CPP: Add a test of ExprHasNoEffect.ql with a call to a 'weak' function.

Author: geoffw0

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/ExprHasNoEffect.expected

@@ -33,3 +33,5 @@
 | volatile.c:12:5:12:9 | access to array | This expression has no effect. | volatile.c:12:5:12:9 | access to array |  |
 | volatile.c:16:5:16:7 | * ... | This expression has no effect. | volatile.c:16:5:16:7 | * ... |  |
 | volatile.c:20:5:20:13 | * ... | This expression has no effect. | volatile.c:20:5:20:13 | * ... |  |
+| weak.c:18:2:18:18 | call to myNothingFunction | This expression has no effect (because $@ has no external side effects). | weak.c:2:5:2:21 | myNothingFunction | myNothingFunction |
+| weak.c:19:2:19:22 | call to myWeakNothingFunction | This expression has no effect (because $@ has no external side effects). | weak.c:9:31:9:51 | myWeakNothingFunction | myWeakNothingFunction |

cpp/ql/test/query-tests/Likely Bugs/Likely Typos/ExprHasNoEffect/weak.c

@@ -0,0 +1,20 @@
+
+int myNothingFunction()
+{
+	// does nothing
+
+	return 0;
+}
+
+int __attribute__((__weak__)) myWeakNothingFunction()
+{
+	// does nothing, but we could be overridden at the linker stage with a non-weak definition
+	// from elsewhere in the program.
+
+	return 0;
+}
+
+void testWeak() {
+	myNothingFunction(); // BAD
+	myWeakNothingFunction(); // GOOD [FALSE POSITIVE]
+}