gh-116093: ABCMeta.__subclasscheck__ now initializes cls if it is not yet initialized #119719
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ABCMeta.__subclasscheck__
now initializes cls if it is not yet initializedThis fixes the issues reported in gh-82266, gh-116093 and gh-119699.
Previously calling
issubclass
within__init_subclass__
may result in the ABC cache of the parent class of the subclass being instantiated to be mutated since the cache of the subclass is not yet initialized.This is fixed by forcing
__subclasscheck__
to initialize the current class if it is found to not have been initialized.