diff --git a/NEWS.md b/NEWS.md
index 7f2debd7..92be8da2 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # collapse 2.0.15
 
+* Some changes on the C-side to move the package closer to C API compliance (demanded by R-Core). One notable change is that `gsplit()` no longer supports S4 objects (because `SET_S4_OBJECT` is not part of the API and `asS4()` is too expensive for tight loops). I cannot think of a single example where it would be necessary to split an S4 object, but if you do have applications please file an issue. 
+
 * `pivot()` has new arguments `FUN = "last"` and `FUN.args = NULL`, allowing wide and recast pivots with aggregation (default last value as before). `FUN` currently supports a single function returning a scalar value. *Fast Statistical Functions* receive vectorized execution. `FUN.args` can be used to supply a list of function arguments, including data-length arguments such as weights. There are also a couple of internal functions callable using function strings: `"first"`, `"last"`, `"count"`, `"sum"`, `"mean"`, `"min"`, or `"max"`. These are built into the reshaping C-code and thus extremely fast. Thanks @AdrianAntico for the request (#582).
 
 * `join()` now provides enhanced verbosity, indicating the average order of the join between the two tables, e.g.
diff --git a/man/join.Rd b/man/join.Rd
index 4328200b..61c2ec97 100644
--- a/man/join.Rd
+++ b/man/join.Rd
@@ -29,7 +29,7 @@ join(x, y,
 
   \item{y}{a data frame-like object to join with \code{x}.}
 
-  \item{on}{character. vector of columns to join on. \code{NULL} uses \code{union(names(x), names(y))}. Use a named vector to match columns named differently in \code{x} and \code{y}, e.g. \code{c("x_id" = "y_id")}.}
+  \item{on}{character. vector of columns to join on. \code{NULL} uses \code{intersect(names(x), names(y))}. Use a named vector to match columns named differently in \code{x} and \code{y}, e.g. \code{c("x_id" = "y_id")}.}
 
   \item{how}{character. Join type: \code{"left"}, \code{"right"}, \code{"inner"}, \code{"full"}, \code{"semi"} or \code{"anti"}. The first letter suffices. }
 
diff --git a/src/base_radixsort.h b/src/base_radixsort.h
index 2d5283f3..b23e8d79 100644
--- a/src/base_radixsort.h
+++ b/src/base_radixsort.h
@@ -8,7 +8,7 @@
 // #define IS_ASCII(x) ((x)->sxpinfo.gp & ASCII_MASK)
 // #define IS_ASCII(x) (LEVELS(x) & ASCII_MASK)
 
-#define SEXPPTR(x) ((SEXP *)DATAPTR(x))  // Replacing STRING_PTR
+#define SEXPPTR(x) ((SEXP *)DATAPTR(x))
 
 // NOTE: All of this is copied from Defn.h: https://github.com/wch/r-source/blob/28de75af0541f93832c5899139b969d290bf422e/src/include/Defn.h
 // to avoid checking for ALTREP in TRUELENGTH, which slows down the code unnecessarily...
@@ -25,6 +25,8 @@
 #define MYLEV(x)	(((SEXPREC_partial *)(x))->sxpinfo.gp)
 #define IS_ASCII(x) (MYLEV(x) & 64) // from data.table.h
 
+#define SETTOF(x,v)	((((SEXPREC_partial *)(x))->sxpinfo.type)=(v))
+
 // NOTE: All of this is copied from Defn.h: https://github.com/wch/r-source/blob/28de75af0541f93832c5899139b969d290bf422e/src/include/Defn.h
 // to avoid checking for ALTREP in TRUELENGTH, which slows down the code unnecessarily...
 
diff --git a/src/data.table_subset.c b/src/data.table_subset.c
index 45d5e6fa..0645f52f 100644
--- a/src/data.table_subset.c
+++ b/src/data.table_subset.c
@@ -82,7 +82,7 @@ static SEXP shallow(SEXP dt, SEXP cols, R_len_t n)
   SEXP newdt = PROTECT(allocVector(VECSXP, n)); protecti++;   // to do, use growVector here?
   SET_ATTRIB(newdt, shallow_duplicate(ATTRIB(dt)));
   SET_OBJECT(newdt, OBJECT(dt));
-  IS_S4_OBJECT(dt) ? SET_S4_OBJECT(newdt) : UNSET_S4_OBJECT(newdt);  // To support S4 objects that include data.table
+  if(IS_S4_OBJECT(dt)) newdt = asS4(newdt, TRUE, 1);  // To support S4 objects that include data.table
   //SHALLOW_DUPLICATE_ATTRIB(newdt, dt);  // SHALLOW_DUPLICATE_ATTRIB would be a bit neater but is only available from R 3.3.0
 
