Python: Fix FP in py/unused-local-variable

This is only a temporary fix, as indicated by the TODO comment.

The real underlying issue is the fact that `isUnused` is defined in
terms of the underlying SSA variables (as these are only created
for variables that are actually used), and the fact that annotated
assignments are always considered to redefine their targets, which may
not actually be the case.

Thus, the correct fix would be to change the extractor to _disregard_
mere type annotations for the purposes of figuring out whether an
SSA variable should be created or not.

However, in the short term the present fix is likely sufficient.

Author: tausbn

python/ql/src/Variables/UnusedLocalVariable.ql

@@ -19,13 +19,25 @@ predicate unused_local(Name unused, LocalVariable v) {
     def.getVariable() = v and
     def.isUnused() and
     not exists(def.getARedef()) and
+    not exists(annotation_without_assignment(v)) and
     def.isRelevant() and
     not v = any(Nonlocal n).getAVariable() and
     not exists(def.getNode().getParentNode().(FunctionDef).getDefinedFunction().getADecorator()) and
     not exists(def.getNode().getParentNode().(ClassDef).getDefinedClass().getADecorator())
   )
 }
 
+/**
+ * Gets any annotation of the local variable `v` that does not also reassign its value.
+ *
+ * TODO: This predicate should not be needed. Rather, annotated "assignments" that do not actually
+ * assign a value should not result in the creation of an SSA variable (which then goes unused).
+ */
+private AnnAssign annotation_without_assignment(LocalVariable v) {
+  result.getTarget() = v.getAStore() and
+  not exists(result.getValue())
+}
+
 from Name unused, LocalVariable v
 where
   unused_local(unused, v) and

python/ql/test/query-tests/Variables/unused/UnusedLocalVariable.expected

@@ -1,4 +1,3 @@
-| type_annotation_fp.py:5:5:5:7 | foo | The value assigned to local variable 'foo' is never used. |
 | variables_test.py:29:5:29:5 | x | The value assigned to local variable 'x' is never used. |
 | variables_test.py:89:5:89:5 | a | The value assigned to local variable 'a' is never used. |
 | variables_test.py:89:7:89:7 | b | The value assigned to local variable 'b' is never used. |