message: Issue Window Masks
  Fix lingering deprecated-copy warning.
diff: @@ -66,6 +66,8 @@ public:
  public:
  friend BitArray;
  + reference(const reference &r) = default;
  +
  reference &operator=(bool x) noexcept {
  auto b = parent.Block(pos);
  auto s = parent.Shift(pos);
  