Pre-SIP: warn against unidiomatic features (e.g., null value and unsafe casts)

[AMatveev]

I agree with you.
I just do not understand why null can be an unidiomatic feature.

@sjrd

Option is not a better way to deal with null ! It is a better way to deal with optional values

So there is no alternative to null

var a:String = _  // it is good 
a = null   // is it  really unidiomatic?