Specialize `match` syntax error for unary addition in pattern

[johnslavik]

Feature or enhancement

Proposal:

Consider a specialized syntax error for these cases:

# Unary addition isn't allowed but we parse it for better error recovery.
match subject:
    case +1:
        pass
    case 1 | +2 | -3:
        pass
    case [1, +2, -3]:
        pass
    case Foo(x=+1, y=-2):
        pass
    case {True: +1, False: -2}:
        pass

This was taken from https://github.com/astral-sh/ruff/blob/6ff4da454c448f01eb0b71728f3941870c811bfc/crates/ruff_python_parser/resources/invalid/statements/match/unary_add_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 +1:
         ^
-SyntaxError: invalid syntax
+SyntaxError: cannot use unary addition in a literal pattern

The code doesn't look incorrect syntactically, which is why I think it's a good idea to add a more specific message (although it is not extremely unclear as-is assuming a pointer ^ is shown).

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