[PEP 696] default substitution not applied correctly when parametrizing a generic type.

Bug report

Bug description:

PEP 696 allows the default of a type-parameter to depend on other type parameters, however, this substitution is not applied at runtime:

class Bar[T, S=T]: ...

print(Bar[int])  # __main__.Bar[int, T]

The result should be __main__.Bar[int, int][^1]

This is likely the root cause for #140596. Tested with 3.13.8, 3.14.0 and 3.15.0a1+

[^1]: pyright playground, mypy-playground, ty-playground, pyrefly sandbox

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-151811

從 PEP 696 中關於使用另一個型別參數作為預設值的部分開始，並在 CPython main 上重現報告中的 Bar[int] 範例。將執行時期的泛型別名與預期的 main.Bar[int, int] 結果進行比較，並查看相關的 PR gh-151811；完成的標準是相依的預設值在執行時期被替換。