fix(typing): correctly type `cached` decorator

logilab/common/decorators.py

@@ -25,7 +25,7 @@ import sys
 
 from time import process_time, time
 from inspect import isgeneratorfunction
-from typing import Any, Optional, Callable, Union
+from typing import Any, Optional, Callable, overload, TypeVar
 
 from inspect import getfullargspec
 
@@ -121,9 +121,22 @@ class _MultiValuesKeyArgCache(_MultiValuesCache):
             return _cache[key]
 
 
+_T = TypeVar("_T", bound=Callable)
+
+
+@overload
 def cached(
-    callableobj: Optional[Callable] = None, keyarg: Optional[int] = None, **kwargs: Any
-) -> Union[Callable, cached_decorator]:
+    callableobj: None = None, keyarg: Optional[int] = None, **kwargs: Any
+) -> Callable[[_T], _T]:
+    ...
+
+
+@overload
+def cached(callableobj: _T = None, keyarg: Optional[int] = None, **kwargs: Any) -> _T:
+    ...
+
+
+def cached(callableobj=None, keyarg=None, **kwargs):
     """Simple decorator to cache result of method call."""
     kwargs["keyarg"] = keyarg
     decorator = cached_decorator(**kwargs)