   // TO DO: keepattr() would be faster, but can't because shallow isn't merely a shallow copy. It
diff --git a/src/data.table_utils.c b/src/data.table_utils.c
index 9d49c7df..0357051f 100644
--- a/src/data.table_utils.c
+++ b/src/data.table_utils.c
@@ -179,7 +179,7 @@ SEXP dt_na(SEXP x, SEXP cols, SEXP Rprop, SEXP Rcount) {
       }
     }
     if(count) {
-      SET_TYPEOF(ans, INTSXP);
+      SETTOF(ans, INTSXP);
     } else {
       // This computes the result
       if(prop < 1.0) {
diff --git a/src/flag.cpp b/src/flag.cpp
index 30e029a5..0cef506f 100644
--- a/src/flag.cpp
+++ b/src/flag.cpp
@@ -1,6 +1,9 @@
 #include <Rcpp.h>
 using namespace Rcpp;
 
+LogicalVector intToLogical(IntegerVector x) {
+  return LogicalVector(x.begin(), x.end());
+}
 
 // 7th version: Irregular time series and panels supported !
 template <int RTYPE>
@@ -569,8 +572,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
               while(i != np) outjp[i++] = ff;
               for( ; i != row; ++i) outjp[i] = column[i - np];
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else if(np<0) {
               IntegerVector outjp = no_init_vector(row);
               if(names) nam[pos] = "F" + nc[p] + "." + na[j];
@@ -578,8 +580,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
               while(i != st) outjp[--i] = ff;
               for( ; i--; ) outjp[i] = column[i - np];
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else {
               if(names) nam[pos] = na[j];
               out[pos] = x[j];
@@ -702,8 +703,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
                 }
               }
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else if(np<0) {
               IntegerVector outjp = no_init_vector(os);
               if(names) nam[pos] = "F" + nc[p] + "." + na[j];
@@ -715,8 +715,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
                 }
               }
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else {
               if(names) nam[pos] = na[j];
               out[pos] = x[j];
@@ -840,8 +839,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
                 }
               }
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else if(np<0) {
               IntegerVector outjp = no_init_vector(gss);
               if(names) nam[pos] = "F" + nc[p] + "." + na[j];
@@ -855,8 +853,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
                 }
               }
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else {
               if(names) nam[pos] = na[j];
               out[pos] = x[j];
@@ -1009,8 +1006,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
                 }
               }
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else if(np<0) {
               IntegerVector outjp = no_init_vector(gss);
               if(names) nam[pos] = "F" + nc[p] + "." + na[j];
@@ -1022,8 +1018,7 @@ List flagleadlCpp(const List& x, const IntegerVector& n = 1, const SEXP& fill =
                 }
               }
               SHALLOW_DUPLICATE_ATTRIB(outjp, column);
-              if(txj == LGLSXP) SET_TYPEOF(outjp, LGLSXP);
-              out[pos] = outjp;
+              out[pos] = txj == LGLSXP ? intToLogical(outjp) : outjp;
             } else {
               if(names) nam[pos] = na[j];
               out[pos] = x[j];
diff --git a/src/gsplit.c b/src/gsplit.c
index 41041e5f..6c8d5c5f 100644
--- a/src/gsplit.c
+++ b/src/gsplit.c
@@ -24,12 +24,12 @@ SEXP gsplit(SEXP x, SEXP gobj, SEXP toint) {
     int ox = OBJECT(x);
     // FAZIT: Need to use SET_VECTOR_ELT!! pres[i] = allocVector() doesn't work!!
     if(TYPEOF(ax) != NILSXP && ox != 0) {
-      for(int i = 0, s4o = IS_S4_OBJECT(x); i != ng; ++i) {
+      for(int i = 0; i != ng; ++i) { // , s4o = IS_S4_OBJECT(x)
         SEXP resi;
         SET_VECTOR_ELT(res, i, resi = allocVector(tx, pgs[i]));
         SET_ATTRIB(resi, ax);
         SET_OBJECT(resi, ox);
-        if(s4o) SET_S4_OBJECT(resi);
+        // if(s4o) SET_S4_OBJECT(resi);
       }
     } else if(TYPEOF(ax) != NILSXP) {
       for(int i = 0; i != ng; ++i) {
@@ -38,11 +38,11 @@ SEXP gsplit(SEXP x, SEXP gobj, SEXP toint) {
         SET_ATTRIB(resi, ax);
       }
     } else if(ox != 0) { // Is this even possible? Object bits but no attributes?
-      for(int i = 0, s4o = IS_S4_OBJECT(x); i != ng; ++i) {
+      for(int i = 0; i != ng; ++i) { // , s4o = IS_S4_OBJECT(x)
         SEXP resi;
         SET_VECTOR_ELT(res, i, resi = allocVector(tx, pgs[i]));
         SET_OBJECT(resi, ox);
-        if(s4o) SET_S4_OBJECT(resi);
+        // if(s4o) SET_S4_OBJECT(resi);
       }
     } else {
       for(int i = 0; i != ng; ++i) SET_VECTOR_ELT(res, i, allocVector(tx, pgs[i]));
diff --git a/src/kit.h b/src/kit.h
index cf4e4103..1de2c883 100644
--- a/src/kit.h
+++ b/src/kit.h
@@ -55,7 +55,7 @@
 #define C_ISNAN(x, y) (B_ISNAN(x, y) || (N_ISNAN(x, y) && x == y))
 #define REQUAL(x, y)  (N_ISNAN(x, y) ? (x == y) : (B_IsNA(x, y) || B_IsNaN(x, y)))
 #define CEQUAL(x, y) ((N_ISNAN(x.r, x.i) && N_ISNAN(y.r, y.i)) ? (x.r == y.r && x.i == y.i) : (C_IsNA(x) ? C_IsNA(y) : (C_IsNA(y) ? 0 : (C_ISNAN(x.r, y.r) && C_ISNAN(x.i, y.i)))))
-#define SEXPPTR(x) ((SEXP *)DATAPTR(x))  // To replace STRING_PTR
+#define SEXPPTR(x) ((SEXP *)DATAPTR(x))
 // #define STR_DF mkString("data.frame")
 // #define MAX(a,b) (((a)>(b))?(a):(b))
 // #define IS_LOGICAL(x) (isLogical(x) && LENGTH(x)==1)
diff --git a/src/programming.c b/src/programming.c
index 8b88229f..b1152018 100644
--- a/src/programming.c
+++ b/src/programming.c
@@ -1,4 +1,5 @@
 #include "collapse_c.h"
+#include "base_radixsort.h"
 
