$ cat Structural.scala


// for Gilles

def f[T](p: { def x[U <: T](t: U): Int }, t: T): Int = p.x(t)

class Base { val m = 9 }
class Derived extends Base {}

// either is sufficient
f[Int](new Object { def x[U <: Int](t: U) = t*t }, 4)
f[Derived](new Object { def x[V <: Base](t: V) = t.m }, new Derived())



$ scala -version
Scala code runner version 2.6.0-RC2 -- (c) 2002-2007 LAMP/EPFL

$ scala -nocompdaemon Structural.scala
Exception in thread "main" java.lang.Error: U cannot be instantiated from AnyRef{def x[U <: Int](U): Int}


(or
> Exception in thread "main" java.lang.Error: U cannot be instantiated from AnyR
ef{def x[U <: Derived](U): Int}
)


	at scala.tools.nsc.symtab.Types$AsSeenFromMap.throwError$0(Types.scala:2263)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.instParam$0(Types.scala:2266)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.toInstance$0(Types.scala:2281)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:2290)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:2221)
	at scala.List$.loop$0(List.scala:244)
	at scala.List$.mapConserve(List.scala:261)
	at scala.tools.nsc.symtab.Types$TypeMap.mapOver(Types.scala:2132)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:2292)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:2221)
	at scala.tools.nsc.symtab.Types$TypeMap.mapOver(Types.scala:2141)
	at scala.tools.nsc.symtab.Types$AsSeenFromMap.apply(Types.scala:2292)
	at scala.tools.nsc.symtab.Types$Type.asSeenFrom(Types.scala:381)
	at scala.tools.nsc.symtab.Types$Type.memberInfo(Types.scala:388)
	at scala.tools.nsc.symtab.Types$class.scala$tools$nsc$symtab$Types$$specializesSym(Types.scala:3076)
	at scala.tools.nsc.symtab.Types$$anonfun$75.apply(Types.scala:3069)
	at scala.tools.nsc.symtab.Types$$anonfun$75.apply(Types.scala:3069)
	at scala.List.exists(List.scala:912)
	at scala.tools.nsc.symtab.Types$class.specializesSym(Types.scala:3068)
	at scala.tools.nsc.symtab.SymbolTable.specializesSym(SymbolTable.scala:11)
	at scala.tools.nsc.symtab.Types$Type.specializes(Types.scala:486)
	at scala.tools.nsc.symtab.Types$$anonfun$68.apply(Types.scala:3004)
	at scala.tools.nsc.symtab.Types$$anonfun$68.apply(Types.scala:3004)
	at scala.List.forall(List.scala:896)
	at scala.tools.nsc.symtab.Types$class.isSubType0(Types.scala:3004)
	at scala.tools.nsc.symtab.Types$class.isSubType(Types.scala:2879)
	at scala.tools.nsc.symtab.SymbolTable.isSubType(SymbolTable.scala:11)
	at scala.tools.nsc.symtab.Types$Type.$less$colon$less(Types.scala:469)
	at scala.tools.nsc.typechecker.Typers$Typer.adapt(Typers.scala:732)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2940)
	at scala.tools.nsc.typechecker.Typers$Typer.typedBlock(Typers.scala:1315)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2703)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
	at scala.tools.nsc.typechecker.Typers$Typer.typedArg(Typers.scala:1497)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$61.apply(Typers.scala:1513)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$61.apply(Typers.scala:1513)
	at scala.List$.map2(List.scala:277)
	at scala.tools.nsc.typechecker.Typers$Typer.typedArgs(Typers.scala:1513)
	at scala.tools.nsc.typechecker.Typers$Typer.doTypedApply(Typers.scala:1567)
	at scala.tools.nsc.typechecker.Typers$Typer.typedApply$0(Typers.scala:2276)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2836)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2971)
	at scala.tools.nsc.typechecker.Typers$Typer.typedStat$0(Typers.scala:1463)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
	at scala.List$.loop$0(List.scala:244)
	at scala.List$.mapConserve(List.scala:261)
	at scala.List$.loop$0(List.scala:248)
	at scala.List$.mapConserve(List.scala:261)
	at scala.List$.loop$0(List.scala:248)
	at scala.List$.mapConserve(List.scala:261)
	at scala.List$.loop$0(List.scala:248)
	at scala.List$.mapConserve(List.scala:261)
	at scala.tools.nsc.typechecker.Typers$Typer.typedStats(Typers.scala:1493)
	at scala.tools.nsc.typechecker.Typers$Typer.typedBlock(Typers.scala:1314)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2703)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2982)
	at scala.tools.nsc.typechecker.Typers$Typer.transformedOrTyped(Typers.scala:3031)
	at scala.tools.nsc.typechecker.Typers$Typer.typedDefDef(Typers.scala:1241)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2679)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2971)
	at scala.tools.nsc.typechecker.Typers$Typer.typedStat$0(Typers.scala:1463)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
	at scala.List$.loop$0(List.scala:244)
	at scala.List$.mapConserve(List.scala:261)
	at scala.tools.nsc.typechecker.Typers$Typer.typedStats(Typers.scala:1493)
	at scala.tools.nsc.typechecker.Typers$Typer.typedTemplate(Typers.scala:1111)
	at scala.tools.nsc.typechecker.Typers$Typer.typedModuleDef(Typers.scala:1013)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2673)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2971)
	at scala.tools.nsc.typechecker.Typers$Typer.typedStat$0(Typers.scala:1463)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
	at scala.tools.nsc.typechecker.Typers$Typer$$anonfun$57.apply(Typers.scala:1493)
	at scala.List$.loop$0(List.scala:244)
	at scala.List$.mapConserve(List.scala:261)
	at scala.tools.nsc.typechecker.Typers$Typer.typedStats(Typers.scala:1493)
	at scala.tools.nsc.typechecker.Typers$Typer.typed1(Typers.scala:2666)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2938)
	at scala.tools.nsc.typechecker.Typers$Typer.typed(Typers.scala:2971)
	at scala.tools.nsc.typechecker.Analyzer$typerFactory$$anon$1.apply(Analyzer.scala:39)
	at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:243)
	at scala.tools.nsc.Global$GlobalPhase$$anonfun$2.apply(Global.scala:232)
	at scala.tools.nsc.Global$GlobalPhase$$anonfun$2.apply(Global.scala:232)
	at scala.Iterator$class.foreach(Iterator.scala:375)
	at scala.collection.mutable.ListBuffer$$anon$0.foreach(ListBuffer.scala:255)
	at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:232)
	at scala.tools.nsc.Global$Run.compileSources(Global.scala:528)
	at scala.tools.nsc.ScriptRunner.compile$0(ScriptRunner.scala:302)
	at scala.tools.nsc.ScriptRunner.withCompiledScript(ScriptRunner.scala:337)
	at scala.tools.nsc.ScriptRunner.runScript(ScriptRunner.scala:362)
	at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:165)
	at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)