Fix #13011 (False positive: returnByReference should not be recommended for non const method marked with &&) (#6689)

Author: danmar

lib/checkclass.cpp

@@ -3345,6 +3345,9 @@ void CheckClass::checkReturnByReference()
             if (const Library::Container* container = mSettings->library.detectContainer(func.retDef))
                 if (container->view)
                     continue;
+            if (!func.isConst() && func.hasRvalRefQualifier())
+                // this method could be used by temporary objects, return by value can be dangerous
+                continue;
             if (const Variable* var = getSingleReturnVar(func.functionScope)) {
                 if (!var->valueType())
                     continue;

test/testclass.cpp

@@ -9045,6 +9045,13 @@ class TestClass : public TestFixture {
                                "};\n"
                                "U<std::string> u;\n");
         ASSERT_EQUALS("", errout_str());
+
+        checkReturnByReference("struct S {\n" // #13011
+                               "    std::string s;\n"
+                               "    const std::string& foo() const & { return s; }\n"
+                               "    std::string foo() && { return s; }\n" // <- used for temporary objects
+                               "};\n");
+        ASSERT_EQUALS("", errout_str());
     }
 };