Mypy fails on Python 3.12 TypeAliasType compatibility branch

src/browserbase/_utils/_typing.py の54-56行目から始め、リポジトリの開発依存関係を使って Python 3.12 で失敗を再現します。タプルのアノテーションと type guard が両方の TypeAliasType 実装をどのように表現しているかを確認し、その後 python -m mypy src を実行します。Python 3.12 で厳密な型チェックに成功し、意図した narrowing が維持されれば完了です。

Description

Running the repository's pinned mypy (1.17) under supported Python 3.12 fails in the shared typing helper because _TYPE_ALIAS_TYPES is annotated as containing type[typing_extensions.TypeAliasType], then appends the distinct stdlib typing.TypeAliasType class.

Code reference

src/browserbase/_utils/_typing.py:54-56

Reproduction

With Python 3.12 and the repository dev dependencies:

python -m mypy src

Actual error:

src/browserbase/_utils/_typing.py:56: error: Argument 2 to <tuple> has incompatible type
type[typing.TypeAliasType]; expected type[typing_extensions.TypeAliasType] [arg-type]

Expected behavior

The strict type-check command should pass on supported Python versions, including Python 3.12 where typing.TypeAliasType exists. The tuple annotation/type guard should model both runtime implementations (or use a suitably common type[object] representation while preserving narrowing).

Actual behavior

Mypy reports a source error on Python 3.12. The current CI lint job uses the repository's minimum Python selected by .python-version, so this version-dependent branch is not checked.

Why it matters

This is a core/shared typing utility used to recognize both stdlib and backport aliases. A Python-version matrix for type checking would catch this class of conditional typing regression.