Irrefutable pattern before the last case in a match statement passes type checking but is a SyntaxError at runtime

Bắt đầu với các ví dụ trong repro.py và so sánh mypy repro.py với quá trình biên dịch bằng Python 3.13, bao gồm các capture có guard và các or-pattern lồng nhau. Truy vết phần xác thực or-pattern hiện có và thêm kiểm tra pattern không thể đạt tới còn thiếu; hoàn thành khi mypy báo cáo các pattern irrefutable không phải final không hợp lệ, đồng thời chấp nhận các trường hợp có guard hoặc final hợp lệ.

Bug Report

mypy reports nothing for match statements that CPython rejects at compile time with SyntaxError: name capture 'y' makes remaining patterns unreachable. An unguarded capture or wildcard pattern in any case except the last one makes the remaining cases unreachable, so the compiler refuses the file, but mypy checks it clean. That means a file that cannot even be imported passes type checking.

The same applies to an irrefutable alternative in a non-final position of an or pattern, e.g. case _ | 1:, which CPython rejects wherever the or pattern appears, including nested inside sequence, mapping and class patterns.

To Reproduce

def f(x: int) -> str:
    match x:
        case y:
            return "capture"
        case _:
            return "wildcard"

Expected Behavior

An error on case y:, matching the runtime behavior:

$ python repro.py
  File "repro.py", line 3
    case y:
         ^
SyntaxError: name capture 'y' makes remaining patterns unreachable

For reference, a case is fine if it has a guard (case y if y > 0:), and an irrefutable pattern in the last case is fine. Inside an or pattern an irrefutable alternative is only allowed in the last position, regardless of guards or which case it is in.

Actual Behavior

$ mypy repro.py
Success: no issues found in 1 source file

mypy already implements the closely related compile time check for or patterns ("Alternative patterns bind different names"), so I think this one is missing rather than intentionally skipped. I have a fix ready and will put up a PR shortly.

Your Environment

• Mypy version used: mypy 2.4.0+dev.3ad6157f9b66e71738bf39cae939aef7c3fd7ea8 (current master, also with --native-parser)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.13.13