Confusing error message when "expand" hits grouped data

[bholtdwyer]

When you apply "expand" to data that has been grouped, a somewhat confusing error message suggests incorrectly that the variable being expanded on does not exist. A more helpful error message would say "you cannot expand on a variable by which the data is grouped. Run 'groups()' on your tibble to find variable groupings, or apply ungroup() first".

> bob = tibble(x=c(1,2,3), y=c("abe","bob","abe")) 
> bob = bob %>% group_by(x) 
> expand(bob, x, y) 
Error in `reframe()`: 
ℹ In argument: `expand(data = pick(everything()), ..., .name_repair = .name_repair)`. 
ℹ In group 1: `x = 1`. 
Caused by error: ! object 'x' not found 
Run `rlang::last_trace()` to see where the error occurred.
>expand(ungroup(bob),x,y)

[bholtdwyer]

This also happens when you try to complete data that have been grouped:

> ## Example for bug report ###############################################
> df <- tibble(
+   group = c(1:2, 1, 2),
+   item_id = c(1:2, 2, 3),
+   item_name = c("a", "a", "b", "b"),
+   value1 = c(1, NA, 3, 4),
+   value2 = 4:7
+ )
> df = df %>% group_by(group)
> df %>% complete(item_id, item_name)
# A tibble: 8 × 5
# Groups:   group [2]
  group item_id item_name value1 value2
  <dbl>   <dbl> <chr>      <dbl>  <int>
1     1       1 a              1      4
2     1       1 b             NA     NA
3     1       2 a             NA     NA
4     1       2 b              3      6
5     2       2 a             NA      5
6     2       2 b             NA     NA
7     2       3 a             NA     NA
8     2       3 b              4      7
> df = df %>% group_by(item_id, item_name)
> df %>% complete(item_id, item_name)
Error in `reframe()`:
ℹ In argument: `complete(data = pick(everything()), ..., fill = fill, explicit =
  explicit)`.
ℹ In group 1: `item_id = 1`, `item_name = "a"`.
Caused by error:
! object 'item_id' not found
Run `rlang::last_trace()` to see where the error occurred.
> df <- tibble(
+   group = c(1:2, 1, 2),
+   item_id = c(1:2, 2, 3),
+   item_name = c("a", "a", "b", "b"),
+   value1 = c(1, NA, 3, 4),
+   value2 = 4:7
+ )
> df = df %>% group_by(item_id, item_name)
> 
> df %>% complete(item_id, item_name)
Error in `reframe()`:
ℹ In argument: `complete(data = pick(everything()), ..., fill = fill, explicit =
  explicit)`.
ℹ In group 1: `item_id = 1`, `item_name = "a"`.
Caused by error:
! object 'item_id' not found
Run `rlang::last_trace()` to see where the error occurred.
> df %>% ungroup() %>% complete(item_id, item_name)
# A tibble: 6 × 5
  item_id item_name group value1 value2
    <dbl> <chr>     <dbl>  <dbl>  <int>
1       1 a             1      1      4
2       1 b            NA     NA     NA
3       2 a             2     NA      5
4       2 b             1      3      6
5       3 a            NA     NA     NA
6       3 b             2      4      7

[hadley]

Could you please rework your reproducible example to use the reprex package ? That makes it easier to see both the input and the output, formatted in such a way that I can easily re-run in a local session.