Add test for known future annotations failure

tests/test_custom_converters_future.py

@@ -1,27 +1,31 @@
-# pylint: disable=arguments-differ
+# pylint: disable=arguments-differ,no-member
 
 from __future__ import annotations  # this breaks custom converter lookup
 
-from datetime import datetime
+from typing import Optional
 
 import pytest
 
-from datafiles import Missing, datafile
+from datafiles import datafile
 
 from .test_custom_converters import MyDateTime
 
 
 @pytest.mark.flaky
 def test_extension(expect):
     @datafile("../tmp/sample.yml")
-    class Timestamp:
-        dt: MyDateTime
+    class MyObject:
+        value: MyDateTime
 
-    ts = Timestamp(MyDateTime(2019, 1, 29))
-    expect(ts.datafile.text) == "dt: '2019-01-29T00:00:00'\n"
+    x = MyObject(MyDateTime(2019, 1, 29))
+    expect(x.datafile.text) == "value: '2019-01-29T00:00:00'\n"
 
-    ts = Timestamp(Missing)  # type: ignore
-    expect(ts.dt) == datetime(2019, 1, 29)
 
-    ts.datafile.text = "dt: '2019-01-11T00:00:00'\n"
-    expect(ts.dt.day) == 11
+@pytest.mark.xfail(reason='https://github.com/jacebrowning/datafiles/issues/131')
+def test_optional(expect):
+    @datafile("../tmp/sample.yml")
+    class MyObject:
+        value: Optional[int]
+
+    x = MyObject(42)
+    expect(x.datafile.text) == "value: 42'\n"