WTF does "Enforcement failed" actually mean?

[Ali Çehreli via Digitalmars-d-learn]

On 09/30/2015 10:46 PM, Russel Winder via Digitalmars-d-learn wrote:
> I have the code:
>
> 	reduce!"a+b"(x)
>
> where x is a int[] and I get an exception "Enforcement failed" at run
> time. This gives me enough information to say ¿que?
>

It's coming from the following no-message enforce():

             enforce(!r.empty);

 
https://github.com/D-Programming-Language/phobos/blob/master/std/algorithm/iteration.d#L2481

You are using the no-seed version of reduce(), which uses the first 
element as seed, which means that the range cannot be empty.

Ali