Can anyone illuminate me as to where this destructor call is coming from?

struct Thing
{
  static int x;
  this(typeof(null)) pure {} // <-- comment this line and the error goes
away
  ~this() { ++x; }
}

Thing fun() pure
{
   return Thing(null);
}

error : `pure` function `urt.string.string.fun` cannot call impure destructor `urt.string.string.Thing.~this`

Shouldn't NVRO construct the result in place and elide the copy/move? I
would not expect any call to the destructor in fun()...
Also surprisingly, if I comment out the constructor, the compile error
about the destructor goes away. I can't see why the constructor's existence
affects the destruction semantics in fun()?

On Wed, 14 Aug 2024 at 21:56, Dennis via Digitalmars-d < digitalmars-d@puremagic.com> wrote:

> On Wednesday, 14 August 2024 at 09:19:55 UTC, Manu wrote:
> > Also surprisingly, if I comment out the constructor, the
> > compile error
> > about the destructor goes away. I can't see why the
> > constructor's existence
> > affects the destruction semantics in fun()?
>
> The compiler's logic goes as follows:
>
> ```D
> return Thing(null);
> ```
>
> Since `Thing` has a constructor, this gets rewritten to:
>
> ```D
> return Thing().this(null);
> ```
>
> Now we have a 'DotVarExp' with a struct literal on the left, and the struct has a destructor. Since internally expression temporaries can't have destructors, it gets extracted into a temporary variable:
>
> ```D
> return ((Thing __slThing3 = Thing();) , __slThing3).this(null);
> ```
>
> Then `__slThing3` goes out of scope and needs a destructor call.
>
> So clearly, in this case you don't want the temporary, but in
> other cases (`return Thing(null).field;`) you do need it, so I'm
> thinking about what the right conditions should be for the
> temporary.
>

Well, the condition is that NRVO should elide the copy/move... it should be
constructed at the caller's scope in this case.
Your example `Thing(null).field` is not the same thing, because it's not
NRVO at all.

I guess this is a bug then?

On Thu, 15 Aug 2024 at 00:31, kinke via Digitalmars-d < digitalmars-d@puremagic.com> wrote:

> On Wednesday, 14 August 2024 at 13:56:08 UTC, Manu wrote:
> > Well, the condition is that NRVO should elide the copy/move...
> > it should be
> > constructed at the caller's scope in this case.
> > Your example `Thing(null).field` is not the same thing, because
> > it's not
> > NRVO at all.
> >
> > I guess this is a bug then?
>
> When actually using NRVO (`auto r = Thing(null); return r;`), one
> even gets the error *twice*. So that's definitely a bug, the
> destruction is handled by the caller; just for the attributes
> check etc., there's no real dtor call in `fun()`.
>
> What you have is a case for RVO; AFAIK, LDC and GDC implement that (for non-POD types at least), no idea about DMD. Meaning that the temporary in the return expression isn't destructed by `foo` either; it's emplaced directly into the caller-allocated return value, as the NRVO case.
>

My understanding is that RVO is in the D spec and NOT simply an optimisation. The language shouldn't attempt to call a destructor here in my case under any circumstances...

Walter?