C++: Add a test showing the motivation.

Author: geoffw0

cpp/ql/test/query-tests/Likely Bugs/Format/WrongNumberOfFormatArguments/WrongNumberOfFormatArguments.expected

@@ -4,6 +4,7 @@
 | custom_printf.cpp:29:5:29:12 | call to myPrintf | Format expects 2 arguments but given 1 |
 | macros.cpp:14:2:14:37 | call to printf | Format expects 4 arguments but given 3 |
 | macros.cpp:21:2:21:36 | call to printf | Format expects 4 arguments but given 3 |
+| macros.cpp:32:2:32:25 | call to printf | Format expects 1 arguments but given 0 |
 | test.c:9:2:9:7 | call to printf | Format expects 1 arguments but given 0 |
 | test.c:12:2:12:7 | call to printf | Format expects 2 arguments but given 1 |
 | test.c:15:2:15:7 | call to printf | Format expects 3 arguments but given 2 |

cpp/ql/test/query-tests/Likely Bugs/Format/WrongNumberOfFormatArguments/macros.cpp

@@ -20,3 +20,14 @@ void testMacros(int a, int b, int c)
 	BADPRINTF("%i %i %i\n", a, b, c); // GOOD
 	BADPRINTF("%i %i %i %i\n", a, b, c); // BAD: too few format arguments
 }
+
+#define DOTHING(x) \
+	printf("doing thing: " #x); x
+
+void testMacros2()
+{
+	int x;
+
+	DOTHING(x++); // GOOD
+	DOTHING(printf("%i", x)); // BAD: the printf inside the macro has too few format arguments
+}