Add a check in ReferenceProperty constructor for an impossible

to satisfy type constraint: empty whitelist.  It would be silly
for anyone to do that, but I should check just in case I guess.

stix2/properties.py

@@ -511,6 +511,9 @@ class ReferenceProperty(Property):
         elif invalid_types and not isinstance(invalid_types, list):
             invalid_types = [invalid_types]
 
+        if valid_types is not None and len(valid_types) == 0:
+            raise ValueError("Impossible type constraint: empty whitelist")
+
         self.types = set(valid_types or invalid_types)
         self.auth_type = self._WHITELIST if valid_types else self._BLACKLIST
 

stix2/test/v20/test_properties.py

@@ -229,6 +229,11 @@ def test_reference_property_hybrid_constraint_type():
         ReferenceProperty(invalid_types=["a", "SCO"], spec_version="2.0")
 
 
+def test_reference_property_impossible_constraint():
+    with pytest.raises(ValueError):
+        ReferenceProperty(valid_types=[], spec_version="2.0")
+
+
 @pytest.mark.parametrize(
     "d", [
         {'description': 'something'},

stix2/test/v21/test_properties.py

@@ -250,6 +250,11 @@ def test_reference_property_hybrid_constraint_type():
         ReferenceProperty(invalid_types=["a", "SCO"], spec_version="2.1")
 
 
+def test_reference_property_impossible_constraint():
+    with pytest.raises(ValueError):
+        ReferenceProperty(valid_types=[], spec_version="2.1")
+
+
 @pytest.mark.parametrize(
     "d", [
         {'description': 'something'},