add fix for s4

Author: ben-schwen

inst/tests/S4.Rraw

@@ -126,3 +126,8 @@ setMethod(`%foo%`, c("Date", "CustomDurationClass"), function (e1, e2) e1 - e2@.
 test(8, as.IDate("2025-03-01") %foo% CustomDurationClass(1), as.IDate("2025-02-28"))
 removeGeneric("%foo%")
 removeClass("CustomDurationClass")
+
+# data.table(s4) #6874 should work
+s4cl = setClass("s4cl", slots = list(x="integer"))
+DT = setalloccol(structure(list(a = new("s4cl", x = 1L)), row.names = c(NA, -1L), class = c("data.table", "data.frame")))
+test(9, data.table(a=s4cl(x=1L)), DT)

src/assign.c

@@ -232,7 +232,7 @@ SEXP setdt_nrows(SEXP x)
       }
       len_xi = INTEGER(dim_xi)[0];
     } else {
-      len_xi = LENGTH(xi);
+      len_xi = length(xi);
     }
     if (!base_length) {
       base_length = len_xi;