Ruby

class C
  include Enumerable
  def each(&b)
    $b = b
    yield 1
  end
end

Set[1, 2, 3] & C.new
$b.call(1) # [BUG] Segmentation fault at 0x00007f21bfa67f60

The cause is essentially the same as #5801, we're initializing an ifunc pointing to a stack-allocated struct. The solution I think is to only ever use rb_block_call with a GC managed object like an imemo_memo.

In addition to set_intersection_block this likely also affects, lazy_flat_map_i, nmin_i, enum_sum_i, and product_each_i all of which are passed a stack buffer.

It might also be helpful to prevent the ifunc from being called after return. Some iterators (ex. sort_by) will raise a runtime error when this happens, but it's done ad-hoc (and detection depends on the variable still being accessible).