Closes #1672, chmatch handles length=0 values in nomatch arg correctly.

NEWS.md

@@ -189,6 +189,8 @@
 
   53. `fread` won't use `wget` for file:// input, [#1668](https://github.com/Rdatatable/data.table/issues/1668); thanks @MichaelChirico for FR&PR.
 
+  54. `chmatch()` handles `nomatch = integer(0)` properly, [#1672](https://github.com/Rdatatable/data.table/issues/1672).
+
 #### NOTES
 
   1. Updated error message on invalid joins to reflect the new `on=` syntax, [#1368](https://github.com/Rdatatable/data.table/issues/1368). Thanks @MichaelChirico.

R/data.table.R

@@ -2458,7 +2458,7 @@ set <- function(x,i=NULL,j,value)  # low overhead, loopable
 }
 
 chmatch <- function(x,table,nomatch=NA_integer_)
-    .Call(Cchmatchwrapper,x,table,as.integer(nomatch),FALSE)
+    .Call(Cchmatchwrapper,x,table,as.integer(nomatch[1L]),FALSE) # [1L] to fix #1672
 
 "%chin%" <- function(x,table) {
     # TO DO  if table has 'ul' then match to that

inst/tests/tests.Rraw

@@ -8809,6 +8809,9 @@ test(1667.1, capture.output(dcast(DT, id ~ rowid(id), value.var = "V1")),
 DT <- data.table(a = 1:4, id = 1:4, id = rep(1:4, 2), V1 = 8:1)
 test(1667.2, dcast(DT, id ~ rowid(id), value.var = "V1"), error = "data.table to cast")
 
+# fix for #1672
+test(1668, chmatch(c("a","b"), c("a","c"), nomatch = integer()), c(1L, NA_integer_))
+
 ##########################
 
 # TODO: Tests involving GForce functions needs to be run with optimisation level 1 and 2, so that both functions are tested all the time.