Template constraints and opAdd

[bearophile]

> John:
>
>> 	Mass!(T,S) opAdd(Mass!(T,S) other) {
>
> If you are using D2 then don't use opAdd, use opBinary:
>
> http://dlang.org/operatoroverloading.html#Binary

Time ago I added an enhancement request for a warning (that later 
is meant to become a deprecation) that helps avoid your problem:

http://d.puremagic.com/issues/show_bug.cgi?id=10320

Bye,
bearophile