parser.addoption has a buggy argument parser (relative path-like strings)

If I provide an existing relative path to pytest via parser.addoption then I get the following error message:

ERROR: usage: pytest [options] [file_or_dir] [file_or_dir] [...]
pytest: error: unrecognized arguments: --var-file
  inifile: None
  rootdir: /home/user/workspace

This does not happen to a relative path that is non-existent.

If you add an equal sign (=) instead of a blank space, this error does not occur.

This is slightly confusing to me as I do not see why parser.addoption cares about whether we're passing in a file (I would have thought it was just deal with the string). And if it does care about whether something is a file, why is it OK with a nonexistent file?

A minimal bit of code that will reproduce this, conftest.py

def pytest_addoption(parser):
    parser.addoption(
        "--var-file",
        action="store",
    )

Place this in a folder with an existing relative file above it, e.g.:

foo.txt
test/conftest.py
test/bar.txt

Enter the test folder and execute pytest --var-file ../foo.txt and observe the error message above.

Running pytest --var-file bar.txt will not produce an error message
Running pytest --var-file ../notthere.txt will not produce the error message
Running pytest --var-file=../foo.txt will not produce the error message

Output of pip freeze:

pip freeze
atomicwrites==1.3.0
attrs==19.3.0
importlib-metadata==1.5.0
more-itertools==8.2.0
packaging==20.3
pluggy==0.13.1
py==1.8.1
pyparsing==2.4.6
pytest==5.3.5
six==1.14.0
wcwidth==0.1.8
zipp==3.1.0

lsb_release -a
LSB Version:	n/a
Distributor ID:	ManjaroLinux
Description:	Manjaro Linux
Release:	18.1.5
Codename:	Juhraya

python --version
Python 3.8.1

Note, observed this in pytest 5.0.1 as well.

Start with the minimal conftest.py reproducer and the parser.addoption entry point, then run the three relative-path commands shown in the issue to compare their behavior. Done means an existing relative path passed with a space is accepted like the nonexistent path and equals-sign form, with regression coverage for the reported case.