-preview=in might break code

[Walter Bright]

On 10/2/2020 7:32 AM, Andrei Alexandrescu wrote:
> Finally, my "told you so" moment has come! :o)
> 
> https://forum.dlang.org/post/rhmst4$1vmc$1@digitalmars.com

This should be diagnosed by -preview=dip1021. However, #dip1021 will only stop 
the obvious cases of it. It can be more subtle, which can only be stopped by @live.

https://github.com/dlang/DIPs/blob/master/DIPs/accepted/DIP1021.md