'Double free' even with @disabled this(this)

[monarch_dodra]

On Friday, 25 October 2013 at 18:24:51 UTC, Maurice wrote:
> Hey,
>
> I'm trying to use a struct for RAII (like C++'s unique_ptr), so 
> i @disabled this(this). However, it's still getting copied 
> somehow:
>
> struct A {
> 	int some_resource = 0;
> 	~this() { if (some_resource) writeln("Freeing resource."); }
> 	@disable this(this); // Prevent copying
> }
>
> struct B {
> 	A a;
> 	alias a this;
> }
>
> void main() {
> 	B b;
> 	b.some_resource = 100;
> 	A a = move(b);
> }
>
> The above code prints 'Freeing resource.' twice!
>
> See it in action on http://dpaste.dzfl.pl/6461df03
>
> I guess the first one is from the temporary B that move(b) 
> gives. However, that temporary should be moved into a and then 
> left in B.init state, but that doesn't happen.
>
> Is this a bug?

Yes, it seems to be a bug related to alias this. If we remove the
alias this, and type the call explicitly, then we get this:

//----
struct B {
      A a;
      //alias a this; //Let's do things explicitly.
}

void main() {
      B b;
      b.a.some_resource = 100;
      A a = move(b).a;
}
//----
Error: struct main.A is not copyable because it is annotated with
@disable
//----

I don't think you should get a *different* behavior with an
explicit call, no matter what said behavior is. You should file 
it in buzilla: http://d.puremagic.com/issues/

In this case, not compiling is the correct behavior I think: If
have an B rvalue you can move, then doesn't mean you can
selectivelly move its members (eg A). Think of it in terms of C++
like struct inheritance: If A is a base of B, you can't move a B
into an A.

The issue here is you are trying to move a B into an A, and that
*can't* work. However, you can move the b.a into the a, that's
fine.

This does what you want:
//----
void main() {
      B b;
      b.a.some_resource = 100;
      A a = move(b.a); //Move b.a, *NOT* b itself
}
//----
Freeing resource. //(Once)
//----