gettext: fix unconstrained TypeVar

[JelleZijlstra]

Part of #7928

[AlexWaygood]

Looking at the 3.11 branch: which calls will be incorrectly inferred (or incorrectly disallowed) if we omit overloads 2-4 here? Could we not just do the following, using only 4 overloads instead of 7?

Overloads 2 and 3
These are the Literal[True] overloads, but they're covered by the final bool fallback overload, since the return type is the same.

Overload 4
This can be combined with the final fallback overload, since the return type is the same. For the final overload, we can just have the class_ parameter as Callable[[io.BufferedReader], _T] | None instead of Callable[[io.BufferedReader], _T].

@overloaddeftranslation(
domain: str,
localedir: StrPath|None= ...,
languages: Iterable[str] |None= ...,
class_: None= ...,
fallback: Literal[False] = ...,
) ->GNUTranslations: ...
@overloaddeftranslation(
domain: str,
localedir: StrPath|None= ...,
languages: Iterable[str] |None= ...,
*,
class_: Callable[[io.BufferedReader], _T],
fallback: Literal[False] = ...,
) ->_T: ...
@overloaddeftranslation(
domain: str,
localedir: StrPath|None,
languages: Iterable[str] |None,
class_: Callable[[io.BufferedReader], _T],
fallback: Literal[False] = ...,
) ->_T: ...
@overloaddeftranslation(
domain: str,
localedir: StrPath|None= ...,
languages: Iterable[str] |None= ...,
class_: Callable[[io.BufferedReader], NullTranslations] |None= ...,
fallback: bool= ...,
) ->NullTranslations: ...

Not sure we need so many overloads

[AlexWaygood]

It might also be good to rename the _T TypeVar to _NullTranslationsT now that it's bound to NullTranslations

[JelleZijlstra]

Good point, you're right.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