[bugfix] add explicit cast

If the gcc implementation of std::variant is used instead of the fallback,
the compiler chokes on this without an explicit cast.

dune/subgrid/subgrid/subgridgeometry.hh

@@ -233,12 +233,12 @@ class SubGridLocalGeometry
 
         //! The Jacobian matrix of the mapping from the reference element to this element
         const JacobianTransposed jacobianTransposed (const FieldVector<ctype, mydim>& local) const {
-            return Std::visit([&](auto&& geom) {return geom.jacobianTransposed(local);}, localGeometry_);
+            return Std::visit([&](auto&& geom) {return static_cast<JacobianTransposed>(geom.jacobianTransposed(local));}, localGeometry_);
         }
 
         //! The inverse of the Jacobian matrix of the mapping from the reference element to this element
         const JacobianInverseTransposed jacobianInverseTransposed (const FieldVector<ctype, mydim>& local) const {
-            return Std::visit([&](auto&& geom) {return geom.jacobianInverseTransposed(local);}, localGeometry_);
+            return Std::visit([&](auto&& geom) {return static_cast<JacobianInverseTransposed>(geom.jacobianInverseTransposed(local));}, localGeometry_);
         }
 
     private: