Better handling of skipped predicates

This pins down the semantics of skipping predicates by letting the result propagate up to the initial invocation (that is ``Predicate.test()``)

rules/predicates.py

@@ -151,7 +151,7 @@ def test(self, obj=NO_VALUE, target=NO_VALUE):
         args = tuple(arg for arg in (obj, target) if arg is not NO_VALUE)
         _context.stack.append(Context(args))
         try:
-            return self._apply(*args)
+            return bool(self._apply(*args))
         finally:
             _context.stack.pop()
 
@@ -183,7 +183,7 @@ def INVERT(*args):
     def _combine(self, other, op, args):
         self_result = self._apply(*args)
         if self_result is None:
-            return bool(other._apply(*args))
+            return other._apply(*args)
 
         # short-circuit evaluation
         if op is operator.and_ and not self_result:

tests/testsuite/test_predicates.py

@@ -324,7 +324,7 @@ def skipped_predicate(self):
 
     with warnings.catch_warnings(record=True) as w:
         warnings.simplefilter('always')
-        assert skipped_predicate.test() is None
+        assert skipped_predicate.test() is False
         assert len(w) == 1 and 'deprecated' in str(w[-1].message)
 
 
@@ -365,7 +365,13 @@ def passthrough(a):
     assert (passthrough | ~requires_two_args).test(False) is False
 
     # test that when all predicates are skipped, result is False
+    assert requires_two_args.test(True) is False
     assert (requires_two_args | requires_two_args).test(True) is False
+    assert (requires_two_args & requires_two_args).test(True) is False
+
+    # test that a skipped predicate doesn't alter the result at all
+    assert (requires_two_args | requires_two_args | passthrough).test(True) is True
+    assert (requires_two_args & requires_two_args & passthrough).test(True) is True
 
 
 def test_invocation_context():