Add test for user-defined extension type.

tests/x509/test_x509_ext.py

@@ -18,7 +18,10 @@
 from cryptography.hazmat.primitives import hashes
 from cryptography.hazmat.primitives.asymmetric import ec
 from cryptography.x509 import DNSName, NameConstraints, SubjectAlternativeName
-from cryptography.x509.extensions import _key_identifier_from_public_key
+from cryptography.x509.extensions import (
+    _key_identifier_from_public_key,
+    ExtensionType,
+)
 from cryptography.x509.oid import (
     AuthorityInformationAccessOID,
     ExtendedKeyUsageOID,
@@ -5719,3 +5722,11 @@ def test_all_extension_oid_members_have_names_defined():
         if oid.startswith("__"):
             continue
         assert getattr(ExtensionOID, oid) in _OID_NAMES
+
+
+def test_unknown_extension():
+    class MyExtension(ExtensionType):
+        oid = x509.ObjectIdentifier("1.2.3.4")
+
+    with pytest.raises(NotImplementedError):
+        MyExtension().to_der_string()