Ada-Style Range Types with Value Range Propagation in Arithmetic

[Nordlöw]

I'm trying to extend my wrapper class Bound to support CTFE Value 
Range Propagation in primary in opUnary and opBinary similar to 
what Ada's Range Types.

My current try so far for binary arithmetic is
    auto opBinary(string op, U,
                  U lower_rhs = U.min,
                  U upper_rhs = U.max,
                  string file = __FILE__,
                  int line = __LINE__)(Bound!(U, lower_rhs, 
upper_rhs) rhs)
    {
        Bound!(CommonType!(T, U),
               min(T.min, U.min),
               max(T.max + U.max)) tmp = void; // TODO: Needs fix
        mixin("tmp = _value " ~ op ~ "rhs;");
        mixin(check());
        return tmp;
    }

but this errors as

bound.d(120,19): Error: undefined identifier U and I don't 
understand why I can't parameterize opBinary in this way.

Ideas anyone?

For reference, this older version

https://github.com/nordlow/justd/blob/master/bound.d

compiles but do not do value range propagation in arithmetic.