[Issue 15793] Change !is error to warning

[via Digitalmars-d-bugs]

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

Steven Schveighoffer <schveiguy at yahoo.com> changed:

           What    |Removed                     |Added
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                 CC|                            |schveiguy at yahoo.com
           Severity|regression                  |enhancement

--- Comment #1 from Steven Schveighoffer <schveiguy at yahoo.com> ---
Some backstory:

Originally, if a was actually null, this would cause a segfault (it would try
to call a.opEquals(null), which was a virtual call, etc.). This is why the
error was added -- any usage of != null for classes/interfaces was never what
you really wanted.

But with the current way object comparisons are done, != null would do the
correct thing. I think actually, we can remove the error completely.

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