Fix PyTreeNode + Generic losing __parameters__ when Generic is last in bases

[mohsinm-dev]

Fixes #5233

PyTreeNode.init_subclass was not calling super().init_subclass(), breaking the cooperative inheritance chain that Generic relies on to set parameters. Added the missing super call and a test.

[gemini-code-assist]

Summary of Changes

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This pull request addresses a critical inheritance bug in PyTreeNode that prevented Generic classes from correctly retaining their type parameters (__parameters__) when Generic was positioned last in the base class list. By ensuring super().__init_subclass__() is called, the cooperative inheritance chain is maintained, guaranteeing proper initialization for generic PyTreeNode subclasses.

Highlights

• PyTreeNode Inheritance Fix: Corrected PyTreeNode.__init_subclass__ to call super().__init_subclass__(), resolving an issue where Generic classes would lose their __parameters__ when Generic was the last base class in the inheritance chain.
• Test Coverage: Added a new test case (test_generic_pytreenode_base_order) to verify that PyTreeNode and Generic work correctly together, regardless of the order in which they appear in the base classes, and that subclassing parameterized generic PyTreeNodes functions as expected.

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Changelog

• flax/struct.py
  • Added a call to super().__init_subclass__() within PyTreeNode.__init_subclass__.
• tests/struct_test.py
  • Imported Generic and TypeVar from typing.
  • Added test_generic_pytreenode_base_order to validate PyTreeNode and Generic interaction with different base class orders and subclassing.

Activity

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[gemini-code-assist]

Code Review

This pull request correctly addresses a bug where PyTreeNode would break the method resolution order for __init_subclass__ when used with typing.Generic, causing __parameters__ to be lost. The fix, adding a super().__init_subclass__() call, is appropriate and correctly placed. The accompanying test is comprehensive, covering various inheritance orders and subclassing scenarios to ensure the fix is robust. The changes are of high quality and ready for merging.

[samanklesaria]

Looks good to me.