Can't typehint decorated functions in MatcherDecoratableTransformer with future annotations #870
Description
In flake8-trio we use from future import __annotations__ in order to use more modern typehinting when running python 3.9, but the typechecking that decorators do don't - so this leads to a crash on python3.9:
@m.visit(m.Await() | m.For(asynchronous=m.Asynchronous()))
def checkpoint_node(self, node: cst.Await | cst.For | cst.With):
... File "/home/h/Git/flake8-trio/flake8_trio/visitors/flake8triovisitor.py", line 172, in __init__
super().__init__()
File "/home/h/Git/flake8-trio/.tox/py39/lib/python3.9/site-packages/libcst/matchers/_visitors.py", line 483, in __init__
_check_types(
File "/home/h/Git/flake8-trio/.tox/py39/lib/python3.9/site-packages/libcst/matchers/_visitors.py", line 250, in _check_types
_verify_return_annotation(
File "/home/h/Git/flake8-trio/.tox/py39/lib/python3.9/site-packages/libcst/matchers/_visitors.py", line 123, in _verify_return_annotation
type_hints = get_type_hints(meth)
File "/usr/lib/python3.9/typing.py", line 1497, in get_type_hints
value = _eval_type(value, globalns, localns)
File "/usr/lib/python3.9/typing.py", line 292, in _eval_type
return t._evaluate(globalns, localns, recursive_guard)
File "/usr/lib/python3.9/typing.py", line 554, in _evaluate
eval(self.__forward_code__, globalns, localns),
File "<string>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'ABCMeta' and 'ABCMeta'
I can workaround it by reverting to using Union, but I think it should be possible to add a future import to your eval environment?