immutable ctors, immutable members, and TDPL

[TommiT]

On Thursday, 23 May 2013 at 15:41:32 UTC, Steven Schveighoffer 
wrote:
> An interesting subject is being debated by some of the heavy 
> hitters of D on the latest beta on the announce NG.
>
> However, I have found that there is an inconsistency in TDPL 
> that needs to be addressed.
>
> Forgetting the controversy currently brewing, we have the 
> following current situation:
>
> struct S
> {
>    immutable int x;
>    this(int n)
>    {
>       x = n;
>       int y = x;
>       x = 0;
>    }
> }
>
> This compiles, and shows that one can use an immutable, and 
> then the immutable can be changed.  This is no good, for 
> obvious reasons.

I think immutable is something you cannot change. Also, the 
reverse is true, if you can change something, it's not immutable. 
According to this logic, x is not immutable inside the scope of 
the constructor, because you *can* change it. Therefore it should 
be so that:

import std.traits;

struct S
{
     immutable int x;

     this(int n)
     {
         static assert(isMutable!(typeof(x)));
     }
}
static assert(!isMutable!(typeof(S.init.x)));