-preview=in might break code

[Paul Backus]

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.