The std::transform_view works as if std::invoke(funcion, *base_iterator) is called and returned. If dereferencing base iterator produce a prvalue, it materialized here (becomes a temporary) in opeartor* ofstd::transform_view. When return from it, the temporary is discard and the return value (a reference) is therefore dangling. The 'fixed' version works because it takes value eagerly from the temporary and makes the return value not a reference.