Should class be allowed to extend refinements?

[smarter]

Refining with a type

class A { type T }
type X = A { type T = Int }
class B extends X

This works, is that what we want?

Refining with a term

class A
type X = A { def f: Int }
class B extends X

This throws an exception in normalizeToRef because the normalization to a TypeRef only handles refining with a type. Do we want to allow this?

Exception in thread "main" dotty.tools.dotc.core.Types$TypeError: unexpected parent type: RefinedType(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,<empty>)),O$)),A), f, ExprType(TypeRef(TermRef(ThisType(TypeRef(NoPrefix,<root>)),scala),Int)))
        at dotty.tools.dotc.core.TypeOps$class.normalizeToRef$1(TypeOps.scala:296)
        at dotty.tools.dotc.core.TypeOps$$anonfun$5.apply(TypeOps.scala:298)
        at dotty.tools.dotc.core.TypeOps$$anonfun$5.apply(TypeOps.scala:298)
        at scala.collection.immutable.List.map(List.scala:274)
        at dotty.tools.dotc.core.TypeOps$class.normalizeToClassRefs(TypeOps.scala:298)
        at dotty.tools.dotc.core.Contexts$Context.normalizeToClassRefs(Contexts.scala:51)
        at dotty.tools.dotc.typer.Namer$ClassCompleter.completeInCreationContext(Namer.scala:493)
        at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:427)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:151)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.info(SymDenotations.scala:133)
        at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:234)
        at dotty.tools.dotc.typer.Typer.retrieveSym(Typer.scala:941)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:966)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1026)
        at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1038)
        at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1036)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:147)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:51)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1036)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1060)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1071)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedClassDef$1.apply(Typer.scala:873)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedClassDef$1.apply(Typer.scala:855)
        at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:855)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:980)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1026)
        at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1038)
        at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1036)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:147)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:51)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1036)
        at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:1060)
        at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:1071)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedPackageDef$1.apply(Typer.scala:920)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedPackageDef$1.apply(Typer.scala:911)
        at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
        at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:911)
        at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1017)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1028)
        at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1038)
        at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1036)
        at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:147)
        at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:51)
        at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1036)
        at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:1075)
        at dotty.tools.dotc.typer.FrontEnd$$anonfun$typeCheck$1.apply$mcV$sp(FrontEnd.scala:40)
        at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:17)
        at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:38)
        at dotty.tools.dotc.typer.FrontEnd$$anonfun$runOn$3.apply(FrontEnd.scala:51)
        at dotty.tools.dotc.typer.FrontEnd$$anonfun$runOn$3.apply(FrontEnd.scala:51)
        at scala.collection.immutable.List.foreach(List.scala:383)
        at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:51)
        at dotty.tools.dotc.Run$$anonfun$compileSources$1$$anonfun$apply$mcV$sp$3.apply(Run.scala:49)
        at dotty.tools.dotc.Run$$anonfun$compileSources$1$$anonfun$apply$mcV$sp$3.apply(Run.scala:47)
        at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
        at dotty.tools.dotc.Run$$anonfun$compileSources$1.apply$mcV$sp(Run.scala:47)
        at dotty.tools.dotc.Run$$anonfun$compileSources$1.apply(Run.scala:39)
        at dotty.tools.dotc.Run$$anonfun$compileSources$1.apply(Run.scala:39)
        at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:68)
        at dotty.tools.dotc.Run.compileSources(Run.scala:38)
        at dotty.tools.dotc.Run.compile(Run.scala:29)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:19)
        at dotty.tools.dotc.Main$.doCompile(Main.scala:26)
        at dotty.tools.dotc.Driver.process(Driver.scala:30)
        at dotty.tools.dotc.Driver.main(Driver.scala:44)
        at dotty.tools.dotc.Main.main(Main.scala)

CC @odersky

[odersky]

I believe we should not allow inheriting refined types in classes. Will add code that prevents it.