://gerrit.libreoffice.org/55189
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

3 files changed, 60 insertions, 0 deletions

diff --git a/compilerplugins/clang/test/unreffun.cxx b/compilerplugins/clang/test/unreffun.cxx
new file mode 100644
index 000000000000..d9d56afe3c35
--- /dev/null
+++ b/compilerplugins/clang/test/unreffun.cxx
@@ -0,0 +1,25 @@
+/* -*- 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 "unreffun.hxx"
+
+template <typename> struct S
+{
+    friend void f();
+};
+
+void f() {}
+
+void g(); // expected-error {{Unreferenced function declaration [loplugin:unreffun]}}
+
+void h() // expected-error {{Unreferenced externally visible function definition [loplugin:unreffun]}}
+{
+}
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */
diff --git a/compilerplugins/clang/test/unreffun.hxx b/compilerplugins/clang/test/unreffun.hxx
new file mode 100644
index 000000000000..20d0e50056be
--- /dev/null
+++ b/compilerplugins/clang/test/unreffun.hxx
@@ -0,0 +1,17 @@
+/* -*- 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_COMPILERPLUGINS_CLANG_TEST_UNREFFUN_HXX
+#define INCLUDED_COMPILERPLUGINS_CLANG_TEST_UNREFFUN_HXX
+
+void f();
+
+#endif
+
+/* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */
diff --git a/compilerplugins/clang/unreffun.cxx b/compilerplugins/clang/unreffun.cxx
index d825423bfeaa..bfd48b328300 100644
--- a/compilerplugins/clang/unreffun.cxx
+++ b/compilerplugins/clang/unreffun.cxx
@@ -58,7 +58,18 @@ public:
     void run() override
     { TraverseDecl(compiler.getASTContext().getTranslationUnitDecl()); }
 
+    bool TraverseFriendDecl(FriendDecl * decl) {
+        auto const old = friendFunction_;
+        friendFunction_ = dyn_cast_or_null<FunctionDecl>(decl->getFriendDecl());
+        auto const ret = RecursiveASTVisitor::TraverseFriendDecl(decl);
+        friendFunction_ = old;
+        return ret;
+    }
+
     bool VisitFunctionDecl(FunctionDecl const * decl);
+
+private:
+    FunctionDecl const * friendFunction_ = nullptr;
 };
 
 bool UnrefFun::VisitFunctionDecl(FunctionDecl const * decl) {
@@ -74,6 +85,13 @@ bool UnrefFun::VisitFunctionDecl(FunctionDecl const * decl) {
     {
         return true;
     }
+    if (decl == friendFunction_) {
+        if (auto const lex = dyn_cast<CXXRecordDecl>(decl->getLexicalDeclContext())) {
+            if (lex->isDependentContext()) {
+                return true;
+            }
+        }
+    }
 
     if (!(decl->isThisDeclarationADefinition() || isFriendDecl(decl)
           || decl->isFunctionTemplateSpecialization()))