PLR1701 fix is not safe (especially together with SIM114)

[simonpercivall]

PLR1701 is mentioned in passing in #10245.

PLR1701 will merge repeated isinstance calls and also rewrite literal tuples into unions for modern Python versions. But an isinstance check with a variable bound tuple will also be merged into the union, and that will cause a TypeError at runtime.

This especially interacts badly with SIM114 where you could have defined two if arms, one with a variable bound tuple, and one with a literal tuple, and it'll get merged and wrongly rewritten as a union. Example below.

Ruff 0.45

pyproject.toml:

[tool.ruff]
target-version = "py311"

ruff check --select SIM114,PLR1701 --fix

TYPES = (dict, list)

if isinstance(None, TYPES):
    pass
elif isinstance(None, set | float):
    pass

actual

TYPES = (dict, list)

if isinstance(None, TYPES | set | float):
    pass

# Traceback (most recent call last):
#   File "PLR1701.py", line 3, in <module>
#     if isinstance(None, TYPES | set | float):
#                         ~~~~~~^~~~~
# TypeError: unsupported operand type(s) for |: 'tuple' and 'type'

[charliermarsh]

Thanks - we should mark it as unsafe. We can improve the rule in the future when we support type inference.

[charliermarsh]

Actually, isinstance(None, (TYPES, set | float)) is safe... so in theory we could always rewrite to the tuple variant if we can't be certain that | will work?