Surprising behavior when passing class/function to marker

Consider the following example:

import pytest

class Context:  # actually imported from the module under test

    def __init__(self, config):
        pass

@pytest.mark.use_context(Context)
def test_with_special_context():
    pass

(and imagine a fixture then using the passed context, though that's not so relevant here)

This results in:

============================= test session starts ==============================
platform linux -- Python 3.9.2, pytest-6.3.0.dev324+gca4a7a085, py-1.10.0, pluggy-0.13.1
rootdir: /home/florian/tmp/rep, configfile: pytest.ini
collected 0 items                                                              

============================ no tests ran in 0.00s =============================

which is surprising, isn't it? When I first saw this, I had no idea what was going on (in my case, there were other tests, and when I introduced the marker, tests suddenly started disappearing).

While writing a minimal reproducer, I discovered what's going on when removing the __init__ for Context:

__________________________ ERROR collecting test_x.py __________________________
test_x.py:7: in <module>
    def test_with_special_context():
E   TypeError: Context() takes no arguments

Still weird, albeit slightly less so. This is happening because the @pytest.mark decorator is written in a way it supports being called as both @pytest.mark.mymark and @pytest.mark.mymark(...):

https://github.com/pytest-dev/pytest/blob/ca4a7a085e853636e400ac68f38609e5ed5e8b3d/src/_pytest/mark/structures.py#L360-L368

However, the Context argument I passed to the marker just happens to be a class, so pytest incorrectly assumes that's the test class to decorate, helpfully replacing my original test function with the Context class...

I have no idea how to fix this though, while still correctly supporting our "hybrid" decorators.

I guess a slight improvement would be for pytest to check whether the argument is a test class/function (i.e. follows the naming conventions for it to be collected as test, and also in a test module rather than a different module). That of course makes this issue even more subtle when it appears, but at least it'd solve this particular case.

I really wish Python decorators somehow worked in a less confusing way though...

Start with the hybrid decorator logic in src/_pytest/mark/structures.py at the linked lines, then reproduce the behavior with the Context example from the issue. Done means passing a class or function as marker data no longer replaces the decorated test or causes collection to fail, while the existing hybrid decorator forms continue to work.