why is it a class property cannot be used like a.b ~= c; ?

[someone via Digitalmars-d-learn]

```d
public class cSomething {

   private:
   dstring pstrWhatever = null;

   public:

   @safe dstring whatever() { return pstrWhatever; }
   @safe void whatever(const dstring lstrWhatever) { pstrWhatever 
= lstrWhatever; }

}

void main() {

   cSomething lobjSomething = new cSomething();
   lobjSomething.whatever = r"abc"d;
   lobjSomething.whatever ~= r"def"d; /// Error: 
`lobjSomething.whatever()` is not an lvalue and cannot be modified

}
```