Specialize `match` syntax error for star pattern outside sequence pattern

[johnslavik]

Feature or enhancement

Proposal:

Consider a specialized syntax error for these cases:

# Star pattern is only allowed inside a sequence pattern
match subject:
    case *_:
        pass
    case *_ as x:
        pass
    case *foo:
        pass
    case *foo | 1:
        pass
    case 1 | *foo:
        pass
    case Foo(*_):
        pass
    case Foo(x=*_):
        pass
    case {*_}:
        pass

This was taken from https://github.com/astral-sh/ruff/blob/6ff4da454c448f01eb0b71728f3941870c811bfc/crates/ruff_python_parser/resources/invalid/statements/match/star_pattern_usage.py. 2+ years in Ruff's parser.

Current vs proposed:

❯ ./python.exe t.py
  File "/Users/bartosz.slawecki/Python/cpython/t.py", line 2
    case *_:
           ^
-SyntaxError: invalid syntax
+SyntaxError: star pattern is only allowed inside a sequence pattern

The code doesn't look obviously incorrect syntactically, which is why I think it's a good idea to add a more specific message.

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

[johnslavik]

cc @sobolevn @Eclips4 @pablogsal

[pablogsal]

CC @brandtbucher