[Issue 15793] Change !is error to warning

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=15793

--- Comment #7 from Jonathan M Davis <issues.dlang at jmdavisProg.com> ---
> The problem is not best practices, but usability.

As I said, while it's best practice to use is and !is with null, it is a bit
much to treat it as an error to use == or !=. So, I'm not opposed to it being
changed. I'm just pointing out that what it's telling you to do is what you
really should be doing anyway. It's just being too forceful about it.

> lowering != null to !is null is the right answer.

That would be a good approach.

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