Re: [boost] [outcome] Change semantics on UB from peer review agreed semantics?

On 13/09/2018 12:54, Peter Dimov wrote:

However, I vaguely remember that I convinced myself at one point that the latter (calling a function to convert) is inferior to calling a function that throws. For one thing, you can't convert an exception_ptr to an exception (*); for another, sometimes you want to throw different exceptions based on the runtime value of the error code (bad_alloc on ENOMEM for instance.)

True. I did start writing the email that way, but along the way thought that it might be a good idea to require use of boost::throw_exception via a return value (and thus getting a return-type trait for "free"). But you're right that there are some cases that don't work as well that way, since value returns aren't polymorphic.

(*) This is needed, for instance, when you build an outcome<> on top of a result<> by

struct my_error_code {    std::error_code code_;    std::exception_ptr exc_; };

template<class T> using outcome = result;

Arguably that sort of construction ought to "unpack" the error into the three-type outcome. Other than interop with pre-Outcome code I would think that such cases should be fairly rare.