Let transformed-sndr be the expression
dom.transform_sender(std::forward<Sndr>(sndr), env...)
if that expression is well-formed; otherwise,
default_domain().transform_sender(std::forward<Sndr>(sndr), env...)
Let final-sndr be the expression transformed-sndr
if transformed-sndr and sndr
have the same type ignoring cv-qualifiers;
otherwise, it is
the expression transform_sender(dom, transformed-sndr, env...).