Improve error message for callables with invalid Concatenate

[sobolevn]

This is a bit hacky, but it does solve the problem :)

Closes #13518

[hauntsaninja]

Can you check the primer output? It looks like there are some lines that used to error that have no errors... is that a good thing?

[sobolevn]

Yes, I am looking at it right now!

discord/app_commands/commands.py:113: error: The first argument to Callable must be a list of types, parameter specification, or "..."

Hm 🤔
Code: Callable[Concatenate[GroupT, 'Interaction', P], Coro[T]]
Link: https://github.com/Rapptz/discord.py/blob/7c1dffbf9d62279960a3618676a2c3972cd55207/discord/app_commands/commands.py#L112-L115

Looks like unbound P is not reported now.

discord/ext/commands/hybrid.py:85: error: The first argument to Callable must be a list of types, parameter specification, or "..."

Code: Callable[Concatenate[CogT, ContextT, P], Coro[T]]
Link: https://github.com/Rapptz/discord.py/blob/7c1dffbf9d62279960a3618676a2c3972cd55207/discord/ext/commands/hybrid.py#L85

Unbound P is not reported.

I will fix it and write extra tests tomorrow 🌆

[sobolevn]

Actually, it works better now in my opinion.
Here's the real output:

Code:

from typing_extensions import Concatenate, ParamSpec
from typing import Callable

P = ParamSpec('P')

X = Callable[Concatenate[int, P], None]

It is still not ideal, because of The last parameter to Concatenate needs to be a ParamSpec, but this is out of scope of this PR. I will fix it in the next one.

[github-actions]

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

discord.py (https://github.com/Rapptz/discord.py)
- discord/app_commands/commands.py:113: error: The first argument to Callable must be a list of types, parameter specification, or "..."
- discord/app_commands/commands.py:113: note: See https://mypy.readthedocs.io/en/stable/kinds_of_types.html#callable-types-and-lambdas
- discord/app_commands/commands.py:114: error: The first argument to Callable must be a list of types, parameter specification, or "..."
- discord/app_commands/commands.py:114: note: See https://mypy.readthedocs.io/en/stable/kinds_of_types.html#callable-types-and-lambdas
+ discord/app_commands/commands.py:672: error: Item "function" of "Union[Callable[[VarArg(Any), KwArg(Any)], Any], Callable[[VarArg(Any), KwArg(Any)], Any]]" has no attribute "__self__"
+ discord/app_commands/commands.py:673: error: Item "function" of "Union[Callable[[VarArg(Any), KwArg(Any)], Any], Callable[[VarArg(Any), KwArg(Any)], Any]]" has no attribute "__func__"
- discord/app_commands/commands.py:2408: error: Item "function" of "Union[Any, Callable[[Interaction, Member], Coroutine[Any, Any, Any]], Callable[[Interaction, User], Coroutine[Any, Any, Any]], Callable[[Interaction, Message], Coroutine[Any, Any, Any]]]" has no attribute "__discord_app_commands_checks__"
+ discord/app_commands/commands.py:2408: error: Item "function" of "Union[Callable[[VarArg(Any), KwArg(Any)], Any], Callable[[Interaction, Member], Coroutine[Any, Any, Any]], Callable[[Interaction, User], Coroutine[Any, Any, Any]], Callable[[Interaction, Message], Coroutine[Any, Any, Any]]]" has no attribute "__discord_app_commands_checks__"
- discord/app_commands/commands.py:2410: error: Item "function" of "Union[Any, Callable[[Interaction, Member], Coroutine[Any, Any, Any]], Callable[[Interaction, User], Coroutine[Any, Any, Any]], Callable[[Interaction, Message], Coroutine[Any, Any, Any]]]" has no attribute "__discord_app_commands_checks__"
+ discord/app_commands/commands.py:2410: error: Item "function" of "Union[Callable[[VarArg(Any), KwArg(Any)], Any], Callable[[Interaction, Member], Coroutine[Any, Any, Any]], Callable[[Interaction, User], Coroutine[Any, Any, Any]], Callable[[Interaction, Message], Coroutine[Any, Any, Any]]]" has no attribute "__discord_app_commands_checks__"
- discord/ext/commands/hybrid.py:85: error: The first argument to Callable must be a list of types, parameter specification, or "..."
- discord/ext/commands/hybrid.py:85: note: See https://mypy.readthedocs.io/en/stable/kinds_of_types.html#callable-types-and-lambdas
- discord/ext/commands/hybrid.py:86: error: The first argument to Callable must be a list of types, parameter specification, or "..."
- discord/ext/commands/hybrid.py:86: note: See https://mypy.readthedocs.io/en/stable/kinds_of_types.html#callable-types-and-lambdas
- discord/ext/commands/hybrid.py:793: note:          def command(self, name: Union[str, locale_str] = ..., *args: Any, with_app_command: bool = ..., **kwargs: Any) -> Callable[[Union[Any, Any]], HybridCommand[CogT, P2, U]]
+ discord/ext/commands/hybrid.py:793: note:          def command(self, name: Union[str, locale_str] = ..., *args: Any, with_app_command: bool = ..., **kwargs: Any) -> Callable[[Union[Callable[[VarArg(Any), KwArg(Any)], Any], Callable[[VarArg(Any), KwArg(Any)], Any]]], HybridCommand[CogT, P2, U]]
- discord/ext/commands/hybrid.py:817: note:          def group(self, name: Union[str, locale_str] = ..., *args: Any, with_app_command: bool = ..., **kwargs: Any) -> Callable[[Union[Any, Any]], HybridGroup[CogT, P2, U]]
+ discord/ext/commands/hybrid.py:817: note:          def group(self, name: Union[str, locale_str] = ..., *args: Any, with_app_command: bool = ..., **kwargs: Any) -> Callable[[Union[Callable[[VarArg(Any), KwArg(Any)], Any], Callable[[VarArg(Any), KwArg(Any)], Any]]], HybridGroup[CogT, P2, U]]