Clang: Fix crash when backend is not yet started

Due to DummyBackendSender not overriding requestToolTip() the base
implementation was called which assumed a connected backend.

To avoid this in future, derive DummyBackendSender from
ClangCodeModelServerInterface so the compiler will warn about not
overridden functions.

Task-number: QTCREATORBUG-19644
Change-Id: Ifd846cbe581092e1688719119dad5003da66d0a3
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangbackendcommunicator.cpp

@@ -70,11 +70,9 @@ static QString backendProcessPath()
 namespace ClangCodeModel {
 namespace Internal {
 
-class DummyBackendSender : public BackendSender
+class DummyBackendSender : public ClangBackEnd::ClangCodeModelServerInterface
 {
 public:
-    DummyBackendSender() : BackendSender(nullptr) {}
-
     void end() override {}
     void registerTranslationUnitsForEditor(const RegisterTranslationUnitForEditorMessage &) override {}
     void updateTranslationUnitsForEditor(const UpdateTranslationUnitsForEditorMessage &) override {}
@@ -87,10 +85,10 @@ public:
     void requestDocumentAnnotations(const RequestDocumentAnnotationsMessage &) override {}
     void requestReferences(const RequestReferencesMessage &) override {}
     void requestFollowSymbol(const RequestFollowSymbolMessage &) override {}
+    void requestToolTip(const RequestToolTipMessage &) override {}
     void updateVisibleTranslationUnits(const UpdateVisibleTranslationUnitsMessage &) override {}
 };
 
-
 BackendCommunicator::BackendCommunicator()
     : m_connection(&m_receiver)
     , m_sender(new DummyBackendSender())

src/plugins/clangcodemodel/clangbackendcommunicator.h

@@ -147,7 +147,7 @@ private:
     BackendReceiver m_receiver;
     ClangBackEnd::ClangCodeModelConnectionClient m_connection;
     QTimer m_backendStartTimeOut;
-    QScopedPointer<BackendSender> m_sender;
+    QScopedPointer<ClangBackEnd::ClangCodeModelServerInterface> m_sender;
     int m_connectedCount = 0;
 };