-preview=in might break code

[Joseph Rushton Wakeling]

On Saturday, 3 October 2020 at 16:08:46 UTC, Steven Schveighoffer 
wrote:
> This is not any different than calling a function which has a 
> reference to the data elsewhere. In other words, it's not 
> necessarily the function itself that changes the data, it could 
> be changed outside the function. You don't need concurrency to 
> do it.

Sure.  Concurrency was just one example of how the 
implementation-dependent behaviour could arise.

> But it's not impossible to define this:
>
> "when accepting a parameter by `in ref`, one cannot depend on 
> the value remaining constant, as other references to the data 
> may change it. The compiler can also decide to pass an `in ref` 
> parameter by value for optimization reasons, so one cannot 
> depend on the parameter changing through a different alias."

OK, but that feels rather like it's imposing a cognitive burden 
on the developer as a way to work around the fact that the 
feature itself isn't working in an intuitive way.

It feels as unintuitive that a parameter marked `ref` could fail 
(in an implementation dependent way) to display reference 
semantics, as it does that a non-reference-type parameter _not_ 
marked `ref` could display them.