[backport] [used before def] handle walrus declaration in match subject correctly

[ilinum]

Backport of #14665.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- tanjun/abc.py:2122: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
- https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
- Please report a bug at https://github.com/python/mypy/issues
- version: 1.0.0+dev.0665ce924290dad3f30010b3bb93310a71c8db81
- tanjun/abc.py:2122: : note: use --pdb to drop into pdb
- tanjun/abc.py:92: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:93: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:94: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:95: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:97: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:98: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:99: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:100: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:102: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:103: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:104: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
- tanjun/abc.py:105: error: Cannot resolve TypeVar upper bound (possible cyclic definition)  [misc]
+ tanjun/dependencies/reloaders.py:365: error: Argument 1 to "filter" has incompatible type "Callable[[Path], bool]"; expected "Callable[[Self], TypeGuard[bool]]"  [arg-type]
+ tanjun/dependencies/reloaders.py:365: error: Argument 1 to "map" has incompatible type "Callable[[Self, bool], Self]"; expected "Callable[[Path], Self]"  [arg-type]
+ tanjun/dependencies/reloaders.py:408: error: "ABC" has no attribute "type"  [attr-defined]
+ tanjun/dependencies/reloaders.py:408: error: "ABC" has no attribute "name"  [attr-defined]
+ tanjun/dependencies/reloaders.py:408: error: "ABC" has no attribute "build"  [attr-defined]
+ tanjun/dependencies/reloaders.py:512: error: Incompatible return value type (got "Tuple[Path, Union[Tuple[None, Set[<nothing>]], Tuple[str, Set[<nothing>]]]]", expected "Tuple[Path, Union[Tuple[str, Set[str]], Tuple[None, Set[Path]]]]")  [return-value]
+ tanjun/context/slash.py:995: error: List item 0 has incompatible type "_OtherT"; expected "_T"  [list-item]
+ tanjun/context/slash.py:1044: error: Signature of "command" incompatible with supertype "Context"  [override]
+ tanjun/context/message.py:130: error: Signature of "command" incompatible with supertype "Context"  [override]
+ tanjun/schedules.py:339: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "_CallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, <nothing>], <nothing>]]"  [arg-type]
+ tanjun/schedules.py:365: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "Callable[..., Coroutine[Any, Any, None]]"; expected "Callable[..., Union[Coroutine[Any, Any, <nothing>], <nothing>]]"  [arg-type]
+ tanjun/schedules.py:388: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "Callable[..., Coroutine[Any, Any, None]]"; expected "Callable[..., Union[Coroutine[Any, Any, <nothing>], <nothing>]]"  [arg-type]
+ tanjun/schedules.py:1052: error: Argument 1 to "call_with_async_di" of "Client" has incompatible type "_CallbackSigT"; expected "Callable[..., Union[Coroutine[Any, Any, <nothing>], <nothing>]]"  [arg-type]
+ tanjun/parsing.py:1668: error: Right operand of "and" is never evaluated  [unreachable]
+ tanjun/parsing.py:1669: error: Statement is unreachable  [unreachable]
+ tanjun/parsing.py:1679: error: Statement is unreachable  [unreachable]
+ tanjun/context/menu.py:101: error: Signature of "command" incompatible with supertype "Context"  [override]
+ tanjun/context/menu.py:129: error: Incompatible return value type (got "Unique", expected "Union[InteractionMember, User, Message]")  [return-value]
+ tanjun/context/menu.py:141: error: Incompatible return value type (got "CommandType", expected "Literal[CommandType.USER, CommandType.MESSAGE]")  [return-value]
+ tanjun/context/menu.py:196: error: Argument "default" to "resolve_to_member" of "MenuContext" has incompatible type "None"; expected "User"  [arg-type]
+ tanjun/dependencies/limiters.py:782: error: Argument 1 to "add_pre_execution" of "Hooks" has incompatible type "CooldownPreExecution"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Optional[Coroutine[Any, Any, None]]]"  [arg-type]
+ tanjun/dependencies/limiters.py:782: note: "CooldownPreExecution.__call__" has type "Callable[[Context, Arg(AbstractCooldownManager, 'cooldowns'), DefaultNamedArg(Optional[AbstractLocaliser], 'localiser'), NamedArg(Optional[AbstractOwners], 'owner_check')], Coroutine[Any, Any, None]]"
