Prevent loplugin:unreffun on explicitly defaulted special members

...as started to appear (with sufficiently recent Clang versions only, it
appears) after c5fcb476ac8eab18152a7f6d0487daa56231fcf8 "toolkit: avoid
-Werror=deprecated-copy (GCC trunk towards GCC 9)":

> [CXX] toolkit/source/controls/spinningprogress.cxx
> toolkit/source/controls/spinningprogress.cxx:38:5: error: Unreferenced externally invisible function definition [loplugin:unreffun]
>     SpinningProgressControlModel(SpinningProgressControlModel &&) = default;
>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: Id78cd2d801e9760dde9e0a594e2b62ec20840204
Reviewed-on: https://gerrit.libreoffice.org/58180
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/compilerplugins/clang/unreffun.cxx b/compilerplugins/clang/unreffun.cxx
index bfd48b328300..629e369dadc7 100644
--- a/compilerplugins/clang/unreffun.cxx
+++ b/compilerplugins/clang/unreffun.cxx
@@ -51,6 +51,19 @@ Decl const * getPreviousNonFriendDecl(Decl const * decl) {
     }
 }
 
+bool isSpecialMemberFunction(FunctionDecl const * decl) {
+    if (auto const ctor = dyn_cast<CXXConstructorDecl>(decl)) {
+        return ctor->isDefaultConstructor() || ctor->isCopyOrMoveConstructor();
+    }
+    if (isa<CXXDestructorDecl>(decl)) {
+        return true;
+    }
+    if (auto const meth = dyn_cast<CXXMethodDecl>(decl)) {
+        return meth->isCopyAssignmentOperator() || meth->isMoveAssignmentOperator();
+    }
+    return false;
+}
+
 class UnrefFun: public RecursiveASTVisitor<UnrefFun>, public loplugin::Plugin {
 public:
     explicit UnrefFun(loplugin::InstantiationData const & data): Plugin(data) {}
@@ -141,6 +154,14 @@ bool UnrefFun::VisitFunctionDecl(FunctionDecl const * decl) {
     {
         return true;
     }
+    if (canon->isExplicitlyDefaulted() && isSpecialMemberFunction(canon)) {
+        // If a special member function is explicitly defaulted on the first declaration, assume
+        // that its presence is always due to some interface design consideration, not to explicitly
+        // request a definition that might be worth to flag as unused (and C++20 may extend
+        // defaultability beyond special member functions to comparison operators, therefore
+        // explicitly check here for special member functions only):
+        return true;
+    }
     LinkageInfo info(canon->getLinkageAndVisibility());
     if (info.getLinkage() == ExternalLinkage
         && hasCLanguageLinkageType(canon) && canon->isDefined()