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

czw., 13 wrz 2018 o 01:52 Gavin Lambert via Boost napisał(a):

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template <typename T> using resultC = result; // the above will be *UB* on value from valueless, because default_policy is UB for unknown (to the framework) types.

The fourth case is described as it works today. The proposal in this

On 13/09/2018 09:48, Andrzej Krzemienski wrote: thread

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is to change the fourth case so that default_policy fails to compile when used for unknown types.

That seems sensible to me.

In my view it is always superior to choose fail-to-compile over UB whenever possible, at least for obviously silly code. And "quietly" switching from throw semantics to UB is probably not a good idea, as you pointed out.

*Having said that*, if I'm understanding correctly, this sounds like it would mean that you won't be able to use a custom error_type with Outcome unless you either:

1. Declare some "this is how you throw an exception for this error_type" method (presumably a template specialisation?)

2. Only ever use it with some non-default policy (that either never throws or has user-implemented code to throw).

Is that correct? (Or would #1 always be required regardless of usage?)

If so, while I'm not sure whether that's enough to change my mind, it does make me a little hesitant about adding another hurdle.

On the *other* other hand, it wouldn't be a silly idea to have a generic "throw this error_type" method. system_error itself was somewhat trying to be that generic method, but with the caveat of only supporting plain enum error_types.

Perhaps boost::throw_error, which is expected to internally construct an appropriate exception based on its argument type and then call boost::throw_exception?

This is actually what Outcome is currently doing (modulo the unfortunate UB case in the default policy). For the sake of keeping this discussion focused, I did not describe the the full behavior of the default policy, which is: + Throw system error when `EC` is: * error_code * an enum "plugged" into the error_code framework * a type for which ADL can find function make_error_code() that converts it to error_code + if `EC` is exception_ptr, rethrow the contained exception - If `EC` is void call std::terminate - otherwise UB The last two (marked with `-`) are the ones that I would propose to change. And Outcome is also using an ADL-based customization point outcome_throw_as_system_error_with_payload(), which gets an `EC` and throws the desired exception (quite similar to what Peter suggests). Regards, &rzej;