count()/add_count() should preserve input classes

[yutannihilation]

(Originally reported at #4051 (comment).)

Details

``` r library(dplyr, warn.conflicts = FALSE)

x <- as_tibble(iris)
g <- group_by(x, Species)
class(g) <- c("myclass", class(g))

x %>%
add_tally() %>%
class()
#> [1] "tbl_df" "tbl" "data.frame"

x %>%
add_count() %>%
class()
#> [1] "tbl_df" "tbl" "data.frame"

<sup>Created on 2019-01-07 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1)</sup>
</details>

[romainfrancois]

Note that the original reprex used x instead of g.

library(dplyr, warn.conflicts = FALSE)

x <- as_tibble(iris)
g <- group_by(x, Species)
class(g) <- c("myclass", class(g))
class(g)
#> [1] "myclass"    "grouped_df" "tbl_df"     "tbl"        "data.frame"

g %>%
  add_tally() %>%
  class()
#> [1] "grouped_df" "tbl_df"     "tbl"        "data.frame"

g %>%
  add_count() %>%
  class()
#> [1] "grouped_df" "tbl_df"     "tbl"        "data.frame"

This is part of our design for the 0.9.* series. We essentially need to find a way to let some method of myclass express this. At the moment we have no idea what an object of class myclass would need to do in that situation, so treating it as a grouped_df is "the best we can do"

[yutannihilation]

Ah, I don't know why I didn't used g, sorry... Looking forward to v0.9!

[elinw]

This is really inconsistent with the fact that elsewhere in dplyr 0.8.0 you have stopped stripping classes.

[romainfrancois]

Fixing these things is planned for the 0.9.* series because we’ll benefit from better vctrs support.

[hadley]

Minimal reprex:

library(dplyr, warn.conflicts = FALSE)

x <- data.frame(x = 1:3)
x %>% count() %>% class()
#> [1] "tbl_df"     "tbl"        "data.frame"
x %>% add_count() %>% class()
#> [1] "tbl_df"     "tbl"        "data.frame"

<sup>Created on 2019-12-31 by the reprex package (v0.3.0)</sup>

(tally() and add_tally() don't touch the classes so don't need to be considered further)

[hadley]

We might want to solve this with a generic mechanism for temporarily grouping, performing some operation, and then restoring groups (some sort of with_groups()), which we could also export.

[hadley]

Somewhat more comprehensive reprex:

df <- structure(data.frame(x = 1:3), class = c("myclass", "data.frame"))
df %>% count() %>% class()
df %>% count(x) %>% class()
df %>% add_count() %>% class()
df %>% add_count(x) %>% class()