strange handling of test ids for parametrize

$ cat tests/test_toast.py
import pytest

@pytest.mark.parametrize(
    "ham,spam",
    [
        ("/bacon/../", u"//bacon/../"),
        ("/bacon/../../", u"//bacon/../../"),
        ("/bacon//", u"//bacon//"),
    ],
)
def test_eggs(ham,spam):
    pass
$ pytest -vvv tests/test_eggs.py
==================================== test session starts ====================================
platform linux -- Python 3.7.5, pytest-5.3.2, py-1.8.0, pluggy-0.12.0 -- /home/graingert/.virtualenvs/pytest/bin/python3.7
cachedir: .pytest_cache
rootdir: /home/graingert/projects/sausage/tests, inifile: pytest.ini
collected 3 items                                                                           

tests/test_toast.py::test_eggs[/-/] PASSED                                            [ 33%]
tests/] PASSED                                                                        [ 66%]
tests/test_toast.py::test_eggs[/bacon/-/bacon/] PASSED                                [100%]

===================================== 3 passed in 0.01s =====================================

• a detailed description of the bug or suggestion
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

[graingert]

@blueyed reproduction as xfail test here: https://github.com/pytest-dev/pytest/pull/6394

[blueyed]

I can reproduce it after touch tests/pytest.ini / touch t/pytest.ini… 👍