Closes #1082. Fixed segfault in assignment by reference.

README.md

@@ -169,6 +169,8 @@
 
   45. Fixed another segfault in `melt.data.table` issue that was caught due to issue in Windows. Closes [#1059](https://github.com/Rdatatable/data.table/issues/1059). Thanks again to @ChristK for the minimal report. 
 
+  46. `DT[rows, newcol := NULL]` resulted in a segfault on the next assignment by reference. Closes [#1082](https://github.com/Rdatatable/data.table/issues/1082). Thanks to @stevenbagley for the MRE.
+
 #### NOTES
 
   1. Clearer explanation of what `duplicated()` does (borrowed from base). Thanks to @matthieugomez for pointing out. Closes [#872](https://github.com/Rdatatable/data.table/issues/872).

inst/tests/tests.Rraw

@@ -6196,6 +6196,13 @@ ans2 <- cbind(ans21, ans22[, -1L])
 setkey(setnames(setDT(ans2), names(ans1)), x)
 test(1501.8, ans1, ans2)
 
+# fix for #1082
+dt1 = data.table(x=rep(c("a","b","c"),each=3), y=c(1,3,6), v=1:9, key=c("x", "y"))
+dt2 = copy(dt1)
+test(1502.1, dt1["a", z := NULL], error="When deleting columns, i should not be provided")
+# this shouldn't segfault on 'dt1[...]'
+test(1502.2, dt1["a", z := 42L], dt2["a", z := 42L])
+
 ##########################
 
 

src/assign.c

@@ -367,6 +367,8 @@ SEXP assign(SEXP dt, SEXP rows, SEXP cols, SEXP newcolnames, SEXP values, SEXP v
                 if (newcolnum<0 || newcolnum>=length(newcolnames))
                     error("Internal logical error. length(newcolnames)=%d, length(names)=%d, coln=%d", length(newcolnames), length(names), coln);
                 if (isNull(thisvalue)) {
+                    // fix for #1082
+                    if (!isNull(rows)) error("When deleting columns, i should not be provided");
                     warning("Adding new column '%s' then assigning NULL (deleting it).",CHAR(STRING_ELT(newcolnames,newcolnum)));
                     continue;
                 }