-preview=in might break code

[Walter Bright]

On 10/5/2020 10:46 AM, Iain Buclaw wrote:
> None of the posted examples I've seen here affect the implementation being 
> trialed in GDC.  Though I've already said that I'm likely being more 
> conservative than DMD.

I could implement it in DMD by whenever the spec allows it to be by value, doing 
it by value. Then the problem would never happen. (This is how I implemented 
__restrict__ in Digital Mars C.)

But that would defeat the purpose of the feature. And since the specification 
allows it to be ref, but does not require it, I would be forced to disallow `in` 
in any code under my purview.