eval() not working with := syntax

[GitHunter0]

library(data.table)
new_col <- "test_col"
DT <- data.table(x=1:3)

This works:

DT[, eval(new_col) := x][]

This also works:

DT[, ':=' ("test_col" = x)][]

But this throws "Error: unexpected '=' in "DT[, := (eval(new_col) =" :

DT[, `:=` (eval(new_col) = x)][]

R version 4.0.3 (2020-10-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19042)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252 
[2] LC_CTYPE=English_United States.1252   
[3] LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                          
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices
[4] utils     datasets  methods  
[7] base     

other attached packages:
[1] data.table_1.13.6

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.5          
 [2] compiler_4.0.3      
 [3] prettyunits_1.1.1   
 [4] remotes_2.2.0       
 [5] tools_4.0.3         
 [6] testthat_2.3.2      
 [7] digest_0.6.27       
 [8] pkgbuild_1.2.0      
 [9] pkgload_1.1.0       
[10] evaluate_0.14       
[11] jsonlite_1.7.0      
[12] memoise_1.1.0       
[13] lattice_0.20-41     
[14] rlang_0.4.10        
[15] Matrix_1.2-18       
[16] cli_2.2.0           
[17] rstudioapi_0.13     
[18] yaml_2.2.1          
[19] xfun_0.19           
[20] withr_2.3.0         
[21] knitr_1.30          
[22] desc_1.2.0          
[23] fs_1.5.0            
[24] devtools_2.3.1      
[25] rprojroot_2.0.2     
[26] grid_4.0.3          
[27] reticulate_1.16     
[28] glue_1.4.2          
[29] R6_2.5.0            
[30] processx_3.4.5      
[31] fansi_0.4.1         
[32] rmarkdown_2.6       
[33] librarian_1.7.2     
[34] sessioninfo_1.1.1   
[35] callr_3.4.3         
[36] magrittr_2.0.1      
[37] htmltools_0.5.0.9003
[38] ps_1.5.0            
[39] ellipsis_0.3.1      
[40] usethis_1.6.1       
[41] assertthat_0.2.1    
[42] crayon_1.3.4   

[MichaelChirico]

you can't set the names in a call like this. the issue here will be solved when #4304 is merged; closing here for now.

BTW the usual way would be

DT[, (new_col) := ...]