setDT should check if any column is POSIXlt

Start with the minimal POSIXlt example and the setDT(x) entry point, then compare its behavior with as.data.table.list(), which already emits a conversion warning. Determine the intended handling for POSIXlt columns and add coverage showing that setDT() no longer leaves an object that fails in head(); the issue does not name specific files or tests.

I was doing setDT() of a dataframe (or tibble) with a variable POSIXlt. It seems not to be any problem. Even I can do str() of that object. It does not show anything strange. Then errors started to appear when I tried to manipulate the object or simply do head().

As I have found the problem, I would ask/suggest why do not produce the error when running setDT or, otherwise show a warning?, which would tell about the problem that there are POSIXlt variables and they are not supported. Possibly these variables could be removed or coerced to POSIXct or another better format for data.table and the warning would tell about that.

Update: Minimal example with session info:

> library(tibble)
> library(data.table)
> now <- as.POSIXlt(Sys.time())
> x <- as.data.frame(tibble(now))
> mdt <- data.table(id=1:3, d=strptime(c("06:02:36", "06:02:48", "07:03:12"), "%H:%M:%S"))
Warning message:
In as.data.table.list(x, keep.rownames = keep.rownames, check.names = check.names,  :
  POSIXlt column type detected and converted to POSIXct. We do not recommend use of POSIXlt at all because it uses 40 bytes to store one date.
> # previous is fine; but next...
> setDT(x)
> head(x)
Error in dimnames(x) <- dn : 
  length of 'dimnames' [1] not equal to array extent
> str(x)
Classes ‘data.table’ and 'data.frame':	11 obs. of  1 variable:
 $ now: POSIXlt, format: "2020-11-04 10:24:47"
 - attr(*, ".internal.selfref")=<externalptr> 
> sessionInfo()
R version 4.0.3 (2020-10-10)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.7 LTS

Matrix products: default
BLAS:   /usr/lib/atlas-base/atlas/libblas.so.3.0
LAPACK: /usr/lib/atlas-base/atlas/liblapack.so.3.0

locale:
 [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8        LC_COLLATE=C.UTF-8    
 [5] LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8    LC_PAPER=C.UTF-8       LC_NAME=C             
 [9] LC_ADDRESS=C           LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   

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

other attached packages:
[1] data.table_1.13.0 tibble_3.0.3     

loaded via a namespace (and not attached):
 [1] Rcpp_1.0.5          rstudioapi_0.11     magrittr_1.5.0.9000 tidyselect_1.1.0    munsell_0.5.0      
 [6] colorspace_1.4-1    R6_2.4.1            rlang_0.4.7         fansi_0.4.1         plyr_1.8.6         
[11] dplyr_1.0.2         tools_4.0.3         grid_4.0.3          gtable_0.3.0        cli_2.0.2          
[16] ellipsis_0.3.1      assertthat_0.2.1    lifecycle_0.2.0     crayon_1.3.4        purrr_0.3.4        
[21] ggplot2_3.3.2       vctrs_0.3.4         glue_1.4.2          compiler_4.0.3      pillar_1.4.6       
[26] generics_0.0.2      scales_1.1.1        pkgconfig_2.0.3    

Any case, thank you for this great package.