geom_curve does not use lineend

[Ax3man]

According to the documentation, lineend is a valid parameter for both geom_segment and geom_curve, but geom_curve does not change depending on lineend. This is true for both the CRAN version and the current dev version. Reprex:

ggplot(mapping = aes(x = x1, xend = x2, y = y, yend = y)) +
  geom_curve(data = data.frame(x1 = 0, x2 = 1, y = 0), size = 4, curvature = -1,
             lineend = 'round') +
  geom_curve(data = data.frame(x1 = 1.5, x2 = 2.5, y = 0), size = 4, curvature = -1,
             lineend = 'butt') +
  geom_curve(data = data.frame(x1 = 3, x2 = 4, y = 0), size = 4, curvature = -1,
             lineend = 'square') +
  ylim(0, 1)