Fix ParamSpec constraint for types as callable

[VincentVanlaer]

Most types can be considered as callables, constructing the type itself. When a constraint was created for a ParamSpec variable, the return type would be set to NoneType, which conflicts with assumptions that CallableType makes when it is the constructor of another type, crashing mypy. This patch replaces the return type by UninhabitedType instead, which stops CallableType from considering itself as a constructor.

[hauntsaninja]

Thanks for finding and fixing! This looks great :-)

mypy_primer also looks promising, but haven't yet confirmed it's correct

For my reference, this is the guard mentioned in the PR description:

mypy/mypy/types.py

Line 1773 in 401798f

def is_type_obj(self) -> bool:

[VincentVanlaer]

The mypy_primer diff boils down to the following example:

from typing import Callable, ParamSpec, Union, Any

P = ParamSpec("P")

def wrapper(coro: Callable[P, int]) -> Callable[P, int]:
    return coro

f: Union[Callable[[str], int], Any]

g = wrapper(f)
# Before: error: Argument 1 to "wrapper" has incompatible type "Union[Callable[[str], int], Any]"; expected "Callable[[VarArg(<nothing>), KwArg(<nothing>)], int]"
# After: No error
# --no-strict-optional: No error

reveal_type(g)
# Before: Revealed type is "def (*<nothing>, **<nothing>) -> int"
# After: Revealed type is "def (builtins.str) -> int"
# --no-strict-optional: Revealed type is "def (builtins.str) -> int"

The main difference is that the constraint on P is [132 <: def (builtins.str), 132 <: def (*Any, **Any) -> Any] in the first case and [132 <: def (builtins.str) -> <nothing>, 132 <: def (*Any, **Any) -> Any] in the second case. The second case seems preferable to me.

[hauntsaninja]

Thanks again! I resolved a merge conflict, will merge once tests pass again

[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/ext/commands/context.py:605: error: Argument 1 to "wrap_callback" has incompatible type "Callable[[Cog], Coroutine[Any, Any, None]]"; expected "Callable[[Mapping[Optional[Cog], List[Command[Any, [VarArg(Any), KwArg(Any)], Any]]]], Coroutine[Any, Any, None]]"  [arg-type]
+ discord/ext/commands/context.py:605: error: Incompatible types in assignment (expression has type "Callable[[Cog], Coroutine[Any, Any, None]]", variable has type "Callable[[Mapping[Optional[Cog], List[Command[Any, [VarArg(Any), KwArg(Any)], Any]]]], Coroutine[Any, Any, None]]")  [assignment]
- discord/ext/commands/context.py:608: error: Argument 1 to "wrap_callback" has incompatible type "Callable[[Group[Any, [VarArg(Any), KwArg(Any)], Any]], Coroutine[Any, Any, None]]"; expected "Callable[[Mapping[Optional[Cog], List[Command[Any, [VarArg(Any), KwArg(Any)], Any]]]], Coroutine[Any, Any, None]]"  [arg-type]
+ discord/ext/commands/context.py:608: error: Incompatible types in assignment (expression has type "Callable[[Group[Any, [VarArg(Any), KwArg(Any)], Any]], Coroutine[Any, Any, None]]", variable has type "Callable[[Mapping[Optional[Cog], List[Command[Any, [VarArg(Any), KwArg(Any)], Any]]]], Coroutine[Any, Any, None]]")  [assignment]
- discord/ext/commands/context.py:611: error: Argument 1 to "wrap_callback" has incompatible type "Callable[[Command[Any, [VarArg(Any), KwArg(Any)], Any]], Coroutine[Any, Any, None]]"; expected "Callable[[Mapping[Optional[Cog], List[Command[Any, [VarArg(Any), KwArg(Any)], Any]]]], Coroutine[Any, Any, None]]"  [arg-type]
+ discord/ext/commands/context.py:611: error: Incompatible types in assignment (expression has type "Callable[[Command[Any, [VarArg(Any), KwArg(Any)], Any]], Coroutine[Any, Any, None]]", variable has type "Callable[[Mapping[Optional[Cog], List[Command[Any, [VarArg(Any), KwArg(Any)], Any]]]], Coroutine[Any, Any, None]]")  [assignment]
- discord/ext/commands/core.py:641: error: Argument 1 to "wrap_callback" has incompatible type "Union[Callable[[Context[BotT], Exception], Coroutine[Any, Any, None]], Any]"; expected "Callable[[VarArg(<nothing>), KwArg(<nothing>)], Coroutine[Any, Any, Optional[Any]]]"  [arg-type]
- discord/ext/commands/core.py:642: error: Argument 1 has incompatible type "Context[BotT]"; expected <nothing>  [arg-type]
- discord/ext/commands/core.py:642: error: Argument 2 has incompatible type "CommandError"; expected <nothing>  [arg-type]
- discord/ext/commands/core.py:1271: error: Argument 1 to "maybe_coroutine" has incompatible type "Union[Callable[[Context[BotT]], bool], Any]"; expected "Callable[[VarArg(<nothing>), KwArg(<nothing>)], Union[Any, Awaitable[Any]]]"  [arg-type]
- discord/ext/commands/core.py:1271: error: Argument 2 to "maybe_coroutine" has incompatible type "Context[BotT]"; expected <nothing>  [arg-type]
+ discord/ext/commands/core.py:1271: error: Argument 2 to "maybe_coroutine" has incompatible type "Context[BotT]"; expected "Context[BotT]"  [arg-type]