Auto keyword with const variable

[monarch_dodra]

On Wednesday, 24 July 2013 at 10:37:40 UTC, Artur Skawina wrote:
> On 07/24/13 12:09, monarch_dodra wrote:
>> Keeping it to "same type, 100% of the time" seems like the 
>> best.
>
> No, it's a design bug. The head qualifier(s) should always be 
> stripped
> when copying objects. Stripping is always fine (ie safe)

Nope. Stop. Wrong. Qualification is transitive in D, unlike in 
C++. Even when copying, stripping qualification is not safe:

//----
struct S
{
     int* p;
}

void main()
{
     immutable i = 1;
     auto a = immutable(S)(&i);
     auto b = cast(S)a; //Unsafe cast
     *b.p = 2; //This'll probably crash on a linux
     //Program corrupted by here.
     assert(i == *&i); //This fails (!!!) on my win32
}
//----

Long story short, never cast to Unqual. Always to an implicit 
cast, and let the compiler complain if it is illegal:

//----
struct S1
{
     int a;
}
struct S2
{
     int* p;
}

void main()
{
     immutable i = 1;
     auto a1 = immutable(S1)(i);
     auto a2 = immutable(S2)(&i);
     S1 b1 = a1; //OK!
     S2 b2 = a2; //Error: cannot implicitly convert expression 
(a2) of type immutable(S2) to S2
}
//----