Surprise destructor call...

[Dennis]

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.