Change behavior of intersection: intersecting something with an empty object will be empty

src/CGALEvaluator.cc

@@ -61,6 +61,11 @@ void CGALEvaluator::process(CGAL_Nef_polyhedron &target, const CGAL_Nef_polyhedr
  	if (target.dim != 2 && target.dim != 3) {
  		assert(false && "Dimension of Nef polyhedron must be 2 or 3");
  	}
+    // Intersecting something with nothing results in nothing
+	if (src.isEmpty() && op == CGE_INTERSECTION) {
+        target = src;
+        return;
+    }
 	if (src.isEmpty()) return; // Empty polyhedron. This can happen for e.g. square([0,0])
 	if (target.isEmpty() && op != CGE_UNION) return; // empty op <something> => empty
 	if (target.dim != src.dim) return; // If someone tries to e.g. union 2d and 3d objects

testdata/scad/features/intersection-tests.scad

@@ -39,3 +39,22 @@ translate([-12,12,0]) intersection() {
   cube([10,10,10], center=true);
   translate([0,-9.99,-9.99]) cube([10,10,10], center=true);
 }
+
+// Intersecting something with nothing
+translate([0,-12,0]) intersection() {
+  cylinder(r=4, h=5, center=true);
+  cube(0);
+}
+
+// Intersecting 2D with 3D
+translate([12,-12,0]) intersection() {
+  cube([5,5,5], center=true);
+  circle(r=2);
+}
+
+// Non-geometry (echo) statement as first child should be ignored
+translate([24,-12,0]) intersection() {
+  echo("difference-tests");
+  cube([5,5,5], center=true);
+  cylinder(r=2, h=20, center=true);
+}

tests/regression/dumptest/intersection-tests-expected.csg

@@ -54,4 +54,23 @@ group() {
 			}
 		}
 	}
+	multmatrix([[1, 0, 0, 0], [0, 1, 0, -12], [0, 0, 1, 0], [0, 0, 0, 1]]) {
+		intersection() {
+			cylinder($fn = 0, $fa = 12, $fs = 2, h = 5, r1 = 4, r2 = 4, center = true);
+			cube(size = [0, 0, 0], center = false);
+		}
+	}
+	multmatrix([[1, 0, 0, 12], [0, 1, 0, -12], [0, 0, 1, 0], [0, 0, 0, 1]]) {
+		intersection() {
+			cube(size = [5, 5, 5], center = true);
+			circle($fn = 0, $fa = 12, $fs = 2, r = 2);
+		}
+	}
+	multmatrix([[1, 0, 0, 24], [0, 1, 0, -12], [0, 0, 1, 0], [0, 0, 0, 1]]) {
+		intersection() {
+			group();
+			cube(size = [5, 5, 5], center = true);
+			cylinder($fn = 0, $fa = 12, $fs = 2, h = 20, r1 = 2, r2 = 2, center = true);
+		}
+	}
 }