multibody/joint: add BlockReturn to SizeDepType helper

src/multibody/joint/joint-base.hpp

@@ -154,18 +154,27 @@ struct CastType< NewScalar, JointModelTpl<Scalar,Options> > \
       typedef typename Mat::template FixedSegmentReturnType<NV>::Type Type;
       typedef typename Mat::template ConstFixedSegmentReturnType<NV>::Type ConstType;
     };
+    
     template<class Mat>
     struct ColsReturn
     {
       typedef typename Mat::template NColsBlockXpr<NV>::Type Type;
       typedef typename Mat::template ConstNColsBlockXpr<NV>::Type ConstType;
     };
+    
     template<class Mat>
     struct RowsReturn
     {
       typedef typename Mat::template NRowsBlockXpr<NV>::Type Type;
       typedef typename Mat::template ConstNRowsBlockXpr<NV>::Type ConstType;
     };
+    
+    template<class Mat>
+    struct BlockReturn
+    {
+      typedef Eigen::Block<Mat, NV, NV> Type;
+      typedef const Eigen::Block<const Mat, NV, NV> ConstType;
+    };
   };
   
   template<>
@@ -177,18 +186,27 @@ struct CastType< NewScalar, JointModelTpl<Scalar,Options> > \
       typedef typename Mat::SegmentReturnType Type;
       typedef typename Mat::ConstSegmentReturnType ConstType;
     };
+    
     template<class Mat>
     struct ColsReturn
     {
       typedef typename Mat::ColsBlockXpr Type;
       typedef typename Mat::ConstColsBlockXpr ConstType;
     };
+    
     template<class Mat>
     struct RowsReturn
     {
       typedef typename Mat::RowsBlockXpr Type;
       typedef typename Mat::ConstRowsBlockXpr ConstType;
     };
+    
+    template<class Mat>
+    struct BlockReturn
+    {
+      typedef Eigen::Block<Mat> Type;
+      typedef const Eigen::Block<const Mat> ConstType;
+    };
   };
 
   template<typename Derived>