The issue there has nothing to do with generics. You need overloading for what you want (and, IMO, overloading is a bad idea). Don't write code that way. Since conversion is rarely a generic pattern anyways, you just pass a conversion function in.

     func processStuff(type U, V)(a U, b V, convert func(U) V){
         bleh(a, convert(b))
     }

     processStuff(123, "foo", strconv.Itoa)

Adding the parts needed to do generic conversion functions is not a great idea. That way lies madness and SFINAE.