immutable promise broken in unions?

[John Colvin via Digitalmars-d-learn]

On Saturday, 2 January 2016 at 12:08:48 UTC, Meta wrote:
> On Saturday, 2 January 2016 at 12:07:31 UTC, John Colvin wrote:
>> You are manually breaking immutable by making a union of 
>> immutable and mutable data and then writing to the mutable 
>> reference. This is roughly equivalent to casting away 
>> immutable and then writing to the reference. It's a bug in 
>> your code.
>>
>> All references to the same data should be
>> 1) either immutable or const
>> or all the references should be
>> 2) either mutable or const (assuming the data was never 
>> immutable).
>> Anything else is dangerous.
>
> Surely the compiler should disallow this. It makes it trivial 
> to break the type system otherwise.

Casting away immutable can sometimes be necessary (e.g. when 
talking to other languages), so I'm not sure it should be 
disallowed, but it'd be great if it was somehow easier to catch 
these bugs.