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

在 CPython 3.14 上运行 issue 中的两个类定义示例，并比较它们的行为。阅读链接的 PEP 696 和 PEP 749 讨论，然后检查链接的 PR gh-152208；完成标准是前向定义示例不再引发 NameError，同时现有示例继续正常工作。