Fix signatures if a decorator has no signatures, fixes #1705

jedi/inference/value/decorator.py

@@ -19,3 +19,11 @@ class Decoratee(ValueWrapper):
             Decoratee(v, self._original_value)
             for v in self._wrapped_value.py__get__(instance, class_value)
         )
+
+    def get_signatures(self):
+        signatures = self._wrapped_value.get_signatures()
+        if signatures:
+            return signatures
+        # Fallback to signatures of the original function/class if the
+        # decorator has no signature or it is not inferrable.
+        return self._original_value.get_signatures()

test/test_inference/test_signature.py

@@ -102,6 +102,8 @@ class X:
         (partialmethod_code + 'X().d(', None),
         (partialmethod_code + 'X.c(', 'func(a, b)'),
         (partialmethod_code + 'X.d(', None),
+
+        ('import contextlib\n@contextlib.contextmanager\ndef f(x): pass\nf(', 'f(x)'),
     ]
 )
 def test_tree_signature(Script, environment, code, expected):