Improve code-assist API isSortable

Now IGenericProposalModel::isSortable also takes the current prefix
as a parameter. This is particularly useful for cases like in the C++
completion: For global completions it will only sort if there's
already a content prefix - Notice, however, that the "sorting threshold"
for a maximum number of items is still kept in IGenericProposalModel::sort.

Task-number: QTCREATORBUG-6404
Change-Id: I3c2f282aa565287b8b4989930b6a27093862ef25
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/plugins/cpptools/cppcompletionassist.cpp

@@ -183,16 +183,14 @@ class CppAssistProposalModel : public TextEditor::BasicProposalItemListModel
 public:
     CppAssistProposalModel()
         : TextEditor::BasicProposalItemListModel()
-        , m_sortable(false)
         , m_completionOperator(T_EOF_SYMBOL)
         , m_replaceDotForArrow(false)
         , m_typeOfExpression(new TypeOfExpression)
     {}
 
-    virtual bool isSortable() const { return m_sortable; }
+    virtual bool isSortable(const QString &prefix) const;
     virtual IAssistProposalItem *proposalItem(int index) const;
 
-    bool m_sortable;
     unsigned m_completionOperator;
     bool m_replaceDotForArrow;
     QSharedPointer<TypeOfExpression> m_typeOfExpression;
@@ -229,6 +227,14 @@ private:
 
 Q_DECLARE_METATYPE(CppTools::Internal::CompleteFunctionDeclaration)
 
+bool CppAssistProposalModel::isSortable(const QString &prefix) const
+{
+    if (m_completionOperator != T_EOF_SYMBOL)
+        return true;
+
+    return !prefix.isEmpty();
+}
+
 IAssistProposalItem *CppAssistProposalModel::proposalItem(int index) const
 {
     BasicProposalItem *item =
@@ -699,10 +705,6 @@ IAssistProposal * CppCompletionAssistProcessor::perform(const IAssistInterface *
         if (m_hintProposal)
             return m_hintProposal;
 
-        if (m_model->m_completionOperator != T_EOF_SYMBOL)
-            m_model->m_sortable = true;
-        else
-            m_model->m_sortable = false;
         return createContentProposal();
     }