key(as.data.table(x, key = <key>)) is inconsistent, varies based on x #6859

suppressPackageStartupMessages({
  library(data.table)
  library(dplyr)
})

# One probably expects key="t" to produce a key of "t":
data.frame(t = c(3:1,4:5), y = 1:5) %>% as.data.table(key = "t") %>% key()
#> [1] "t"
tibble(t = c(3:1,4:5), y = 1:5) %>% as.data.table(key = "t") %>% key()
#> NULL
data.table(t = c(3:1,4:5), y = 1:5) %>% as.data.table(key = "t") %>% key()
#> NULL

# One may also expect omitting `key =` arg to produce unkeyed output; however, 
# that's also not always the case.  The "proper"/expected output is probably
# more up for debate here than above, though.
data.table(t = c(3:1,4:5), key = "t") %>%
  as.data.table() %>%
  key()
#> [1] "t"

sessionInfo()
#> R version 4.4.2 (2024-10-31)
#> Platform: x86_64-pc-linux-gnu
#> Running under: openSUSE Tumbleweed
#> 
#> Matrix products: default
#> BLAS:   /home/fullname/files/sw2/R-4.4.2/lib/libRblas.so 
#> LAPACK: /home/fullname/files/sw2/R-4.4.2/lib/libRlapack.so;  LAPACK version 3.12.0
#> 
#> locale:
#>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C              
#>  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8    
#>  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
#>  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                 
#>  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
#> [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
#> 
#> time zone: America/Los_Angeles
#> tzcode source: system (glibc)
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] dplyr_1.1.4        data.table_1.17.99
#> 
#> loaded via a namespace (and not attached):
#>  [1] digest_0.6.37     R6_2.6.1          fastmap_1.2.0     tidyselect_1.2.1 
#>  [5] xfun_0.51         magrittr_2.0.3    glue_1.8.0        tibble_3.2.1     
#>  [9] knitr_1.49        pkgconfig_2.0.3   htmltools_0.5.8.1 generics_0.1.3   
#> [13] rmarkdown_2.29    lifecycle_1.0.4   cli_3.6.4         vctrs_0.6.5      
#> [17] reprex_2.1.1      withr_3.0.2       compiler_4.4.2    tools_4.4.2      
#> [21] pillar_1.10.1     evaluate_1.0.3    yaml_2.3.10       rlang_1.1.5      
#> [25] fs_1.6.5