Converting move constructor.

template<
    typename OtherExecutor>
basic_eager_async_operation(
    basic_eager_async_operation< OtherExecutor, Signatures... > && other,
    constraint_t< is_convertible< OtherExecutor, Executor >::value >  = 0);

Transfers ownership of an operation whose executor is convertible to this object's executor type, leaving other empty.