mutable, const, immutable guidelines

[qznc]

On Wednesday, 16 October 2013 at 17:55:14 UTC, H. S. Teoh wrote:
> Maybe it's helpful to understand how D's const system works. The
> following diagram may help (please excuse the ASCII graphics):
>
> 	       const
> 	      /     \
> 	mutable     immutable

I think people in this thread know how const works, but some 
think it is broken. Scenario is this:

Library code:

struct Foo { int x; }

User code:

Foo f;
immutable f2 = f;

This works, even though the library writer might not have 
anticipated that someone makes Foo immutable. However, now the 
library writer obliviously releases a new version of the library, 
which extends it like this:

struct Foo {
   int x;
   private int[] history;
}

Unfortunately, now the user code is broken due to the freshly 
introduced mutable aliasing. Personally, I think is fine. Upon 
compilation the user code gives a  error message and user 
developer can adapt to the code to the new library version. Some 
think the library writer should have a possibility to make this 
work.