Python: Port UnusedExceptionObject.ql

Depending on whether other queries depend on this, we may end up moving
the exception utility functions to a more central location.

python/ql/src/Statements/UnusedExceptionObject.ql

@@ -12,11 +12,49 @@
  */
 
 import python
-private import LegacyPointsTo
+private import semmle.python.dataflow.new.internal.DataFlowDispatch
+private import semmle.python.dataflow.new.internal.Builtins
+private import semmle.python.ApiGraphs
 
-from Call call, ClassValue ex
+/**
+ * Holds if `cls` is a user-defined exception class, i.e. it transitively
+ * extends one of the builtin exception base classes.
+ */
+predicate isUserDefinedExceptionClass(Class cls) {
+  cls.getABase() =
+    API::builtin(["BaseException", "Exception"]).getAValueReachableFromSource().asExpr()
+  or
+  isUserDefinedExceptionClass(getADirectSuperclass(cls))
+}
+
+/**
+ * Gets the name of a builtin exception class.
+ */
+string getBuiltinExceptionName() {
+  result = Builtins::getBuiltinName() and
+  (
+    result.matches("%Error") or
+    result.matches("%Exception") or
+    result.matches("%Warning") or
+    result =
+      ["GeneratorExit", "KeyboardInterrupt", "StopIteration", "StopAsyncIteration", "SystemExit"]
+  )
+}
+
+/**
+ * Holds if `call` is an instantiation of an exception class.
+ */
+predicate isExceptionInstantiation(Call call) {
+  exists(Class cls |
+    classTracker(cls).asExpr() = call.getFunc() and
+    isUserDefinedExceptionClass(cls)
+  )
+  or
+  call.getFunc() = API::builtin(getBuiltinExceptionName()).getAValueReachableFromSource().asExpr()
+}
+
+from Call call
 where
-  call.getFunc().(ExprWithPointsTo).pointsTo(ex) and
-  ex.getASuperType() = ClassValue::exception() and
+  isExceptionInstantiation(call) and
   exists(ExprStmt s | s.getValue() = call)
 select call, "Instantiating an exception, but not raising it, has no effect."