Error on named tidyselections in `pivot_longer()` and `pivot_wider()`

[mgirlich]

As the cols argument supports the tidy-select syntax I thought I could use it to rename a column. But this didn't work as expected and when using a named vector in cols we get:

• a unclear error message when using a data frame
• a better error message when using a data frame
• a clearly unwanted result when the name actually exists as a column

I think it would be nice to allow renaming via cols

library(tidyr)

# unclear error message for data frame
data.frame(x = 1, y = 2) %>% 
  pivot_longer(c(z = y))
#> Error in `[.data.frame`(data, cols): undefined columns selected

# better for tibble
tibble(x = 1, y = 2) %>% 
  pivot_longer(c(z = y))
#> Error: Can't subset columns that don't exist.
#> x Column `z` doesn't exist.

# when the name actually exists we get something quite unwanted
data.frame(x = 1, y = 2, z = 3) %>% 
  pivot_longer(c(z = y))
#> # A tibble: 1 x 4
#>       x     y name  value
#>   <dbl> <dbl> <chr> <dbl>
#> 1     1     2 z         3

^{Created on 2021-03-08 by the reprex package (v1.0.0)}

[hadley]

Hmmmm, I'm not sure — I feel like that might be overloading pivot_longer() with slightly too much functionality.

[DavisVaughan]

In #1160 we discussed just setting allow_rename = FALSE in the tidyselect call to disable this early on, but #1160 (comment) reminds us of a tidyselect bug preventing this from working well

[DavisVaughan]

We are going to wait until we get a tidyselect release that will fix this, and then we can apply allow_rename = FALSE everywhere all at once. Needed in:

• pivot_longer() + spec helpers
• pivot_wider() + spec helpers