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

Subject: Re: [boost] [outcome] Change semantics on UB from peer review agreed semantics?
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2018-09-13 07:50:48

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

Front line `result<T, E>` would become unusable with UDT E types yes. If
people want UDT E types, they would need to use `result<T, E, Policy>`,
`unchecked<T, E>`, or `checked<T, E>`.

So what's being proposed here is that `result<T, E>` becomes strictly
for E types which are convertible to error code or exception ptr only.
Everything else fails to compile with a suggestion "please go use
checked<T, E> or unchecked<T, E> for UDT E types".

> 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?

Me personally I almost exclusively use Outcome in P1028 SG14
`status_code` mode now. Far superior codegen. SG14 `status_code` permits
arbitrary error coding types, and can throw arbitrary exceptions from
those. Outcome in SG14 `status_code` mode delegates the exception
throwing to `status_code`.

I'm fairly sure a major games studio has rolled out SG14 `status_code`
into its code. They seem very pleased with it. It does indeed produce
very lovely looking assembler.

Niall