BaseExceptionGroup should return ExceptionGroup if initialized with non-base exceptions

When initializing a BaseExceptionGroup with non-base exceptions the stdlib (and the backport) will in fact return an ExceptionGroup. The typing in neither of typeshed nor the backport currently supports this.

from typing_extensions import reveal_type

x = BaseExceptionGroup('', [ValueError()])
reveal_type(x)

$ python foo.py
Runtime type is 'ExceptionGroup'
$ mypy foo.py 
foo.py:30: note: Revealed type is "builtins.BaseExceptionGroup[builtins.ValueError]"
$ pyright foo.py 
foo.py
  foo.py:30:13 - information: Type of "x" is "BaseExceptionGroup[ValueError]"

I have vague recollections that trying to do this was hard-to-impossible, but I currently cannot find any related issues.

まず typeshed と backport にある BaseExceptionGroup の定義を見つけ、コンストラクターの型付けを issue に示されている実行時の動作と比較します。mypy と pyright で例を確認します。非ベース例外だけが渡された場合に、両方の型チェッカーが ExceptionGroup[ValueError] を推論できれば完了です。