pmap_chr() on a 0-column tibble crashes 32-bit R 3.5.1

[wlandau]

Using 43c3eb3:

purrr::pmap_chr(tibble::tibble(x = 1)[, 0], paste)

sessionInfo()
#> R version 3.5.1 (2018-07-02)
#> Platform: i386-w64-mingw32/i386 (32-bit)
#> Running under: Windows 7 x64 (build 7601) Service Pack 1
#> 
#> 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 utils     datasets  methods   base     
#> 
#> loaded via a namespace (and not attached):
#>  [1] compiler_3.5.1  backports_1.1.2 magrittr_1.5    rprojroot_1.3-2
#>  [5] tools_3.5.1     htmltools_0.3.6 yaml_2.2.0      Rcpp_0.12.19   
#>  [9] stringi_1.2.4   rmarkdown_1.10  knitr_1.20      stringr_1.3.1  
#> [13] digest_0.6.18   evaluate_0.12

[wlandau]

I forgot to mention: for Windows, this error appears to happen on 32-bit R 3.5.1 and devel, but not on 64-bit R and not for R 3.4.4.

[lionel-]

Simpler reprex:

pmap(list(), identity)

[lionel-]

It seems that VECTOR_ELT(empty_list, 0) usually returns NULL but crashes on that platform. That's weird, I'd have thought it'd crash on all platforms.

[wlandau]

Just a guess, but I wonder if this could somehow be related to ALTREP. From Rinternals.h, VECTOR_ELT is just ((SEXP *) DATAPTR(x))[i], and DATAPTR now seems to rely on ALTREP functionality. This might explain why it crashes on R 3.5.1 but not 3.4.4. cc @gmbecker.