diff --git a/src/libslic3r/Geometry/ConvexHull.hpp b/src/libslic3r/Geometry/ConvexHull.hpp
index 9ba957824..9e9088f1e 100644
--- a/src/libslic3r/Geometry/ConvexHull.hpp
+++ b/src/libslic3r/Geometry/ConvexHull.hpp
@@ -4,6 +4,10 @@
 #include "../Polygon.hpp"
 
 namespace Slic3r {
+
+class ExPolygon;
+using ExPolygons = std::vector<ExPolygon>;
+
 namespace Geometry {
 
 Pointf3s convex_hull(Pointf3s points);