Autouse and parameterization results in fixtures not being torn down, or even being torn down out of order

[SalmonMode]

$ pip list --format=columns
Package            Version
------------------ -------
attrs              19.3.0 
importlib-metadata 1.3.0  
more-itertools     8.0.2  
packaging          20.0   
pip                9.0.1  
pkg-resources      0.0.0  
pluggy             0.13.1 
py                 1.8.1  
pyparsing          2.4.6  
pytest             5.3.2  
selenium           3.141.0
setuptools         39.0.1 
six                1.13.0 
urllib3            1.25.7 
wcwidth            0.1.8  
zipp               0.6.0

I noticed that when autouse and parameterized fixtures of multiple scope levels, it can result in fixtures not being torn down every time a parameterized fixture is torn down if they were executed after the parameterized fixture was setup (and before it was torn down). I also saw in more extreme cases that fixtures would even be torn down in the wrong order.

Here's a simple example:

import pytest

@pytest.fixture(scope="module", params=["m1", "m2"], autouse=True)
def m_fix(request):
    yield

@pytest.fixture(scope="class")
def c_fix():
    yield

class TestFixtures:
    def test_a(self, c_fix):
        pass

I would expect the setup plan for this to be:

tests/test_auto.py 
    SETUP    M m_fix[m1]
      SETUP    C c_fix
        tests/test_auto.py::TestFixtures::test_a[m1] (fixtures used: c_fix, m_fix, request)
      TEARDOWN C c_fix
    TEARDOWN M m_fix[m1]
    SETUP    M m_fix[m2]
      SETUP    C c_fix
        tests/test_auto.py::TestFixtures::test_a[m2] (fixtures used: c_fix, m_fix, request)
      TEARDOWN C c_fix
    TEARDOWN M m_fix[m2]

but it's actually this:

tests/test_auto.py 
    SETUP    M m_fix[m1]
      SETUP    C c_fix
        tests/test_auto.py::TestFixtures::test_a[m1] (fixtures used: c_fix, m_fix, request)
    TEARDOWN M m_fix[m1]
    SETUP    M m_fix[m2]
        tests/test_auto.py::TestFixtures::test_a[m2] (fixtures used: c_fix, m_fix, request)
      TEARDOWN C c_fix
    TEARDOWN M m_fix[m2]

Notice that the class-level fixture is not torn down along with the parameterized, autouse module-level fixture, despite being affected by it.

[SalmonMode]

I have a working solution plus a new test. I'm just drawing up the PR now.