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 上執行 issue 中的兩個類別定義範例，並比較它們的行為。閱讀連結的 PEP 696 和 PEP 749 討論，然後檢視連結的 PR gh-152208；完成標準是前向定義範例不再引發 NameError，同時現有範例繼續正常運作。