How to get the argument for promise::set_exception(x)?
There is a different name for copy_exception. copy_exception was renamed late in the standardization process over confusion of what it actually did:
template<class E> exception_ptr make_exception_ptr(E e) noexcept;Effects: Creates an
exception_ptrobject that refers to a copy ofe, ...
Use of either make_exception_ptr or current_exception is fine, depending on what exception you're trying to set.