-preview=in might break code

[Steven Schveighoffer]

On 10/2/20 4:06 PM, Steven Schveighoffer wrote:
> "It is undefined behavior to pass a mutable argument to an `in` 
> parameter, and to read from that parameter after that mutable data has 
> been modified while executing the function."

It should actually be "a mutable argument that has no destructor, no
postblit, no copy constructor, is copyable, and is not a reference type" 
As those are defined to be ref or not ref explicitly by the spec.

-Steve