gather() should possibly generate warnings or errors when key and value columns overlap with column in input

[JohnMount]

It seems like some of the examples below should generate warnings or errors.

df <- data.frame(
  x = 1:3, 
  y = 4:6, 
  choice = c("x", "y", "x"), 
  stringsAsFactors = FALSE)
  
tidyr::gather(df, "k", "v")


    ##        k v
    ## 1      x 1
    ## 2      x 2
    ## 3      x 3
    ## 4      y 4
    ## 5      y 5
    ## 6      y 6
    ## 7 choice x
    ## 8 choice y
    ## 9 choice x


tidyr::gather(df, "k", "x")


    ##        k x
    ## 1      y 4
    ## 2      y 5
    ## 3      y 6
    ## 4 choice x
    ## 5 choice y
    ## 6 choice x


tidyr::gather(df, "x", "v")


    ##        x v
    ## 1      y 4
    ## 2      y 5
    ## 3      y 6
    ## 4 choice x
    ## 5 choice y
    ## 6 choice x


tidyr::gather(df, "x", "y")


    ##        x y
    ## 1 choice x
    ## 2 choice y
    ## 3 choice x


tidyr::gather(df, "x", "x")


    ##        x x
    ## 1      y 4
    ## 2      y 5
    ## 3      y 6
    ## 4 choice x
    ## 5 choice y
    ## 6 choice x

[hadley]

More minimal reprex:

``` r
library(tidyr)
df <- tibble::tibble(
  x = 1:2, 
  y = 4:5, 
  choice = c("x", "y"), 
)
  
gather(df, "k", "v")
#> # A tibble: 6 x 2
#>   k      v    
#>   <chr>  <chr>
#> 1 x      1    
#> 2 x      2    
#> 3 y      4    
#> 4 y      5    
#> 5 choice x    
#> 6 choice y
gather(df, "k", "x")
#> # A tibble: 4 x 2
#>   k      x    
#>   <chr>  <chr>
#> 1 y      4    
#> 2 y      5    
#> 3 choice x    
#> 4 choice y
gather(df, "x", "v")
#> # A tibble: 4 x 2
#>   x      v    
#>   <chr>  <chr>
#> 1 y      4    
#> 2 y      5    
#> 3 choice x    
#> 4 choice y
gather(df, "x", "y")
#> # A tibble: 2 x 2
#>   x      y    
#>   <chr>  <chr>
#> 1 choice x    
#> 2 choice y
gather(df, "x", "x")
#> Error: Column name `x` must not be duplicated.
#> Use .name_repair to specify repair.

^{Created on 2019-01-04 by the reprex package (v0.2.1)}

[hadley]

Need to resolve this problem before can check other results:

library(dplyr, warn.conflicts = FALSE)
library(tidyr)

df <- tibble::tibble(
  x = c(1, 2), 
  y = c(10, 20), 
  z = c(100, 200), 
)

# Values are incorrect
df %>% pivot(pivot_spec_long(df, everything()))
#> # A tibble: 6 x 2
#>   variable value
#>   <chr>    <dbl>
#> 1 x            1
#> 2 y            2
#> 3 z           10
#> 4 x           20
#> 5 y          100
#> 6 z          200

^{Created on 2019-03-03 by the reprex package (v0.2.1.9000)}

[hadley]

These now all appear to return reasonable results

library(tidyr)
df <- tibble::tibble(
  x = 1:2, 
  y = 4:5, 
  choice = 6:7, 
)
  
# gather(df, "k", "v")
pivot(df, pivot_spec_long(df, 1:3, "v", "k"))
#> # A tibble: 6 x 2
#>   k          v
#>   <chr>  <int>
#> 1 x          1
#> 2 y          2
#> 3 choice     4
#> 4 x          5
#> 5 y          6
#> 6 choice     7

# gather(df, "k", "x")
pivot(df, pivot_spec_long(df, 1:3, "x", "k"))
#> # A tibble: 6 x 2
#>   k          x
#>   <chr>  <int>
#> 1 x          1
#> 2 y          2
#> 3 choice     4
#> 4 x          5
#> 5 y          6
#> 6 choice     7

# gather(df, "x", "v")
pivot(df, pivot_spec_long(df, 1:3, "v", "x"))
#> # A tibble: 6 x 2
#>   x          v
#>   <chr>  <int>
#> 1 x          1
#> 2 y          2
#> 3 choice     4
#> 4 x          5
#> 5 y          6
#> 6 choice     7

# gather(df, "x", "y")
pivot(df, pivot_spec_long(df, 1:3, "y", "x"))
#> # A tibble: 6 x 2
#>   x          y
#>   <chr>  <int>
#> 1 x          1
#> 2 y          2
#> 3 choice     4
#> 4 x          5
#> 5 y          6
#> 6 choice     7

# gather(df, "x", "x")
pivot(df, pivot_spec_long(df, 1:3, "x", "x"))
#> New names:
#> x -> x..1
#> x -> x..2
#> # A tibble: 6 x 2
#>   x..1    x..2
#>   <chr>  <int>
#> 1 x          1
#> 2 y          2
#> 3 choice     4
#> 4 x          5
#> 5 y          6
#> 6 choice     7

^{Created on 2019-03-03 by the reprex package (v0.2.1.9000)}