Not so covariant sets

sets.scala

scala> class MySet[T](elements: Seq[T])(implicit ordering: Ordering[T]) extends scala.collection.immutable.Set[T] {
     |   // Members declared in scala.collection.GenSetLike
     |   def iterator: Iterator[T] = elements.iterator
     |   
     |   // Members declared in scala.collection.SetLike
     |   def -(elem: T): scala.collection.immutable.Set[T] = ???
     |   def +(elem: T): scala.collection.immutable.Set[T] = ???
     | 
     |   def contains(t: T): Boolean = elements.exists(ordering.equiv(t, _))
     | }
defined class MySet

scala> 

scala> import scala.annotation.unchecked.uncheckedVariance
import scala.annotation.unchecked.uncheckedVariance

scala> 

scala> class T[+A: Ordering](elements: Seq[A]) extends MySet[A @uncheckedVariance](elements)
defined class T

scala> 

scala> val ints = new T(Seq(1, 2, 3, 4, 5))
ints: T[Int] = Set(1, 2, 3, 4, 5)

scala> 

scala> val cov: T[Any] = ints
cov: T[Any] = Set(1, 2, 3, 4, 5)

scala> 

scala> cov.contains("1")
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
	at scala.runtime.BoxesRunTime.unboxToInt(Unknown Source)
	at scala.math.Ordering$Int$.compare(Ordering.scala:256)
	at scala.math.Ordering$class.equiv(Ordering.scala:100)
	at scala.math.Ordering$Int$.equiv(Ordering.scala:256)
	at MySet$$anonfun$contains$1.apply(<console>:15)
	at MySet$$anonfun$contains$1.apply(<console>:15)
	at scala.collection.LinearSeqOptimized$class.exists(LinearSeqOptimized.scala:80)
	at scala.collection.immutable.List.exists(List.scala:84)
	at MySet.contains(<console>:15)
	at .<init>(<console>:13)
	at .<clinit>(<console>)
	at .<init>(<console>:7)
	at .<clinit>(<console>)
	at $print(<console>)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:734)
	at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:983)
	at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:573)
	at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:604)
	at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:568)
	at scala.tools.nsc.interpreter.ILoop.reallyInterpret$1(ILoop.scala:745)
	at scala.tools.nsc.interpreter.ILoop.interpretStartingWith(ILoop.scala:790)
	at scala.tools.nsc.interpreter.ILoop.command(ILoop.scala:702)
	at scala.tools.nsc.interpreter.ILoop.processLine$1(ILoop.scala:566)
	at scala.tools.nsc.interpreter.ILoop.innerLoop$1(ILoop.scala:573)
	at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:576)
	at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:867)
	at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:822)
	at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:822)
	at scala.tools.nsc.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:135)
	at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:822)
	at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:83)
	at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:96)
	at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:105)
	at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)