[Issue 15793] Change !is error to warning

[via Digitalmars-d-bugs]

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

--- Comment #8 from Sobirari Muhomori <dfj1esp02 at sneakemail.com> ---
A little comparison: in java and C# Object.Equals is provided, but not called
implicitly. It's provided, but is opt-in and explicit, it's all somewhat rooted
in java, C# and D just copied it. C# uses C++-style operator overloads, it's up
to them to call Object.Equals or not. Also C# requires x.Equals(null) to always
return false.

(In reply to Steven Schveighoffer from comment #6)
> lowering != null to !is null is the right answer.
That would be awesome.

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