How to break const

[deadalnix]

Le 20/06/2012 16:55, Timon Gehr a écrit :
>>> It works if implicit conversions from 'R delegate(A)' to
>>> 'R delegate(A) const' are allowed, as well as looking up an
>>> 'R delegate(A) in a const receiver object.
>>
>> What about conversion from/to immutable ?
>
> Simple application of transitivity. This part is the same for 1./2.
>
> 'R delegate(A)' and 'R delegate(A) const' are not convertible to
> 'R delegate(A) immutable', but 'R delegate(A) immutable' is
> convertible to 'R delegate(A)' and 'R delegate(A) const'.
>
> 'R delegate(A)' and 'R delegate(A) const' decay to
> 'R delegate(A) immutable' when stored in an immutable object.
>

I've gone through a similar reasoning. It does not work. Here are 
implicit casts summed up :

mutable -> const
immutable -> const
immutable -> mutable

which implicitely imply immutable -> const

And then you add when storing in immutable object.

mutable -> immutable
const -> immutable

It means that you can store a delegate that have a mutable frame pointer 
into an immutable object, which break transitivity.