Ada-Style Sub-Typing

[via Digitalmars-d-learn]

Is there some elegant way of creating "isolated" types in D 
similar to the semantics of `subtype` in Ada.

Something like this

struct A
{
     this(int value) { this._value = value; }
     private int _value;
     alias _value this;
}

struct B
{
     this(int value) { this._value = value; }
     private int _value;
     alias _value this;
}

void main(string[] args)
{
     A a = 11;
     B b = 12;
     a = b;
}

except that

the final statement should error.