[Issue 15793] Change !is error to warning

[via Digitalmars-d-bugs]

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

--- Comment #3 from Sobirari Muhomori <dfj1esp02 at sneakemail.com> ---
In reality I'm comparing COM interfaces. I removed the check, it was defensive
anyway. That's my concern actually: such checks should be a no-brainer or it
will cause very nasty consequences in the long run, so yeah, it's disturbing.

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