C++: Work around issue with 'Unknown literal'.

Author: geoffw0

cpp/ql/src/Best Practices/Unused Entities/UnusedLocals.ql

@@ -57,5 +57,9 @@ where
   not declarationHasSideEffects(v) and
   not exists(AsmStmt s | f = s.getEnclosingFunction()) and
   not v.getAnAttribute().getName() = "unused" and
-  not any(ErrorExpr e).getEnclosingFunction() = f // unextracted expr likely used `v`
+  not any(ErrorExpr e).getEnclosingFunction() = f and // unextracted expr may use `v`
+  not exists(Literal l |
+    l.getEnclosingFunction() = f and
+    not exists(l.getValue())
+  )
 select v, "Variable " + v.getName() + " is not used"

cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/UnusedLocals.expected

@@ -8,7 +8,6 @@
 | code2.cpp:108:11:108:12 | v2 | Variable v2 is not used |
 | code2.cpp:128:9:128:9 | b | Variable b is not used |
 | code2.cpp:141:18:141:18 | b | Variable b is not used |
-| code2.cpp:162:14:162:16 | obj | Variable obj is not used |
 | code.c:10:18:10:18 | y | Variable y is not used |
 | code.c:11:18:11:18 | z | Variable z is not used |
 | code.c:18:7:18:7 | x | Variable x is not used |