Calling the usual rb_iclass_classext_free() causes SEGV because duplicating a newer classext of iclass had set the reference from superclass to the newer classext, but calling rb_iclass_classext_free() deletes it.
cause:
Thread#raise
Fiber#raise
SET_SHAREABLE
set_table->bins
set_table->entries
rb_gc_mark_and_move
rb_gc_location