Undefined type and AssertionError: class has non-class parent ErrorType

[panacekcz]

Def.scala:

class A[T]
class B extends A[C]
class C extends B

Use.scala

class D[U <: B] extends C

First, dotc Def.scala succeeds.
Then, compiling dotc -classpath . Use.scala:

undefined: new B # 103: TermRef(TypeRef(ThisType(TypeRef(ThisType(TypeRef(NoPrefix,module class <root>)),module class <empty>)),B),<init>) at frontend
exception occurred while typechecking Use.scala
exception occurred while compiling Use.scala
java.lang.AssertionError: assertion failed: class C has non-class parent: dotty.tools.dotc.core.Types$ErrorType$$anon$1@907f2b7 while compiling Use.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: class C has non-class parent: dotty.tools.dotc.core.Types$ErrorType$$anon$1@907f2b7
	at scala.Predef$.assert(Predef.scala:219)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.$anonfun$computeBaseData$1(SymDenotations.scala:1523)
	at scala.collection.immutable.List.foreach(List.scala:388)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.computeBaseData(SymDenotations.scala:1520)
	at dotty.tools.dotc.core.SymDenotations$BaseDataImpl.apply(SymDenotations.scala:2243)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.baseData(SymDenotations.scala:1501)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.baseClassSet(SymDenotations.scala:1512)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.derivesFrom(SymDenotations.scala:1532)
	at dotty.tools.dotc.core.Types$Type.loop$1(Types.scala:208)
	at dotty.tools.dotc.core.Types$Type.derivesFrom(Types.scala:224)
	at dotty.tools.dotc.typer.Namer$ClassCompleter.checkedParentType$1(Namer.scala:889)
	at dotty.tools.dotc.typer.Namer$ClassCompleter.$anonfun$completeInCreationContext$5(Namer.scala:936)
	at scala.collection.immutable.List.map(List.scala:282)
	at dotty.tools.dotc.typer.Namer$ClassCompleter.completeInCreationContext(Namer.scala:936)
	at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:739)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:232)
	at dotty.tools.dotc.core.Denotations$Denotation.completeInfo$1(Denotations.scala:180)
	at dotty.tools.dotc.core.Denotations$Denotation.info(Denotations.scala:182)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.ensureCompleted(SymDenotations.scala:338)
	at dotty.tools.dotc.typer.Typer.retrieveSym(Typer.scala:1841)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1866)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1942)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1974)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1970)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1986)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2005)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2038)
	at dotty.tools.dotc.typer.Typer.$anonfun$typedPackageDef$1(Typer.scala:1715)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:37)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1708)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1921)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1943)
	at dotty.tools.dotc.typer.Typer.$anonfun$typed$2(Typer.scala:1974)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1970)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1986)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2049)
	at dotty.tools.dotc.typer.FrontEnd.$anonfun$typeCheck$1(FrontEnd.scala:60)
	at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:34)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:58)
	at dotty.tools.dotc.typer.FrontEnd.$anonfun$runOn$6(FrontEnd.scala:88)
	at dotty.tools.dotc.typer.FrontEnd.$anonfun$runOn$6$adapted(FrontEnd.scala:88)
	at scala.collection.immutable.List.foreach(List.scala:388)
	at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:88)
	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.compileSources(Run.scala:134)
	at dotty.tools.dotc.Run.compile(Run.scala:118)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:30)
	at dotty.tools.dotc.Driver.process(Driver.scala:136)
	at dotty.tools.dotc.Driver.process(Driver.scala:105)
	at dotty.tools.dotc.Driver.process(Driver.scala:117)
	at dotty.tools.dotc.Driver.main(Driver.scala:144)
	at dotty.tools.dotc.Main.main(Main.scala)

Possibly related: #3625