feature and command decorators loose type annotations

[perrinjerome]

If I use this patch on json-extension example:

diff --git a/examples/json-extension/server/server.py b/examples/json-extension/server/server.py
index efd6087..70fd3bf 100644
--- a/examples/json-extension/server/server.py
+++ b/examples/json-extension/server/server.py
@@ -90,7 +90,7 @@ def _validate_json(source):
 
 
 @json_server.feature(COMPLETION, trigger_characters=[','])
-def completions(params: CompletionParams = None):
+def completions(params: CompletionParams = None) -> CompletionList:
     """Returns completion items."""
     return CompletionList(False, [
         CompletionItem('"'),
diff --git a/examples/json-extension/server/tests/unit/test_features.py b/examples/json-extension/server/tests/unit/test_features.py
index b2ff762..3ec98d7 100644
--- a/examples/json-extension/server/tests/unit/test_features.py
+++ b/examples/json-extension/server/tests/unit/test_features.py
@@ -50,8 +50,9 @@ def _reset_mocks():
     server.show_message_log.reset_mock()
 
 
-def test_completions():
-    completion_list = completions()
+def test_completions() -> None:
+    completion_list = completions("type error")
+    reveal_type(completion_list)
     labels = [i.label for i in completion_list.items]
 
     assert '"' in labels

I would expect that mypy report type error for wrong "type error" argument and reveal type of completion_list as CompletionList, instead, I have:

$ mypy examples/json-extension/server/tests/unit/test_features.py 
...
examples/json-extension/server/tests/unit/test_features.py:55: note: Revealed type is 'Any'

I would have expected something like:

$ mypy examples/json-extension/server/tests/unit/test_features.py
...
examples/json-extension/server/tests/unit/test_features.py:54: error: Argument 1 to "completions" has incompatible type "str"; expected "Optional[CompletionParams]"
examples/json-extension/server/tests/unit/test_features.py:55: note: Revealed type is 'pygls.types.CompletionList'