abs and minimum values

[kyle]

```
void main()
{
     import std.math : abs, sgn;

     alias n_type = short; //or int, long, byte, whatever

     assert(n_type.min == abs(n_type.min));
     assert(sgn(abs(n_type.min)) == -1);
}
```
I stumbled into this fun today. I understand why abs yields a 
negative value here with overflow and no promotion. I just want 
to know if it should. Should abs ever return a negative number? 
Thanks.