Generic extension method (<<<) crashes in REPL but non-generic version doesn't

[travisbrown]

minimized code

given [A] { def (a: A) <<< : A = a }

1.<<<

Stack trace

java.lang.ClassFormatError: Method "<<<_of_A_given" in class rs$line$2$ has illegal signature "()Lrs$line$1$$$$_of_A_given;"
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:760)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
	at dotty.tools.repl.AbstractFileClassLoader.findClass(AbstractFileClassLoader.scala:28)
	at dotty.tools.repl.AbstractFileClassLoader.loadClass(AbstractFileClassLoader.scala:32)
	at rs$line$2.res0(rs$line$2)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at dotty.tools.repl.Rendering.$anonfun$3(Rendering.scala:57)
	at scala.Option.map(Option.scala:163)
	at dotty.tools.repl.Rendering.valueOf(Rendering.scala:57)
	at dotty.tools.repl.Rendering.renderVal(Rendering.scala:80)
	at dotty.tools.repl.ReplDriver.displayMembers$3$$anonfun$3(ReplDriver.scala:274)
	at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:237)
	at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:62)
	at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:55)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:49)
	at scala.collection.TraversableLike.map(TraversableLike.scala:237)
	at scala.collection.TraversableLike.map$(TraversableLike.scala:230)
	at scala.collection.AbstractTraversable.map(Traversable.scala:108)
	at dotty.tools.repl.ReplDriver.displayMembers$7(ReplDriver.scala:274)
	at dotty.tools.repl.ReplDriver.displayDefinitions$$anonfun$3$$anonfun$2(ReplDriver.scala:301)
	at scala.Option.map(Option.scala:163)
	at dotty.tools.repl.ReplDriver.displayDefinitions$$anonfun$1(ReplDriver.scala:301)
	at dotty.tools.dotc.core.Periods.atPhase(Periods.scala:25)
	at dotty.tools.dotc.core.Phases.atPhase(Phases.scala:35)
	at dotty.tools.dotc.core.Contexts$Context.atPhase(Contexts.scala:71)
	at dotty.tools.repl.ReplDriver.displayDefinitions(ReplDriver.scala:306)
	at dotty.tools.repl.ReplDriver.compile$$anonfun$2(ReplDriver.scala:230)
	at scala.util.Either.fold(Either.scala:191)
	at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:230)
	at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:182)
	at dotty.tools.repl.ReplDriver.loop$1(ReplDriver.scala:127)
	at dotty.tools.repl.ReplDriver.runUntilQuit$$anonfun$1(ReplDriver.scala:130)
	at dotty.tools.repl.ReplDriver.withRedirectedOutput$$anonfun$2$$anonfun$1(ReplDriver.scala:143)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:62)
	at scala.Console$.withErr(Console.scala:196)
	at dotty.tools.repl.ReplDriver.withRedirectedOutput$$anonfun$1(ReplDriver.scala:143)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:62)
	at scala.Console$.withOut(Console.scala:167)
	at dotty.tools.repl.ReplDriver.withRedirectedOutput(ReplDriver.scala:143)
	at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:130)
	at xsbt.ConsoleInterface.run(ConsoleInterface.java:52)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at sbt.internal.inc.AnalyzingCompiler.call(AnalyzingCompiler.scala:248)
	at sbt.internal.inc.AnalyzingCompiler.console(AnalyzingCompiler.scala:210)
	at sbt.Console.console0$1(Console.scala:48)
	at sbt.Console.$anonfun$apply$2(Console.scala:51)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
	at sbt.util.InterfaceUtil$$anon$1.get(InterfaceUtil.scala:10)
	at sbt.TrapExit$App.run(TrapExit.scala:257)
	at java.lang.Thread.run(Thread.java:745)

expectation

It seems like this should work, since the non-generic version is just fine:

scala> given { def (a: Int) <<< : Int = a }
// defined object <<<_of_Int_given

scala> 1.<<<
val res0: Int = 1

[smarter]

Fixed by #6903, meanwhile you can work around the problem by giving a name to your given.