alias this - am I using it wrong?

[Johann Lermer]

Hi all,

I have a little problem understanding alias this. I always 
thought, that alias this only makes implicit conversions from the 
aliased object to this. Then, why do lines 18 and 22 compile in 
the code below? And, btw, line 22 crashes with a segmentation 
fault.

```d
01 struct Test_Struct {long t;}
02
03 class Alias_Class
04 {
05     Test_Struct ts;
06     alias ts this;
07 }
08
09 class Test_Class
10 {
11     Alias_Class ac;
12 }
13
14 void main ()
15 {
16     auto ac = new Alias_Class;
17     Test_Struct ts = ac;  // compiles
18     ac = ts;              // compiles as well - why?
19
20     auto tc = new Test_Class;
21     ts = tc.ac;           // compiles
22     tc.ac = ts;           // again this compiles, but seg 
faults
23 }
```

Johann