str(arrow()) seems broken

arrow.R

library(grid)
(a <- arrow())
#> $angle
#> [1] 30
#> 
#> $length
#> [1] 0.25inches
#> 
#> $ends
#> [1] 2
#> 
#> $type
#> [1] 1
#> 
#> attr(,"class")
#> [1] "arrow"

names(a)
#> [1] "angle"  "length" "ends"   "type"  

mode(a)
#> [1] "list"

a[[2]]
#> [1] 0.25inches

# not what I'd expect!
str(a)
#> List of 1
#> $ angle : num 30
#> - attr(*, "class")= chr "arrow"


str(unclass(a))
#> List of 4
#> $ angle : num 30
#> $ length:Class 'unit'  atomic [1:1] 0.25
#> .. ..- attr(*, "valid.unit")= int 2
#> .. ..- attr(*, "unit")= chr "inches"
#> $ ends  : int 2
#> $ type  : int 1


#' Proposal arrow method for str()
#' @export
str.arrow <- function(object, ...) {
  o <- oldClass(object)
  oldClass(object) <- setdiff(o, "arrow")
  str(object)
}