Omit type parameter in extend clause

[allanrenucci]

As opposed to scalac, Dotty lets you write:

trait Foo[A]
class Bar[A] extends Foo

Note that in Dotty and Scala 2, you can write

class Foo[A]
class Bar[A] extends Foo

Here Dotty infers the type parameter toi be Any and scalac infers Nothing.

Does it make sense to infer a type argument when there is no call to the super constructor?

[allanrenucci]

Maybe a separate issue: the compiler crashes when compiling

trait SetOps[A, +C <: SetOps[A, C]]  {
  def concat(that: Iterable[A]): C = ???
}

class Set1[A] extends SetOps //should be SetOps[A, Set1[A]]

Exception in thread "main" dotty.tools.dotc.core.TypeError: bad parameter reference Set1.this.C at tryCatchPatterns
the parameter is type C in trait SetOps but the prefix Set1[A](Set1.this)
does not define any corresponding arguments.
	at dotty.tools.dotc.core.Types$NamedType.argDenot(Types.scala:1832)
	at dotty.tools.dotc.core.Types$NamedType.fromDesignator$1(Types.scala:1749)
	at dotty.tools.dotc.core.Types$NamedType.computeDenot(Types.scala:1767)
	at dotty.tools.dotc.core.Types$NamedType.denot(Types.scala:1723)
	at dotty.tools.dotc.core.Types$NamedType.info(Types.scala:1712)
	at dotty.tools.dotc.core.Types$TermLambda$$anon$8.apply(Types.scala:2826)
	at dotty.tools.dotc.core.Types$TermLambda$$anon$8.apply(Types.scala:2818)
	at dotty.tools.dotc.core.Types$TermLambda.dotty$tools$dotc$core$Types$TermLambda$$depStatus(Types.scala:2836)
	at dotty.tools.dotc.core.Types$TermLambda.dependencyStatus(Types.scala:2850)
	at dotty.tools.dotc.core.Types$TermLambda.resultType(Types.scala:2793)
	at dotty.tools.dotc.core.Types$TermLambda.resultType$(Types.scala:2792)
	at dotty.tools.dotc.core.Types$MethodType.resultType(Types.scala:2897)
	at dotty.tools.dotc.core.Types$MethodicType.resultSignature(Types.scala:2622)
	at dotty.tools.dotc.core.Types$MethodicType.resultSignature$(Types.scala:2622)
	at dotty.tools.dotc.core.Types$MethodOrPoly.resultSignature(Types.scala:2765)
	at dotty.tools.dotc.core.Types$MethodType.computeSignature(Types.scala:2917)
	at dotty.tools.dotc.core.Types$SignatureCachingType.signature(Types.scala:2614)
	at dotty.tools.dotc.core.Types$SignatureCachingType.signature$(Types.scala:2612)
	at dotty.tools.dotc.core.Types$MethodOrPoly.signature(Types.scala:2765)
	at dotty.tools.dotc.core.TypeComparer.compareMethod$1(TypeComparer.scala:538)
	at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:544)
	at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:368)
	at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:290)
	at dotty.tools.dotc.core.TypeComparer.$anonfun$recur$2(TypeComparer.scala:873)
	at scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.java:12)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:40)
	at dotty.tools.dotc.reporting.trace$.apply(trace.scala:44)
	at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:133)
	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:124)
	at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:131)
	at dotty.tools.dotc.core.ConstraintHandling.isSubTypeWhenFrozen(ConstraintHandling.scala:173)
	at dotty.tools.dotc.core.ConstraintHandling.isSubTypeWhenFrozen$(ConstraintHandling.scala:170)
	at dotty.tools.dotc.core.TypeComparer.isSubTypeWhenFrozen(TypeComparer.scala:19)
	at dotty.tools.dotc.core.Types$Type.$anonfun$frozen_$less$colon$less$1(Types.scala:807)
	at scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.java:12)
	at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
	at dotty.tools.dotc.core.Types$Type.frozen_$less$colon$less(Types.scala:807)
	at dotty.tools.dotc.core.Types$Type.overrides(Types.scala:843)
	at dotty.tools.dotc.typer.RefChecks$.$anonfun$checkAllOverrides$52(RefChecks.scala:640)
	at dotty.tools.dotc.typer.RefChecks$.$anonfun$checkAllOverrides$52$adapted(RefChecks.scala:637)
	at scala.collection.immutable.List.foreach(List.scala:389)
	at dotty.tools.dotc.typer.RefChecks$.$anonfun$checkAllOverrides$51(RefChecks.scala:637)
	at dotty.tools.dotc.typer.RefChecks$.$anonfun$checkAllOverrides$51$adapted(RefChecks.scala:636)
	at scala.collection.Iterator.foreach(Iterator.scala:944)
	at scala.collection.Iterator.foreach$(Iterator.scala:944)
	at dotty.tools.dotc.util.HashSet$$anon$1.foreach(HashSet.scala:102)
	at dotty.tools.dotc.util.Set.foreach(Set.scala:17)
	at dotty.tools.dotc.typer.RefChecks$.checkMemberTypesOK$1(RefChecks.scala:636)
	at dotty.tools.dotc.typer.RefChecks$.dotty$tools$dotc$typer$RefChecks$$checkAllOverrides(RefChecks.scala:668)
	at dotty.tools.dotc.typer.RefChecks.transformTemplate(RefChecks.scala:963)
	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:922)
	at dotty.tools.dotc.transform.MegaPhase.goTemplate(MegaPhase.scala:923)

[Blaisorblade]

Oh, #5010, #5027 and scala/bug#11111 overlap/coincide with both issues. Looks like the first one works only because A is not used in the body of Foo?