The relationship of invariants and alias this

[Mark Isaacson via Digitalmars-d]

I've encountered something which is probably intentional 
behavior, but that I think is interesting and may merit 
discussion. When you do "alias this" and have an invariant, any 
methods that are forwarded to the aliased member do not invoke 
your invariant methods.

This prevents me from writing a really sleek 10-liner to the tune 
of:

struct ValueRestrictedInteger(int lowerBound, int upperBound) {
   int value;
   alias value this;

   this (int rhs) { value = rhs; }

   invariant() {
     assert (value >= lowerBound && value <= upperBound);
   }

   void forDemonstrationOnly() {}
}

unittest {
   ValueRestrictedInteger!(0, 100) x = 0;
   x += 10;
   x -= 100; //This works, but I don't think it should
   x.forDemonstrationOnly(); //This causes the assertion to fire

   ValueRestrictedInteger!(0, 100) y = -100; //This also would hit 
the assertion
}

I realize that this is not the most efficient way to implement 
this concept and that the invariant gets compiled out in a 
-release build... I still think this is a pretty nifty piece of 
code whose semantics are not unreasonable. Why shouldn't an 
aliased method be subject to our invariants?