`group_by()` + `transmute()` has multiple issues

[mgirlich]

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

df <- tibble(x = 1) %>% group_by(x)

# grouping variable cannot be selected
df %>% 
  lazy_dt() %>% 
  transmute(y = 2) %>% 
  .$vars
#> [1] "y"

df %>% 
  transmute(y = 2)
#> # A tibble: 1 x 2
#> # Groups:   x [1]
#>       x     y
#>   <dbl> <dbl>
#> 1     1     2

# transmuting a grouping variable 
df %>% 
  lazy_dt() %>% 
  transmute(x = x + 1, y = 2)
#> Source: local data table [1 x 3]
#> Groups: x
#> Call:   `_DT2`[, .(x = x + 1, y = 2), keyby = .(x)]
#> 
#>       x     x     y
#>   <dbl> <dbl> <dbl>
#> 1     1     2     2
#> 
#> # Use as.data.table()/as.data.frame()/as_tibble() to access results

df %>% 
  transmute(x = x + 1, y = 2)
#> # A tibble: 1 x 2
#> # Groups:   x [1]
#>       x     y
#>   <dbl> <dbl>
#> 1     2     2

# mutating works correctly
df %>% 
  lazy_dt() %>% 
  mutate(x = x + 1, y = 2)
#> Source: local data table [1 x 2]
#> Groups: x
#> Call:   copy(`_DT3`)[, `:=`(x = x + 1, y = 2), by = .(x)]
#> 
#>       x     y
#>   <dbl> <dbl>
#> 1     2     2
#> 
#> # Use as.data.table()/as.data.frame()/as_tibble() to access results

df %>% 
  mutate(x = x + 1, y = 2)
#> # A tibble: 1 x 2
#> # Groups:   x [1]
#>       x     y
#>   <dbl> <dbl>
#> 1     2     2

# grouping variable cannot be renamed
lazy_dt(tibble(x = 1, y = 2)) %>% 
  group_by(z = x) %>% 
  summarise(y = mean(y)) %>% 
  show_query()
#> `_DT4`[, .(y = mean(y)), keyby = .(z)]

tibble(x = 1, y = 2) %>% 
  group_by(z = x) %>% 
  summarise(y = mean(y))
#> # A tibble: 1 x 2
#>       z     y
#>   <dbl> <dbl>
#> 1     1     2

^{Created on 2021-05-19 by the reprex package (v2.0.0)}