cas and interfaces

[Johannes Loher]

I recently played around with atomic operations. While doing so, 
I noticed a problem with the interaction of interfaces and cas. 
Consider the following program:

```
import core.atomic;
import std.stdio;

interface TestInterface
{
}

class TestClass : TestInterface
{
}

void main()
{
     shared TestInterface testInterface = new shared TestClass;
     cas(&testInterface, testInterface, new shared 
TestClass).writeln;
     writeln(typeid(testInterface));
}
```
(https://run.dlang.io/is/9P7PAb)

This program compiles successfully and outputs

```
true
Error: program killed by signal 11
```

i.e. a segmentation fault happens.

When replacing "interface" with "abstract class" 
(https://run.dlang.io/is/sFaO1k), everything works as expected 
and the program outputs

```
true
onlineapp.TestClass
```

Is this actually a bug or a fundamental limitation of cas and 
interfaces? Did I do anything wrong?