C++: Add tests for implicit downcast involving references

cpp/ql/test/query-tests/Likely Bugs/Conversion/ImplicitDowncastFromBitfield/test.cpp

@@ -2,22 +2,42 @@ typedef struct {
 	int x : 24;
 } my_struct;
 
-int getX1(my_struct m) {
+unsigned int getX1(my_struct m) {
 	return m.x;
 }
 
 short getX2(my_struct m) {
-	return m.x;
+	return m.x; // BAD
 }
 
 short getX3(my_struct m) {
-	return (short) m.x;
+	return (short) m.x; // GOOD
 }
 
 bool getX4(my_struct m) {
-	return m.x;
+	return m.x; // GOOD
 }
 
 short getX5(my_struct m) {
-	return (char) m.x;
+	return (char) m.x; // GOOD
+}
+
+const char& getx6(my_struct& m) {
+	const char& result = m.x; // BAD [NOT DETECTED]
+	return result;
+}
+
+const short& getx7(my_struct& m) {
+	const short& result = (short) m.x; // GOOD
+	return result;
+}
+
+const int& getx8(my_struct& m) {
+	const int& result = m.x; // GOOD
+	return result;
+}
+
+const bool& getx9(my_struct& m) {
+	const bool& result = m.x; // GOOD
+	return result;
 }