Propagating Errors

[Nicholas Wilson]

On Wednesday, 6 March 2019 at 12:48:08 UTC, FeepingCreature wrote:
> Personally, I think the problem comes down to this: we have no 
> way of propagating errors from  inside template constraints to 
> the user.
>
> I think the way to go is instead of returning `false`, have 
> __traits(compiles) (which is really the core source of these 
> difficulties)

I find `is` expressions to be more to blame, but the underlying 
reason is the same: the error are gagged. This is good most of 
the time (because otherwise it would be like always using 
verrors=spec), but it misses the small fraction of the time when 
that is extremely useful.

> return an "error object" that evaluates to false, but also 
> encodes additional information about the problem, recursively.
>
> Propagate this through short-circuiting - ErrorObject && bla = 
> true, bla && ErrorObject = ErrorObject, etc.
>
> The compiler can then pick up on the fact that an error object 
> was the reason why a template constraint failed to give a 
> highly informative error message.
>
> For instance:
> *snip*

This works nicely for __traits compiles, I'm not so sure about is 
expressions (though I'd love to be shown otherwise).