REPL crash when compiling lambda to anonymous class

[Duhemm]

scala> def x[A, B]: ImplicitConverter[A, B] = _ => ???                                                                                                                                                                                                                                                                        
Exception in thread "main" java.lang.AssertionError: assertion failed: private method $anonfun in rs$line$1 accessed from method apply in null                                                                                                                                                                                
        at scala.Predef$.assert(Predef.scala:219)                                                                                                                                                                                                                                                                             
        at dotty.tools.dotc.transform.ExpandPrivate.ensurePrivateAccessible(ExpandPrivate.scala:89)                                                                                                                                                                                                                           
        at dotty.tools.dotc.transform.ExpandPrivate.transformSelect(ExpandPrivate.scala:99)                                                                                                                                                                                                                                   
        at dotty.tools.dotc.transform.ExpandPrivate.transformSelect(ExpandPrivate.scala:32)                                                                                                                                                                                                                                   
        at dotty.tools.dotc.transform.MegaPhase.goSelect(MegaPhase.scala:562)                                                                                                                                                                                                                                                 
        at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:229)                                                                                                                                                                                                                                         
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:391)                                                                                                                                                                                                                                            
        at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:270)                                                                                                                                                                                                                                       
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:392)                                                                                                                                                                                                                                            
        at dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:245)                                                                                                                                                                                                                                              
        at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:248)                                                                                                                                                                                                                                         
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:391)                                                                                                                                                                                                                                            
        at dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:400)                                                                                                                                                                                                                                          
        at dotty.tools.dotc.transform.MegaPhase.$anonfun$transformStats$2(MegaPhase.scala:405)                                                                                                                                                                                                                                
        at scala.collection.immutable.List.mapConserve(List.scala:175)
        at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:405)
        at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:339)
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:392)
        at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:251)
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:391)
        at dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:400)
        at dotty.tools.dotc.transform.MegaPhase.$anonfun$transformStats$2(MegaPhase.scala:405)
        at scala.collection.immutable.List.mapConserve(List.scala:175)
        at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:405)
        at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:339)
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:392)
        at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:251)
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:391)
        at dotty.tools.dotc.transform.MegaPhase.transformStat$1(MegaPhase.scala:400)
        at dotty.tools.dotc.transform.MegaPhase.$anonfun$transformStats$2(MegaPhase.scala:405)
        at scala.collection.immutable.List.mapConserve(List.scala:175)
        at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:405)
        at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:356)
        at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:359)
        at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:392)
        at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:411)
        at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:423)
        at dotty.tools.dotc.core.Phases$Phase.$anonfun$runOn$1(Phases.scala:297)
        at scala.collection.immutable.List.map(List.scala:282)
        at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:295)
        at dotty.tools.dotc.core.Phases$Phase.runOn$(Phases.scala:294)
        at dotty.tools.dotc.transform.MegaPhase.runOn(MegaPhase.scala:137)
        at dotty.tools.dotc.Run.$anonfun$compileUnits$3(Run.scala:172)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
        at dotty.tools.dotc.util.Stats$.trackTime(Stats.scala:49)
        at dotty.tools.dotc.Run.$anonfun$compileUnits$2(Run.scala:169)
        at dotty.tools.dotc.Run.$anonfun$compileUnits$2$adapted(Run.scala:167)
        at scala.collection.IndexedSeqOptimized.foreach(IndexedSeqOptimized.scala:32)
        at scala.collection.IndexedSeqOptimized.foreach$(IndexedSeqOptimized.scala:29)
        at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:194)         
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:167)                        
        at dotty.tools.dotc.Run.$anonfun$compileUnits$1(Run.scala:192)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:90)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:147)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:139)
        at dotty.tools.repl.ReplCompiler.runCompilationUnit(ReplCompiler.scala:141)
        at dotty.tools.repl.ReplCompiler.compile(ReplCompiler.scala:150)
        at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:213)
        at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:182)
        at dotty.tools.repl.ReplDriver.loop$1(ReplDriver.scala:127)
        at dotty.tools.repl.ReplDriver.$anonfun$runUntilQuit$2(ReplDriver.scala:130)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
        at scala.Console$.withErr(Console.scala:192)
        at dotty.tools.repl.ReplDriver.$anonfun$withRedirectedOutput$1(ReplDriver.scala:143)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
        at scala.Console$.withOut(Console.scala:163)
        at dotty.tools.repl.ReplDriver.withRedirectedOutput(ReplDriver.scala:143)
        at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:130)
        at dotty.tools.repl.Main$.main(Main.scala:6)
        at dotty.tools.repl.Main.main(Main.scala)

This doesn't happen outside the REPL, or when we don't use ImplicitConverter.

[allanrenucci]

This is not specific to ImplicitConverter. This is another occurence of #4051.

This happens when we exercice the following code path in the REPL: https://github.com/lampepfl/dotty/blob/0fcbfdb752fa22ed1b9c5f5ebf99382aa0f76ead/compiler/src/dotty/tools/dotc/transform/ExpandPrivate.scala#L86-L88
Here ctx.owner.sourceFile happens to be null in the REPL.

We exercice this code path when we compile a lambda to an anonymous class:

scala> trait Foo extends Function1[Int, Int]; (_ => 1): Foo                                                                                                                                            
Exception in thread "main" java.lang.AssertionError: assertion failed: private method $anonfun in rs$line$2 accessed from method apply in null
	at scala.Predef$.assert(Predef.scala:219)
	at dotty.tools.dotc.transform.ExpandPrivate.ensurePrivateAccessible(ExpandPrivate.scala:89)
	at dotty.tools.dotc.transform.ExpandPrivate.transformSelect(ExpandPrivate.scala:99)
	at dotty.tools.dotc.transform.ExpandPrivate.transformSelect(ExpandPrivate.scala:32)
	...

[smarter]

Already fixed, regression test in #5800.