-preview=in might break code

[Ola Fosheim Grøstad]

On Friday, 2 October 2020 at 17:31:06 UTC, Steven Schveighoffer 
wrote:
> Yes, the problem is the "sometimes ref". Because ref changes 
> the semantics.
>
> I read it as, in means by ref, unless the compiler can prove 
> it's the same to pass by value, and that is more efficient. But 
> if it's for *optimization*, it shouldn't change the effective 
> semantics. The optimizer should be invisible.
>
> In practice, I don't think the compiler can prove that.

A good backend most certainly can? This ought to be a pure 
backend issue and not affect the fontend at all if the separation 
front/back is as it should be.

Calling conventions do not belong in a language spec...