ClangCodeModel: Fix mis-highlighting of lambda calls

Calls to lambdas without arguments that are declared as const variables
were erroneously displayed as output arguments.

Change-Id: Ibd914431a34157606694f85d8e00c0dd1db1a618
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/clangcodemodel/clangdclient.cpp

@@ -2683,10 +2683,22 @@ static void semanticHighlighter(QFutureInterface<HighlightingResult> &future,
             // where the user sees that it's being written.
             if (it->kind() == "CXXOperatorCall") {
                 const QList<AstNode> children = it->children().value_or(QList<AstNode>());
+
+                // Child 1 is the call itself, Child 2 is the named entity on which the call happens
+                // (a lambda or a class instance), after that follow the actual call arguments.
                 if (children.size() < 2)
                     return false;
-                if (!children.last().range().contains(range))
+
+                // The call itself is never modifiable.
+                if (children.first().range() == range)
                     return false;
+
+                // The callable is never displayed as an output parameter.
+                // TODO: A good argument can be made to display objects on which a non-const
+                //       operator or function is called as output parameters.
+                if (children.at(1).range() == range)
+                    return false;
+
                 QList<AstNode> firstChildTree{children.first()};
                 while (!firstChildTree.isEmpty()) {
                     const AstNode n = firstChildTree.takeFirst();

src/plugins/clangcodemodel/test/clangdtests.cpp

@@ -1285,6 +1285,14 @@ void ClangdTestHighlighting::test_data()
                                       << QList<int>{C_LOCAL} << 0;
     QTest::newRow("const member as function argument") << 868 << 32 << 868 << 43
                                       << QList<int>{C_FIELD} << 0;
+    QTest::newRow("lambda call without arguments (const var)") << 887 << 5 << 887 << 12
+                                      << QList<int>{C_LOCAL} << 0;
+    QTest::newRow("lambda call without arguments (non-const var)") << 889 << 5 << 889 << 12
+                                      << QList<int>{C_LOCAL} << 0;
+    QTest::newRow("non-const operator()") << 898 << 5 << 898 << 7
+                                      << QList<int>{C_LOCAL} << 0;
+    QTest::newRow("const operator()") << 903 << 5 << 903 << 7
+                                      << QList<int>{C_LOCAL} << 0;
 }
 
 void ClangdTestHighlighting::test()

src/plugins/clangcodemodel/test/data/highlighting/highlighting.cpp

@@ -880,3 +880,25 @@ void constMemberAsFunctionArg()
 #if 0
 #define BAR
 # endif
+
+void lambdaCall()
+{
+    const auto lambda1 = [] {};
+    lambda1();
+    auto lambda2 = [] {};
+    lambda2();
+}
+
+void callOperators()
+{
+    struct Callable1 {
+        void operator()() {};
+    };
+    Callable1 c1;
+    c1();
+    struct Callable2 {
+        void operator()() const {};
+    };
+    Callable2 c2;
+    c2();
+}