[Issue 15793] Change !is error to warning

[via Digitalmars-d-bugs]

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

Sobirari Muhomori <dfj1es...@sneakemail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |spec
                URL|                            |https://dlang.org/spec/expr
                   |                            |ession.html

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