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POWhy there is a ClassCastException when isInstanceOf returns true on a scala type with wildcard parameter?
 

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  1. POWhy there is a ClassCastException when isInstanceOf returns true on a scala type with wildcard parameter?
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    <p>My questions are:</p> <ol> <li>what is this warning about? There is no type parameter for reflect.runtime.universe.TypeRef</li> <li>why this is thrown on method invocation <em>arg</em> rather than <em>asInstanceOf</em>?</li> </ol> <p>Console output:</p> <pre><code>Welcome to Scala version 2.10.0 (Java HotSpot(TM) Client VM, Java 1.6.0_35). Type in expressions to have them evaluated. Type :help for more information. scala&gt; import reflect.runtime.{universe =&gt; ru} import reflect.runtime.{universe=&gt;ru} scala&gt; val t = ru.typeOf[Option[_]] t: reflect.runtime.universe.Type = scala.Option[_] scala&gt; t.isInstanceOf[ru.TypeRef] &lt;console&gt;:10: warning: abstract type reflect.runtime.universe.TypeRef is unchecked since it is eliminated by erasure t.isInstanceOf[ru.TypeRef] ^ res0: Boolean = true scala&gt; t.asInstanceOf[ru.TypeRef] res1: reflect.runtime.universe.TypeRef = scala.Option[_] scala&gt; t.asInstanceOf[ru.TypeRef].args java.lang.ClassCastException: scala.reflect.internal.Types$ExistentialType cannot be cast to scala.reflect.api.Types$TypeRefApi at .&lt;init&gt;(&lt;console&gt;:10) at .&lt;clinit&gt;(&lt;console&gt;) at .&lt;init&gt;(&lt;console&gt;:7) at .&lt;clinit&gt;(&lt;console&gt;) at $print(&lt;console&gt;) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:731) at scala.tools.nsc.interpreter.IMain$Request.loadAndRun(IMain.scala:980) at scala.tools.nsc.interpreter.IMain.loadAndRunReq$1(IMain.scala:570) at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:601) at scala.tools.nsc.interpreter.IMain.interpret(IMain.scala:565) 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.interpreter.ILoop.main(ILoop.scala:889) at org.jetbrains.plugins.scala.compiler.rt.ConsoleRunner.main(ConsoleRunner.java:64) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25) at java.lang.reflect.Method.invoke(Method.java:597) at com.intellij.rt.execution.CommandLineWrapper.main(CommandLineWrapper.java:121) </code></pre>
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