immutable promise broken in unions?

[Marc Schütz 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.

Right, this is a known bug:
https://issues.dlang.org/show_bug.cgi?id=13537