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

repro.py の例から始め、guarded capture とネストされた or-pattern を含めて、mypy repro.py と Python 3.13 でのコンパイル結果を比較します。既存の or-pattern の検証を追跡し、不足している到達不能パターンのチェックを追加します。mypy が無効な非 final の irrefutable pattern を報告し、有効な guarded case または final case を受け入れれば完了です。

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