Support PEP-604 style unions in decorator annotations

These unions were introduced in Python 3.10 and do not define __origin__,
so some extra checks are necessary to identify then. Since there is not
yet a 3.10 build, a somewhat hacky test was added to simulate one of
these new Unions.

Resolves #414.

libcst/matchers/_visitors.py

@@ -79,8 +79,18 @@ def _get_possible_match_classes(matcher: BaseMatcherNode) -> List[Type[cst.CSTNo
         return [getattr(cst, matcher.__class__.__name__)]
 
 
-def _get_possible_annotated_classes(annotation: object) -> List[Type[object]]:
+def _annotation_looks_like_union(annotation: object) -> bool:
     if getattr(annotation, "__origin__", None) is Union:
+        return True
+    # support PEP-604 style unions introduced in Python 3.10
+    return (
+        annotation.__class__.__name__ == "Union"
+        and annotation.__class__.__module__ == "types"
+    )
+
+
+def _get_possible_annotated_classes(annotation: object) -> List[Type[object]]:
+    if _annotation_looks_like_union(annotation):
         return getattr(annotation, "__args__", [])
     else:
         return [cast(Type[object], annotation)]

libcst/matchers/tests/test_decorators.py

@@ -6,6 +6,7 @@
 from ast import literal_eval
 from textwrap import dedent
 from typing import List, Set
+from unittest.mock import Mock
 
 import libcst as cst
 import libcst.matchers as m
@@ -993,3 +994,25 @@ def bar() -> None:
 
         # We should have only visited a select number of nodes.
         self.assertEqual(visitor.visits, ['"baz"'])
+
+
+# This is meant to simulate `cst.ImportFrom | cst.RemovalSentinel` in py3.10
+FakeUnionClass: Mock = Mock()
+setattr(FakeUnionClass, "__name__", "Union")
+setattr(FakeUnionClass, "__module__", "types")
+FakeUnion: Mock = Mock()
+FakeUnion.__class__ = FakeUnionClass
+FakeUnion.__args__ = [cst.ImportFrom, cst.RemovalSentinel]
+
+
+class MatchersUnionDecoratorsTest(UnitTest):
+    def test_init_with_new_union_annotation(self) -> None:
+        class TransformerWithUnionReturnAnnotation(m.MatcherDecoratableTransformer):
+            @m.leave(m.ImportFrom(module=m.Name(value="typing")))
+            def test(
+                self, original_node: cst.ImportFrom, updated_node: cst.ImportFrom
+            ) -> FakeUnion:
+                pass
+
+        # assert that init (specifically _check_types on return annotation) passes
+        TransformerWithUnionReturnAnnotation()