Better error messages - from reddit

[Nicholas Wilson]

On Tuesday, 5 March 2019 at 01:18:05 UTC, H. S. Teoh wrote:
> Well, if you're looking for a perfect solution, then yes this 
> will be very complicated and hairy to implement.

Unfortunately both options are going to be nasty to implement 
because of the way that the compiler works, namely that you need 
to reevaluate the expressions and print their source. This sounds 
trivial, but because the constraint is boolean the result is 
overridden, so you need to look at the template declaration's 
constraint and reevaluate that, but then all the templates that 
are used in the expression are already instantiated so all you 
get from `isInputRange!R && isFoo!Bar` is `true && false` so you 
have to flush the template cache (which fortunately is fine 
because at this point you know the compiler is going to give an 
error). There is possibly a way to get back at the source from 
the location but I haven't got that far.

> But to take care of the most common case, all we have to do is 
> to assume that sig constraints are of the form (A && B && C && 
> ...).  The compiler only needs to report which of these top 
> level conjuncts failed.  If a sig constraint isn't of this 
> form, then fallback to reporting the entire constraint as 
> failed, i.e., treat it as the case (A) (single-argument 
> conjunction).

You don't actually get (A && B && C && ...) you get (A && (B && 
(C && ...))) (at least I think thats what you get I haven't been 
able to confirm the exact AST)
each of which could contain arbitrary conjugation, so you need to 
recurse and that hurts my head.

> It's not a perfect solution, but having this is already a lot 
> better than the current pessimal state of things.  Your DIP 
> represents an improvement over this most basic step, but IMO we 
> should at least have this basic step first.  Don't let the 
> perfect become the enemy of the good yet again.

Fear not, I'm going to use Dconf to get through all the 
bureaucracy (and get help with the implementation).