safe_==

class Safe_==[T](value: T) {
  def safe_==(other: T) = value == other
}
implicit def toSafe_==[T](value: T) = new Safe_==(value)

scala> 5 == "test"
<console>:10: warning: comparing values of types Int and java.lang.String using `==' will always yield false
              5 == "test"
                ^
res0: Boolean = false

scala> 5 safe_== "test"
<console>:10: error: type mismatch;
 found   : java.lang.String("test")
 required: Int
              5 safe_== "test"
                        ^

scala> "hello" safe_== "test"
res2: Boolean = false

scala> "hello" safe_== "hello"
res3: Boolean = true

//  But it goes wrong when inheritance is involved
scala> val o: Option[String] = None
o: Option[String] = None

scala> o safe_== None
res3: Boolean = true

scala> None safe_== o
<console>:11: error: type mismatch;
 found   : o.type (with underlying type Option[String])
 required: None.type
              None safe_== o