Use a custom holder class so we can be sure __pytest_wrapper__ was set by us

src/_pytest/compat.py

@@ -228,6 +228,18 @@ else:
             return val.encode("unicode-escape")
 
 
+class _PytestWrapper(object):
+    """Dummy wrapper around a function object for internal use only.
+
+    Used to correctly unwrap the underlying function object
+    when we are creating fixtures, because we wrap the function object ourselves with a decorator
+    to issue warnings when the fixture function is called directly.
+    """
+
+    def __init__(self, obj):
+        self.obj = obj
+
+
 def get_real_func(obj):
     """ gets the real function object of the (possibly) wrapped object by
     functools.wraps or functools.partial.
@@ -238,8 +250,8 @@ def get_real_func(obj):
         # 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
+        if isinstance(new_obj, _PytestWrapper):
+            obj = new_obj.obj
             break
         new_obj = getattr(obj, "__wrapped__", None)
         if new_obj is None:

src/_pytest/fixtures.py

@@ -31,6 +31,7 @@ from _pytest.compat import (
     safe_getattr,
     FuncargnamesCompatAttr,
     get_real_method,
+    _PytestWrapper,
 )
 from _pytest.deprecated import FIXTURE_FUNCTION_CALL, RemovedInPytest4Warning
 from _pytest.outcomes import fail, TEST_OUTCOME
@@ -981,7 +982,7 @@ def wrap_function_to_warning_if_called_directly(function, fixture_marker):
 
     # 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
+    result.__pytest_wrapped__ = _PytestWrapper(function)
 
     return result