Currently all resource adaptors are templated on their upstream resource.

Transitioning toresource_ref this is superfluous, as we do not need to know the upstream type anymore.

However, we cannot simply remove the template argument, as that would break user code. So we need to first introduce an indirection using an alias:

template<class>structresource_adaptor_impl{};template<classUpstream>[[deprecated("Use resource_adaptor_ref instead")]]using resource_adaptor = resource_adaptor_impl<Upstream>;using resource_adaptor_ref = resource_adaptor_impl<int>;