immutable T.init, and pointers to mutable data

[monarch_dodra via Digitalmars-d-learn]

On Thursday, 25 September 2014 at 13:37:52 UTC, Steven 
Schveighoffer wrote:
> On 9/25/14 9:00 AM, monarch_dodra wrote:
>> On Thursday, 25 September 2014 at 12:46:01 UTC, Steven 
>> Schveighoffer wrote:
>>> On 9/25/14 5:47 AM, monarch_dodra wrote:
>>>> I was playing around with how T.init works. And I think I 
>>>> may have found
>>>> a type loophole.
>>>>
>>>> Given that you may initialize a pointer member to the 
>>>> address to a
>>>> static global:
>>>>
>>>> //----
>>>> __gshared int a = 0;
>>>> struct S
>>>> {
>>>>    int* p = &a;
>>>> }
>>>> //----
>>>>
>>>> Then, in theory, any variable, be they mutable or const, are 
>>>> initialized
>>>> to T.init:
>>>>
>>>> //----
>>>> void main()
>>>> {
>>>>    immutable S s;
>>>> }
>>>> //----
>>>>
>>>> This is an issue, because I now have an immutable pointer 
>>>> that points to
>>>> mutable data:
>>>> //----
>>>>    immutable S s = S.init;
>>>>    immutable int* p = s.p;
>>>>    assert(*p == 0); //OK
>>>>    a = 5;
>>>>    assert(*p == 5); //OK
>>>> //----
>>>>
>>>> So this violates the type system...
>>>>
>>>> The question here is:
>>>> Is this "legit" code? At what point do you think my code 
>>>> should have
>>>> been rejected?
>>>
>>> It should be rejected. The declaration of s (the variable) 
>>> should be
>>> the trigger, since it casts the pointer to immutable.
>>>
>>> Please file a bug report.
>>>
>>> -Steve
>>
>> Hum... So that means certain types just *can't* be initialized 
>> (as
>> immutable) at all?
>
> I wouldn't say that:
>
> immutable s = S(null);
>
> But clearly, any code that results in an immutable pointer to 
> mutable data without casts is incorrect. We should start by 
> outlawing such code, and if there are ways we can carve out 
> certain usages, we can do that.
>
> -Steve

Hum... right, but I meant "T.init" itself would not be valid. As 
in:
alias T = immutable(S);
T t = T.init; //Illegal?

This might be a borderline case, but I kind of figured that that 
piece of code was the *only* one that was universally valid in 
generic code.

The context was this pull request:
https://github.com/D-Programming-Language/phobos/pull/2172

This sucks...