Fix #13845 FP returnByReference when function is used with std::optional::transform (danmar#7526)

Co-authored-by: chrchr-github <noreply@github.com>

Author: chrchr-github

lib/checkclass.cpp

@@ -3380,6 +3380,8 @@ void CheckClass::checkReturnByReference()
                 continue;
             if (func.isOperator())
                 continue;
+            if (func.functionPointerUsage)
+                continue;
             if (const Library::Container* container = mSettings->library.detectContainer(func.retDef))
                 if (container->view)
                     continue;

test/testclass.cpp

@@ -9240,6 +9240,15 @@ class TestClass : public TestFixture {
                                "};\n");
         ASSERT_EQUALS("[test.cpp:10:17]: (performance) Function 'get2()' should return member 'str' by const reference. [returnByReference]\n",
                       errout_str());
+
+        checkReturnByReference("struct S {\n" // #13845
+                               "    std::string m;\n"
+                               "    std::string get() { return m; }\n"
+                               "};\n"
+                               "std::string f(std::optional<S> o) {\n"
+                               "    return o.transform(&S::get).value_or(\"\");\n"
+                               "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 };