LSP: Do not sort completion items if none sets sortText

It is unclear if and how the client should order completion items, but
at least haskell-language-server provides a sensibly sorted list (not
alphabetically) and doesn't set a "sortText".
Sorting the items alphabetically by label messes up the prioritization
of "best" matches that the server correctly does.

Only sort, if any item has a sortText.

Task-number: QTCREATORBUG-26114
Change-Id: I55a966763e21ba8b85085a087c37e6005438d4e3
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/languageclientcompletionassist.cpp

@@ -73,6 +73,7 @@ public:
     quint64 hash() const override;
 
     const QString &sortText() const;
+    bool hasSortText() const;
 
     bool operator <(const LanguageClientCompletionItem &other) const;
 
@@ -208,6 +209,11 @@ const QString &LanguageClientCompletionItem::sortText() const
     return m_sortText;
 }
 
+bool LanguageClientCompletionItem::hasSortText() const
+{
+    return m_item.sortText().has_value();
+}
+
 QString LanguageClientCompletionItem::filterText() const
 {
     if (m_filterText.isEmpty()) {
@@ -249,7 +255,7 @@ class LanguageClientCompletionModel : public GenericProposalModel
 public:
     // GenericProposalModel interface
     bool containsDuplicates() const override { return false; }
-    bool isSortable(const QString &/*prefix*/) const override { return true; }
+    bool isSortable(const QString &/*prefix*/) const override;
     void sort(const QString &/*prefix*/) override;
     bool supportsPrefixExpansion() const override { return false; }
 
@@ -257,6 +263,11 @@ public:
     { return Utils::static_container_cast<LanguageClientCompletionItem *>(m_currentItems); }
 };
 
+bool LanguageClientCompletionModel::isSortable(const QString &) const
+{
+    return Utils::anyOf(items(), &LanguageClientCompletionItem::hasSortText);
+}
+
 void LanguageClientCompletionModel::sort(const QString &/*prefix*/)
 {
     std::sort(m_currentItems.begin(), m_currentItems.end(),