+ tanjun/dependencies/limiters.py:1137: error: Argument 1 to "add_pre_execution" of "Hooks" has incompatible type "ConcurrencyPreExecution"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Optional[Coroutine[Any, Any, None]]]"  [arg-type]
+ tanjun/dependencies/limiters.py:1137: note: "ConcurrencyPreExecution.__call__" has type "Callable[[Context, Arg(AbstractConcurrencyLimiter, 'limiter'), DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], Coroutine[Any, Any, None]]"
+ tanjun/dependencies/limiters.py:1137: error: Argument 1 to "add_post_execution" of "Hooks" has incompatible type "ConcurrencyPostExecution"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Optional[Coroutine[Any, Any, None]]]"  [arg-type]
+ tanjun/dependencies/limiters.py:1137: note: "ConcurrencyPostExecution.__call__" has type "Callable[[Context, Arg(AbstractConcurrencyLimiter, 'limiter')], Coroutine[Any, Any, None]]"
+ tanjun/checks.py:620: error: Argument 2 to "_optional_kwargs" has incompatible type "DmCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:620: note: "DmCheck.__call__" has type "Callable[[Context, DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], bool]"
+ tanjun/checks.py:686: error: Argument 2 to "_optional_kwargs" has incompatible type "GuildCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:686: note: "GuildCheck.__call__" has type "Callable[[Context, DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], bool]"
+ tanjun/checks.py:748: error: Argument 2 to "_optional_kwargs" has incompatible type "NsfwCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:748: note: "NsfwCheck.__call__" has type "Callable[[Context, DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], Coroutine[Any, Any, bool]]"
+ tanjun/checks.py:810: error: Argument 2 to "_optional_kwargs" has incompatible type "SfwCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:810: note: "SfwCheck.__call__" has type "Callable[[Context, DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], Coroutine[Any, Any, bool]]"
+ tanjun/checks.py:872: error: Argument 2 to "_optional_kwargs" has incompatible type "OwnerCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:872: note: "OwnerCheck.__call__" has type "Callable[[Context, Arg(AbstractOwners, 'dependency'), DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], Coroutine[Any, Any, bool]]"
+ tanjun/checks.py:927: error: Argument 2 to "_add_to_command" has incompatible type "AuthorPermissionCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:927: note: "AuthorPermissionCheck.__call__" has type "Callable[[Context, DefaultNamedArg(Optional[AbstractLocaliser], 'localiser')], Coroutine[Any, Any, bool]]"
+ tanjun/checks.py:983: error: Argument 2 to "_add_to_command" has incompatible type "OwnPermissionCheck"; expected "Callable[[_ContextT_contra, VarArg(Any), KwArg(Any)], Union[Coroutine[Any, Any, bool], bool]]"  [arg-type]
+ tanjun/checks.py:983: note: "OwnPermissionCheck.__call__" has type "Callable[[Context, DefaultNamedArg(Optional[AbstractLocaliser], 'localiser'), DefaultNamedArg(OwnUser, 'my_user'), DefaultNamedArg(Optional[GuildBoundCache[Union[Snowflake, int], Member]], 'member_cache')], Coroutine[Any, Any, bool]]"
+ tanjun/checks.py:996: error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]
+ tanjun/checks.py:1003: error: Overloaded function signature 3 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]
+ tanjun/checks.py:1003: error: Overloaded function signature 3 will never be matched: signature 2's parameter type(s) are the same or broader  [misc]
+ tanjun/checks.py:1010: error: Overloaded function signature 4 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]
+ tanjun/checks.py:1010: error: Overloaded function signature 4 will never be matched: signature 2's parameter type(s) are the same or broader  [misc]
+ tanjun/checks.py:1010: error: Overloaded function signature 4 will never be matched: signature 3's parameter type(s) are the same or broader  [misc]
+ tanjun/checks.py:1087: error: Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader  [misc]

... (truncated 92 lines) ...

[JukkaL]

The mypy_primer output doesn't look related to this PR.

[hauntsaninja]

The primer is related to an issue calculating the merge base (it assumed all PRs were targeting master).

I fixed, but the fix was not backported to the 1.0 branch