if_else na()

na <- function (x) 
{
  x <- if (missing(x)) {
    prev_call <- sys.call(sys.nframe() - 2)
    if (identical(prev_call[[1]], quote(if_else)) | 
          identical(prev_call[[1]], quote(dplyr::if_else))) {
      prev_env <- sys.frame(sys.nframe() - 1)
      prev_env$x
    }
    else logical(1)
  }
  else x
  y <- NA
  y <- type.convert(y, typeof(x))
  class(y) <- class(x)
  rep(y, length(x))
}

x <- 1:10
y <- letter[1:10]
z <- rbinom(10,1,0.5) == 1

if_else(x < 5, x, na())
if_else(x > 5, y, na())
if_else(x > 5, z, na())