Latest master doesn't segfault:

Error in dcast.data.table(d, formula = id ~ x, fun.aggregate = agg, value.var = "v") :
STRING_ELT() can only be applied to a 'character vector', not a 'logical'

It looks like this might be an improvement from base R though

Yep, seems to be that recent R itself errors more gracefully; 3.1.0 still segfaults

docker run -it jangorecki/r-3.1.0
install.packages('data.table', type = 'source', repos = 'http://Rdatatable.github.io/data.table')
library(data.table)
agg <- function(x) if(length(x) > 0) min(x) else NA
d <- data.table(id = c(1,1,2,2), x = c('y','y','y','z'), v = c('a','b','c','d'))
dcast(d, formula = id ~ x, fun.aggregate = agg, value.var = 'v')

 *** caught segfault ***
address 0x80000000, cause 'memory not mapped'

Traceback:
 1: dcast.data.table(d, formula = id ~ x, fun.aggregate = agg, value.var = "v")
 2: dcast(d, formula = id ~ x, fun.aggregate = agg, value.var = "v")

So, not sure if we should try and build a patch to this while we still depend on 3.1.0, or slap a wontfix on it, or bring forward our R dependency (I haven't traced which version of R fixed this)

No point in raising dep. If someone is affected and then can upgrade R. Otherwise we kill all non affected environments running older R.

i.e. wontfix then?

we could eventually improve error message to mention that, now it is not very useful

I came across a related problem having to do with NA vs NA_real_. It doesn't generate a Segfault or other error, but it does put a garbage number where NA should appear. Nearly the same example as above, but with numeric value.var instead of character:

library(data.table)
agg <- function(x) if(length(x) > 0) min(x) else NA
d <- data.table(id = c(1,1,2,2), x = c('y','y','y','z'), v = (1:4)/10)
dcast(d, formula = id ~ x, fun.aggregate = agg, value.var = 'v')

   id   y             z
1:  1 0.1 6.927149e-310
2:  2 0.3  4.000000e-01

Note that if I set fill = NA, I get the result I expect, and that is my current workaround.

dcast(d, formula = id ~ x, fun.aggregate = agg, value.var = 'v', fill=NA)

   id   y   z
1:  1 0.1  NA
2:  2 0.3 0.4

> sessionInfo()
R version 3.6.1 (2019-07-05)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS Mojave 10.14.5

...
other attached packages:
[1] data.table_1.12.8
...

@bpolacco can you try running this on current master? i'm actually not getting any error

Not sure how (it's a big commit) but this commit fixed this issue as far as I can tell; I'll add a test & file to close:

4aadde8

@MichaelChirico Yes, latest master doesn't break for me. Sorry for not trying that before commenting. To my naive eyes, I'd say the changed if statement in file fcast.c line 27 (was 29) is the 'how' of the fix. Thanks!

I ran this test in v1.12.8 where it failed as described, but returns correct result in v1.13.0. So yes it does look like 4aadde8 in v1.13.0 fixed it.
I'll modify the news item in @MichaelChirico's #4251 to include this detail and merge.