-preview=in might break code

[Per Nordlöw]

On Friday, 2 October 2020 at 17:16:46 UTC, Max Haughton wrote:
> Could we be ambitious and aim to have ownership taken to the 
> max and catch this statically? This particular case is 
> relatively low hanging fruit but having the in parameter work 
> this way would be nice if it was safe.

For which parameter types should such ownership checking be 
performed?

All kinds of references types including ref params, classes and 
pointers or a subset of them?