just, just_error, and just_stopped are sender factories
whose asynchronous operations complete synchronously in their start operation
with a value completion operation,
an error completion operation, or
a stopped completion operation, respectively.
just-cpo is just_stopped and
sizeof.(ts)==0 is false.
Otherwise, it is expression-equivalent to
make-sender(just-cpo, product-type{ts..}).
For just, just_error, and just_stopped,
let set-cpo be
set_value, set_error, and set_stopped, respectively.
The exposition-only class template impls-for ([exec.snd.expos])
is specialized for just-cpo as follows:
namespace std::execution {template<>structimpls-for<decayed-typeof<just-cpo>>:default-impls{staticconstexprautostart=[](auto& state, auto& rcvr)noexcept->void{auto&[.ts]= state;
set-cpo(std::move(rcvr), std::move(ts).);
};
};
}