-preview=in might break code

[Walter Bright]

On 10/5/2020 4:32 AM, Iain Buclaw wrote:
> I don't consider there to be any difference between the two as far as parameter 
> passing is concerned.  As I understood from the review, the point of ref passing 
> is to elide copies.
I see a major difference, as relying on the number of copies is not the same as 
memory corruption. Eliding copies is the bread and butter of optimizers, btw.


 > Because this is allowed as an optimization only, none of
 > what it does should spill out into user code.  If people notice then something
 > has gone wrong in the implementation.

The examples posted here shows it DOES.

If an `in` passes by `const ref`, and another mutable reference to the same 
memory object decides to free the memory, the `in` reference now is a live 
dangling pointer.