Mandates: For each type D in Domains...,
the expression
D().transform_sender(Tag(), std::forward<Sndr>(sndr), env)
is either ill-formed or has the same decayed type as
default_domain().transform_sender(Tag(), std::forward<Sndr>(sndr), env).
Remarks: For a pack of types Ds,
common_type_t<indeterminate_domain<Domains...>, indeterminate_domain<Ds...>>
is indeterminate_domain<Us...>
where Us is a pack that contains each type in
Domains..., Ds... except with duplicate types removed.
For a type D that is not a specialization of indeterminate_domain,
common_type_t<indeterminate_domain<Domains...>, D>
is D if Domains is an empty pack, and
common_type_t<indeterminate_domain<Domains...>, indeterminate_domain<D>>
otherwise.