Specialize syntax error for non-`case` statement inside `match` statement

Feature or enhancement

Proposal:

Right now using a non-case statement inside a match block gives a rather opaque syntax error:

match a:
    case 1:
        pass
    if 2:
        pass

gives:

  File "<python-input-0>", line 4
    if 2:
    ^^
SyntaxError: invalid syntax

It's easy to accidentally write something like this when dedenting code too much inside a case block or when not dedenting code enough after the match block.

I propose a specialized syntax error that more clearly points out what the problem is:

SyntaxError: expected 'case' statement inside 'match' statement

PR forthcoming

cc @pablogsal @lysnikolaou

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-146583

使用所示的 match 语句复现该问题，并查看链接的 PR gh-146583 以了解当前的实现方向。当 match 块中的非 case 语句产生提议的专用 SyntaxError 消息，而不是通用的无效语法错误时，即表示完成。