 SEXP Cna_rm(SEXP x) {
   const int n = LENGTH(x);
@@ -894,20 +895,20 @@ SEXP vtypes(SEXP x, SEXP isnum) {
         pans[i] = is_num;
       }
     }
-    SET_TYPEOF(ans, LGLSXP);
+    SETTOF(ans, LGLSXP);
     break;
     }
   case 2: // is.factor
     for(int i = 0; i != n; ++i) pans[i] = (int)isFactor(px[i]);
-    SET_TYPEOF(ans, LGLSXP);
+    SETTOF(ans, LGLSXP);
     break;
   case 3: // is.list, needed for list processing functions
     for(int i = 0; i != n; ++i) pans[i] = TYPEOF(px[i]) == VECSXP;
-    SET_TYPEOF(ans, LGLSXP);
+    SETTOF(ans, LGLSXP);
     break;
   case 4: // is.sublist, needed for list processing functions
     for(int i = 0; i != n; ++i) pans[i] = TYPEOF(px[i]) == VECSXP && !isFrame(px[i]);
-    SET_TYPEOF(ans, LGLSXP);
+    SETTOF(ans, LGLSXP);
     break;
   case 7: // is.atomic(x), needed in atomic_elem()
     // is.atomic: do_is with op = 200:  https://github.com/wch/r-source/blob/9f9033e193071f256e21a181cb053cba983ed4a9/src/main/coerce.c
@@ -927,7 +928,7 @@ SEXP vtypes(SEXP x, SEXP isnum) {
         pans[i] = 0;
       }
     }
-    SET_TYPEOF(ans, LGLSXP);
+    SETTOF(ans, LGLSXP);
     break;
   case 5: // is.atomic(x) || is.list(x), needed in reg_elem() and irreg_elem()
     for(int i = 0; i != n; ++i) {
@@ -949,7 +950,7 @@ SEXP vtypes(SEXP x, SEXP isnum) {
         pans[i] = 0;
       }
     }
-    SET_TYPEOF(ans, LGLSXP);
+    SETTOF(ans, LGLSXP);
     break;
   case 6:
     // Faster object type identification, needed in unlist2d: