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[1.20 regression] value pattern match statement narrowing broken for typevars with values #21356
Description
Bug Report
I think I've found a regression in 1.20 - was recently trying to migrate a larger codebase from 1.18 and noticed this issue in a few places which didn't make sense. I've isolated it down to a small example which is a bit silly, but hopefully clear. I think this is a consequence of a deliberate change in #20908. Happy to try and put together a PR to resolve if useful, but I'm not exactly across the mypy internals, so might need a bit of guidance.
To Reproduce
https://mypy-play.net/?mypy=1.20.2&python=3.12&gist=ae5055b7e174f0ffd34db390a192aef5
import builtins
from typing import TypeVar
T = TypeVar("T", int, str)
def f(x: T) -> T:
match type(x):
case builtins.int:
return 42 # mypy 1.20: error: Incompatible return value type (got "int", expected "str")
case builtins.str:
return "hello" # mypy 1.20: error: Incompatible return value type (got "str", expected "int")
case _:
raise TypeErrorExpected Behavior
Expected there to be no issues
Actual Behavior
uv run --with mypy==1.20.2 mypy mre.py
mre.py:24: error: Incompatible return value type (got "int", expected "str") [return-value]
mre.py:26: error: Incompatible return value type (got "str", expected "int") [return-value]
edit: I did a bit more testing, there's nothing special about builtins here, but it needs to be a value pattern and not a class pattern for this example to even work in 1.19
Your Environment
- Mypy version used: 1.20.2 fails, 1.19 works, a quick bisect suggests afb99eb is where this changes
- Mypy command-line flags: None in minimal example
- Mypy configuration options from
mypy.ini(and other config files): None in minimal example - Python version used: 3.13 / 3.12 in playground link above
Activity
[-]REGR: match statement builtins narrowing in 1.20[/-][+]REGR: match statement narrowing with typevars in 1.20[/+][-]REGR: match statement narrowing with typevars in 1.20[/-][+]REGR: value pattern match statement narrowing with typevars in 1.20[/+]brianschubert commentedon Apr 28, 2026
Thanks, I notice this only happens when
--warn-unreachableis not set. Setting--warn-unreachablegives the old expected behavior: https://mypy-play.net/?mypy=1.20.2&python=3.12&gist=c9f38f7eb2f7fb045aad1ecc5cb42cbb&flags=warn-unreachable. I guess something went wrong with how we're treating unreachable blocks when checking constrained typevar cases.brianschubert commentedon Apr 28, 2026
Bisects to #20146, cc @sterliakov
brianschubert commentedon Apr 28, 2026
(note that this is only true as of #20660 - at #20146 it manifests regardless of whether
--warn-unreachableis set)[-]REGR: value pattern match statement narrowing with typevars in 1.20[/-][+][1.20 regression] value pattern match statement narrowing broken for typevars with values[/+]hauntsaninja commentedon Apr 30, 2026
Oh, interesting case...
So #20146 is a correct change. It make mypy's match narrowing use the same code paths as its equality narrowing. This makes mypy match Python semantics better.
Unfortunately, that means it is also a bit of an all-or-nothing change... differences between the equality narrowing logic and the (incorrect) logic mypy used to use for match narrowing, can show up as regressions like this one.
But fortunately, I spent a lot of time improving equality narrowing in the last release, which is where the
--warn-unreachablecomes into play. #20660 makes mypy's reachability logic a little stricter when--warn-unreachableis set. This happens to solve your false positive because of how mypy's logic for constrained type variables works.Ideally, I would like for this reachability logic to not depend on
--warn-unreachable. But as I mention in that PR, and as based on mypy_primer results, we need to make mypy check unreachable code for this to be a good idea.So I'm not quite sure how best to help you. If
--warn-unreachableis usable for you, that is great. Otherwise, we might just have to wait for changes to make mypy check unreachable code.m-richards commentedon Apr 30, 2026
Thanks for the reply! Appreciate this is all super complicated and sometimes there is a price in pushing things forward - and I'm very glad to see continual improvement in mypy. I think in our case we can migrate to warn unreachable so that should work well enough for us.