Make FlatRegionVisitor take FlatRegions only.

src/main/java/com/sk89q/worldedit/EditSession.java

@@ -1162,7 +1162,7 @@ public class EditSession {
         RegionOffset offset = new RegionOffset(new Vector(0, 1, 0), replace);
         GroundSearch search = new MaskingGroundSearch(this, new ExistingBlockMask());
         GroundFunction groundFunction = new GroundFunction(search, lowerY, upperY, offset);
-        FlatRegionVisitor operation = new FlatRegionVisitor(region, groundFunction);
+        FlatRegionVisitor operation = new FlatRegionVisitor(Regions.asFlatRegion(region), groundFunction);
         OperationHelper.completeLegacy(operation);
 
         return operation.getAffected();
@@ -1966,7 +1966,7 @@ public class EditSession {
         FlatRegionMaskingFilter filter = new FlatRegionMaskingFilter(this, mask, groundFunction);
 
         // Generate those patches!
-        FlatRegionVisitor operation = new FlatRegionVisitor(region, filter);
+        FlatRegionVisitor operation = new FlatRegionVisitor(Regions.asFlatRegion(region), filter);
         OperationHelper.completeLegacy(operation);
 
         return operation.getAffected();

src/main/java/com/sk89q/worldedit/commands/RegionCommands.java

@@ -561,7 +561,7 @@ public class RegionCommands {
         FlatRegionMaskingFilter filter = new FlatRegionMaskingFilter(editSession, mask, groundFunction);
 
         // Execute
-        FlatRegionVisitor operation = new FlatRegionVisitor(region, filter);
+        FlatRegionVisitor operation = new FlatRegionVisitor(Regions.asFlatRegion(region), filter);
         OperationHelper.complete(operation);
 
         player.print(operation.getAffected() + " trees created.");
@@ -591,7 +591,7 @@ public class RegionCommands {
         FlatRegionMaskingFilter filter = new FlatRegionMaskingFilter(editSession, mask, groundFunction);
 
         // Execute
-        FlatRegionVisitor operation = new FlatRegionVisitor(region, filter);
+        FlatRegionVisitor operation = new FlatRegionVisitor(Regions.asFlatRegion(region), filter);
         OperationHelper.complete(operation);
 
         player.print(operation.getAffected() + " flora created.");

src/main/java/com/sk89q/worldedit/function/visitor/FlatRegionVisitor.java

@@ -23,12 +23,12 @@ import com.sk89q.worldedit.Vector2D;
 import com.sk89q.worldedit.WorldEditException;
 import com.sk89q.worldedit.function.FlatRegionFunction;
 import com.sk89q.worldedit.function.operation.Operation;
-import com.sk89q.worldedit.regions.CuboidRegion;
 import com.sk89q.worldedit.regions.FlatRegion;
-import com.sk89q.worldedit.regions.Region;
+
+import static com.google.common.base.Preconditions.checkNotNull;
 
 /**
- * Utility class to apply region functions to {@link com.sk89q.worldedit.regions.Region}.
+ * Applies region functions to columns in a {@link FlatRegion}.
  */
 public class FlatRegionVisitor implements Operation {
 
@@ -36,14 +36,18 @@ public class FlatRegionVisitor implements Operation {
     private final FlatRegionFunction function;
     private int affected = 0;
 
-    public FlatRegionVisitor(Region region, FlatRegionFunction function) {
+    /**
-        this.function = function;
+     * Create a new visitor.
+     *
+     * @param flatRegion a flat region
+     * @param function a function to apply to columns
+     */
+    public FlatRegionVisitor(FlatRegion flatRegion, FlatRegionFunction function) {
+        checkNotNull(flatRegion);
+        checkNotNull(function);
 
-        if (region instanceof FlatRegion) {
+        this.flatRegion = flatRegion;
-            flatRegion = (FlatRegion) region;
+        this.function = function;
-        } else {
-            flatRegion = CuboidRegion.makeCuboid(region);
-        }
     }
 
     /**