-preview=in might break code

[Andrei Alexandrescu]

On 10/3/20 2:36 PM, Paul Backus wrote:
> On Saturday, 3 October 2020 at 16:56:06 UTC, Mathias LANG wrote:
>>
>> To put it differently: Why is `auto ref` acceptable but `in` is not ?
> 
> The issue with `in`, compared to `auto ref`, is that, because its 
> behavior is implementation-defined, it invites programmers to write code 
> that "works on their machine," but is not portable to other environments 
> (including future versions of the same compiler). It's the same issue 
> that C has with features like variable-sized integer types and 
> implementation-defined signedness of `char`.
> 
> Yes, *technically* it's your fault if you write C code that relies on an 
> `int` being 32 bits, or a `char` being unsigned, just like it would 
> *technically* be your fault if you wrote D code that relied in an `in` 
> parameter being passed by reference. But making these things 
> implementation-defined in the first place is setting the programmer up 
> for failure.

That's a very good comparison, thank you.