group-by.md
In dplyr: A Grammar of Data Manipulation

can't rename while partially `ungroup()`-ing (#6606)

Code
  ungroup(gdf, g2 = g)
Condition
  Error in `ungroup()`:
  ! Can't rename variables in this context.

select(group_by(.)) implicitly adds grouping variables (#170)

Code
  res <- mtcars %>% group_by(vs) %>% select(mpg)
Message
  Adding missing grouping variables: `vs`

group_by works with zero-row data frames (#486)

Code
  x <- select(dfg, a)
Message
  Adding missing grouping variables: `g`

group_by() and ungroup() give meaningful error messages

Code
  df <- tibble(x = 1, y = 2)
  (expect_error(df %>% group_by(unknown)))
Output
  <error/rlang_error>
  Error in `group_by()`:
  ! Must group by variables found in `.data`.
  x Column `unknown` is not found.
Code
  (expect_error(df %>% ungroup(x)))
Output
  <error/rlib_error_dots_nonempty>
  Error in `ungroup()`:
  ! `...` must be empty.
  x Problematic argument:
  * ..1 = x
  i Did you forget to name an argument?
Code
  (expect_error(df %>% group_by(x, y) %>% ungroup(z)))
Output
  <error/vctrs_error_subscript_oob>
  Error in `ungroup()`:
  ! Can't subset columns that don't exist.
  x Column `z` doesn't exist.
Code
  (expect_error(df %>% group_by(z = a + 1)))
Output
  <error/dplyr:::mutate_error>
  Error in `group_by()`:
  i In argument: `z = a + 1`.
  Caused by error:
  ! object 'a' not found