C++: exclude uninitialized uses that are cast to void

This eliminates FPs caused by casting a variable explicitly to
void type. Developers use this cast to suppress compiler warnings
on unused variables, e.g.
   (void) x;

Author: szsam

cpp/ql/src/Likely Bugs/Memory Management/UninitializedLocal.ql

@@ -72,6 +72,13 @@ VariableAccess commonException() {
   or
   result.getParent() instanceof BuiltInOperation
   or
+  // Ignore the uninitialized use that is explicitly cast to void and
+  // is also an expression statement.
+  (
+    result.getActualType() instanceof VoidType and
+    result.getParent() instanceof ExprStmt
+  )
+  or
   // Finally, exclude functions that contain assembly blocks. It's
   // anyone's guess what happens in those.
   containsInlineAssembly(result.getEnclosingFunction())

cpp/ql/src/change-notes/2023-07-03-improve-uninitialized-local.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* The `cpp/uninitialized-local` query now excludes uninitialized uses that are explicitly cast to void and are expression statements. As a result, the query will report less false positives.