[dragonwriter]

except .find_all.with_index passes the index to the predicate block, but still only returns a view of the matching elements from the list.

What I think is being sought is more like:

  module Enumerable
    def find_indices
      each.with_index
          .find_all {|x,_| yield x}
          .map {|_,y| y}
    end
  end