S3 method does not show parent params in autocomplete

S3_parent_params.R

# There are two humans: `human` and `angry_human`. They only differ in the fact
# that angry humans have a frown on their face.

# Example code:
# dean <- human("Dean")
# ex <- angry_human("Ex")
# plot(dean)
# plot(ex)

# When plotting `dean`, RStudio shows me all the available parameters.
# But when plotting `ex`, RStudio only shows the params of the immediate class,
# but it would be very useful to also see what other params I can use (all
# the params of the parent class)

library(ggplot2)

human <- function(name) {
  structure(name, class = "human")
}

angry_human <- function(name) {
  structure(name, class = c("angry_human", "human"))
}

plot.human <- function(x, face_col = "black", eye_col = "blue",
                       mouth_col = "white", background_col = "transparent",
                       ...)
  {
  face <- circle()
  eyes <- rbind(
    circle(c(0.4, 0.3), 0.1),
    circle(c(-0.4, 0.3), 0.1)
  )
  mouth <- dplyr::data_frame(x = seq(-0.5, 0.5, length.out = 500),
                             y = x ^ 2 - 0.6)
  ggplot() +
    geom_polygon(data = face, aes(x, y), fill = face_col) + 
    geom_polygon(data = eyes, aes(x, y), fill = eye_col) +
    geom_path(data = mouth, aes(x, y), color = mouth_col, size = 2) +
    theme(axis.text = element_blank(), axis.title = element_blank(),
          line = element_blank(),
          panel.background = element_rect(fill = background_col)) +
    ggtitle(x)
}

plot.angry_human <- function(x, frown_col = "white", ...) {
  res <- NextMethod("plot")
  res +
    geom_segment(aes(x = -0.6, y = 0.6, xend = -0.2, yend = 0.45),
                 color = frown_col, size = 1, show_guide = FALSE) +
    geom_segment(aes(x = 0.2, y = 0.45, xend = 0.6, yend = 0.6),
                 color = frown_col, size = 1, show_guide = FALSE)
}

circle <- function(center = c(0, 0), radius = 1, npoints = 500){
  t <- seq(0, 2 * pi, length.out = npoints)
  x <- center[1] + radius * cos(t)
  y <- center[2] + radius * sin(t)
  return(data.frame(x = x, y = y))
}

# example
dean <- human("Dean")
ex <- angry_human("Ex")
plot(dean)
plot(ex)