False unreachable warning on match statement

[delfick]

Bug Report

Hello,

I want to do something like:

def match(info: RequestContext):
    match info:
        case RequestContext.ForPopup(data_only_in_popup_context=data):
            print(data)
        case RequestContext.ForDetailPage(data_only_in_detail_page=data):
            print(data)

However I'm getting unreachable warnings when --warn-unreachable is turned on. But the code I have does indeed reach that line.

To Reproduce

https://gist.github.com/delfick/e7974027a422f8f212887e490e5d3a75

Expected Behavior

I expect no errors when running with mypy and the warn-unreachable option.

Actual Behavior

When I run python example.py I get expected output

For popup <__main__.Request object at 0x10326a620> (1,) {'1': '2'}
For details page <__main__.Request object at 0x10326a0b0> {'3': '4'}
For popup {'1': '2'}
For details page {'3': '4'}

When I run mypy example.py --warn-unreachable I get

example.py:49: error: Statement is unreachable  [unreachable]
example.py:65: error: Statement is unreachable  [unreachable]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.10.8

[delfick]

for the benefit of anyone who comes across this, I solved my original problem with a slightly different approach, https://gist.github.com/delfick/dddd2e59638cd807584d0d6fe99a7d13. But I still think it would be nice if my other approach didn't fail the way it does :)

[thierry-martinez]

I have the following example that is shorter and exhibits a very similar issue (I don't know if I should fill another issue, because the cause is probably the same; if you think I should open another issue, feel free to ask!):

import typing


class C(typing.NamedTuple):
    v: str


def f(x: typing.Any) -> None:
    match x:
        case C(v=v):
            print(v)
        case [y]:
            print("HERE: " + y)


f(["hello"])

The code prints HERE: hello as expected, so the line print("HERE: " + y) is definitively reachable, but mypy --warn-unreachable says the contrary.