mypy incorrectly identifies statements as unreachable when matching against constants

[menma1234]

Bug Report

When using a match statement with cases that are constants, it incorrectly identifies the later cases as being unreachable.

To Reproduce

I have the following file structure:

project_root
    util
        __init__.py
        consts.py
        test.py

In consts.py:

BLANK = ""
SPECIAL = "asdf"

In test.py:

from util import consts


def test_func(test_str: str) -> str:
    match test_str:
        case consts.BLANK:
            return "blank"
        case consts.SPECIAL:
            return "special"
        case _:
            return "other"

Expected Behavior

mypy succeeds as the code is reachable.

Actual Behavior

Running mypy --warn-unreachable .\util\test.py gives:

util\test.py:9: error: Statement is unreachable
util\test.py:11: error: Statement is unreachable
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10
• Operating system and version: Windows 10

[Kurt-von-Laven]

Not sure whether this is related or separate, but I experience the same symptom in the following scenario:

from typing import Any

def test_structural_pattern_matching(x: Any) -> Any:
    match x:
        case {"x": {"y": [value]}}:
            print("mypy believes this line is unreachable")

Happy to file a separate issue if desired. Intriguingly, if you remove the outer dictionary, mypy stops complaining. This bug particularly impacts the use of structural pattern matching in code that handles JSON, which obviously frequently contains complex nested data structures.

[Kurt-von-Laven]

Evidently the issue I mentioned was a separate one, so I filed #12770.