Fix crash on prefixed paramspec with deferral

[ilevkivskyi]

Fixes #14565

The fix looks simple, looks like an obvious omission.

[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:2202: 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.90168b8396bf53f7573e8f0ca198ab33a855e3d2
- tanjun/abc.py:2202: : 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/_internal/__init__.py:539: error: Incompatible types in assignment (expression has type "Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]", variable has type "Dict[Union[str, _IndexKeys], Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]]")  [assignment]
+ tanjun/_internal/__init__.py:552: error: Item "Dict[Union[str, _IndexKeys], Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]]" of "Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]" has no attribute "append"  [union-attr]
+ tanjun/_internal/__init__.py:616: error: Incompatible types in assignment (expression has type "Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]", variable has type "Dict[Union[str, _IndexKeys], Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]]")  [assignment]
+ tanjun/_internal/__init__.py:664: error: Incompatible types in assignment (expression has type "Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]", variable has type "Dict[Union[str, _IndexKeys], Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]]")  [assignment]
+ tanjun/_internal/__init__.py:677: error: Item "Dict[Union[str, _IndexKeys], Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]]" of "Union[_TreeT, List[Tuple[List[str], MessageCommand[Any]]]]" has no attribute "remove"  [union-attr]
+ 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:1043: 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]

... (truncated 100 lines) ...

[hauntsaninja]

Looks great, thank you!

[JukkaL]

Thanks for the quick fix!