Test case selection cannot work if parametrised id generation contains parentheses

Hi,

First of all, thanks for maintaining pytest.

When using fixture parametrisation with ids=repr, the resulting test case name may contain parentheses. Unfortunately, it doesn't look like I can select them using -k because that expects expressions.

Maybe there should be some restrictions on what the ids callable can output? If so, it'd be nice to have that documented or checked.

Minimal test case:

# test_pytest.py
import pytest


@pytest.fixture(params=[(1, 1), (2, 2)], ids=repr)
def a(request):
    return 1


@pytest.fixture(params=[1, 2])
def b(request):
    return 2


def test_1(a, b):
    assert True

Now, run:

pytest test_pytest.py -k "test_1[(2, 2)-2]"

You will observe:

...<snip>...
    return eval(keywordexpr, {}, mapping)
  File "<string>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'tuple' and 'int'

Environment:

Python 3.6.8 :: Anaconda, Inc.

pytest                        5.3.5               
pytest-asyncio                0.10.0              
pytest-bdd                    3.2.0               
pytest-cov                    2.8.1               
pytest-mock                   1.13.0

Workaround:

Something like this works

@pytest.fixture(ids=lambda t: ",".join(str(x) for x in t))

The reproduction is in test_pytest.py and uses pytest's -k selection with parametrized fixture IDs from repr; start by running the shown command and tracing the keyword-expression evaluation reported in the traceback. Determine whether the behavior should be constrained, documented, or handled differently, then add coverage showing the intended selection behavior without the TypeError.