the patch adds the &lt;iostream> include to `elasticity_upscale.hpp`. it can't be included here, because `elasticity_upscale.hpp` includes ``elasticity_upscale_impl.hpp` from within the Opm::Elasticity namespace which would result that everything declared in &lt;iostream>  would end up in Opm::Elasticity::std if the guard macro would not kick in because of a previous inclusion of iostream (which it does and thus the whole stuff works).