Closes #1549. Fixed a case when fill=TRUE didn't work as expected.

README.md

@@ -115,6 +115,8 @@
 
   29. `last` function will now properly redirect method if `xts` is not installed or not attached on search path. Closes [#1560](https://github.com/Rdatatable/data.table/issues/1560).
 
+  30. `rbindlist` (and `rbind`) works as expected when `fill = TRUE` and the first element of input list doesn't have columns present in other elements of the list, [#1549](https://github.com/Rdatatable/data.table/issues/1549). Thanks to @alexkowa.
+
 #### 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.

inst/tests/tests.Rraw

@@ -7418,6 +7418,10 @@ if(!requireNamespace("chron", quietly = TRUE)){
     test(1602.4, class(as.chron.ITime(tt)), "times")
 }
 
+# fix for #1549
+d1 <- data.table(v1=1:2,x=x)
+d2 <- data.table(v1=3:4)
+test(1603.1, rbindlist(list(d2, d1), fill=TRUE), rbindlist(list(d1,d2), fill=TRUE)[c(3:4, 1:2)])
 
 ##########################
 

src/rbindlist.c

@@ -585,7 +585,7 @@ static void preprocess(SEXP l, Rboolean usenames, Rboolean fill, struct preproce
             } else {
                 // Fix for #705, check attributes and error if non-factor class and not identical
                 if (!data->is_factor[i] && 
-                    !R_compute_identical(thisClass, getAttrib(thiscol, R_ClassSymbol), 0)) {
+                    !R_compute_identical(thisClass, getAttrib(thiscol, R_ClassSymbol), 0) && !fill) {
                     error("Class attributes at column %d of input list at position %d does not match with column %d of input list at position %d. Coercion of objects of class 'factor' alone is handled internally by rbind/rbindlist at the moment.", i+1, j+1, i+1, data->first+1);
                 }
                 type = TYPEOF(thiscol);