∀ {α : Type u} {x : BEq α} {x_1 : Hashable α} {β : Type v} {m : Std.DHashMap α fun x => β} [inst : EquivBEq α]
  [inst_1 : LawfulHashable α] [inst_2 : Inhabited α] {f : α → β → Bool} {k : α},
  (Std.DHashMap.filter f m).getKey! k =
    ((m.getKey? k).pfilter fun x_2 h' => f x_2 (Std.DHashMap.Const.get m x_2 ⋯)).get!