Fix issue where fixtures would lose the decorated functionality

Fix #3774

src/_pytest/compat.py

@@ -234,6 +234,13 @@ def get_real_func(obj):
     """
     start_obj = obj
     for i in range(100):
+        # __pytest_wrapped__ is set by @pytest.fixture when wrapping the fixture function
+        # to trigger a warning if it gets called directly instead of by pytest: we don't
+        # want to unwrap further than this otherwise we lose useful wrappings like @mock.patch (#3774)
+        new_obj = getattr(obj, "__pytest_wrapped__", None)
+        if new_obj is not None:
+            obj = new_obj
+            break
         new_obj = getattr(obj, "__wrapped__", None)
         if new_obj is None:
             break

src/_pytest/fixtures.py

@@ -954,9 +954,6 @@ def _ensure_immutable_ids(ids):
 def wrap_function_to_warning_if_called_directly(function, fixture_marker):
     """Wrap the given fixture function so we can issue warnings about it being called directly, instead of
     used as an argument in a test function.
-
-    The warning is emitted only in Python 3, because I didn't find a reliable way to make the wrapper function
-    keep the original signature, and we probably will drop Python 2 in Pytest 4 anyway.
     """
     is_yield_function = is_generator(function)
     msg = FIXTURE_FUNCTION_CALL.format(name=fixture_marker.name or function.__name__)
@@ -982,6 +979,10 @@ def wrap_function_to_warning_if_called_directly(function, fixture_marker):
     if six.PY2:
         result.__wrapped__ = function
 
+    # keep reference to the original function in our own custom attribute so we don't unwrap
+    # further than this point and lose useful wrappings like @mock.patch (#3774)
+    result.__pytest_wrapped__ = function
+
     return result