PEP696 + PEP749: Defer evaluation of defaults when parametrizing

Bug report

Bug description:

With PEP 696 + PEP 749, the following works just fine:

class Scalar[CompatT, TruthT = BoolScalar]: ...
class BoolScalar(Scalar): ...
class IntScalar(Scalar[int]): ...  # OK ✅️

However, if we switch around the order of definition, the code breaks:

class Scalar[CompatT, TruthT = BoolScalar]: ...
class IntScalar(Scalar[int]): ...  # ❌️ NameError: name 'BoolScalar' is not defined
class BoolScalar(Scalar): ...

I guess since the default gets evaluated here. Not sure whether this is a bug or feature request, but it would be nice if the evaluation of the default could be deferred in is case as well.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-152208

issue にある2つのクラス定義の例を CPython 3.14 で実行し、動作を比較します。リンクされている PEP 696 と PEP 749 の議論を読み、リンクされている PR gh-152208 を確認します。前方定義の例で NameError が発生しなくなり、既存の例が引き続き動作すれば完了です。