Type disambiguation error in safe init checker caused by `-Yexplicit-nulls`

[Xavientois]

Ever since explicit nulls have been enabled, the error occurs when compiling the following code with both -Ysafe-init and -Yexplicit-nulls.

Compiler version

3.1.2-RC1

Minimized code

class Outer:
  sealed abstract class Name(val encoded: String) {
    type ThisName <: Name
    def compareTo(that: ThisName): Int = ???
  }

  class LocalName (encoded: String)
    extends Name(encoded) with Comparable[LocalName] {
    type ThisName = LocalName
  }

  val localName = LocalName("hello")
  println(localName)
  var count = 0

Output

Exception in thread "main" dotty.tools.dotc.core.TypeError: Failure to disambiguate overloaded reference with
  method compareTo in trait Comparable: (x$0: Outer.this.LocalName | Null): Int  and
  method compareTo in class Name: (that: Outer.this.LocalName#ThisName): Int
Failure to disambiguate overloaded reference with
  method compareTo in trait Comparable: (x$0: Outer.this.LocalName | Null): Int  and
  method compareTo in class Name: (that: Outer.this.LocalName#ThisName): Int while compiling example/Outer.scala
	at dotty.tools.dotc.core.Denotations$MultiDenotation.suchThat(Denotations.scala:1243)
	at dotty.tools.dotc.core.Denotations$Denotation.matchingDenotation(Denotations.scala:358)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.matchingMember(SymDenotations.scala:1300)
	at dotty.tools.dotc.transform.init.Semantic$.resolve(Semantic.scala:1578)
	at dotty.tools.dotc.transform.init.Semantic$.call(Semantic.scala:627)
	at dotty.tools.dotc.transform.init.Semantic$.tryPromote$$anonfun$1$$anonfun$1(Semantic.scala:949)
	at dotty.tools.dotc.core.Scopes$Scope.filter(Scopes.scala:102)
	at dotty.tools.dotc.core.Scopes$Scope.exists(Scopes.scala:109)
	at dotty.tools.dotc.transform.init.Semantic$.tryPromote$$anonfun$1(Semantic.scala:956)
	at scala.collection.immutable.List.exists(List.scala:395)
	at dotty.tools.dotc.transform.init.Semantic$.tryPromote(Semantic.scala:956)
	at dotty.tools.dotc.transform.init.Semantic$.promote(Semantic.scala:895)
	at dotty.tools.dotc.transform.init.Semantic$ArgInfo.promote(Semantic.scala:1051)
	at dotty.tools.dotc.transform.init.Semantic$.checkArgs$1$$anonfun$1(Semantic.scala:591)
	at scala.collection.immutable.List.flatMap(List.scala:293)
	at dotty.tools.dotc.transform.init.Semantic$.checkArgs$1(Semantic.scala:591)
	at dotty.tools.dotc.transform.init.Semantic$.call(Semantic.scala:613)
	at dotty.tools.dotc.transform.init.Semantic$Result.call(Semantic.scala:414)
	at dotty.tools.dotc.transform.init.Semantic$.cases(Semantic.scala:1151)
	at dotty.tools.dotc.transform.init.Semantic$.eval$$anonfun$1(Semantic.scala:1071)
	at dotty.tools.dotc.transform.init.Semantic$Cache$Cache.assume(Semantic.scala:314)
	at dotty.tools.dotc.transform.init.Semantic$.eval(Semantic.scala:1071)
	at dotty.tools.dotc.transform.init.Semantic$.init$$anonfun$4(Semantic.scala:1487)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.transform.init.Semantic$.init(Semantic.scala:1488)
	at dotty.tools.dotc.transform.init.Semantic$.cases(Semantic.scala:1265)
	at dotty.tools.dotc.transform.init.Semantic$.eval$$anonfun$1(Semantic.scala:1071)
	at dotty.tools.dotc.transform.init.Semantic$Cache$Cache.assume(Semantic.scala:314)
	at dotty.tools.dotc.transform.init.Semantic$.eval(Semantic.scala:1071)
	at dotty.tools.dotc.transform.init.Semantic$.dotty$tools$dotc$transform$init$Semantic$WorkList$$_$iterate$1(Semantic.scala:1008)
	at dotty.tools.dotc.transform.init.Semantic$WorkList.doTask(Semantic.scala:1019)
	at dotty.tools.dotc.transform.init.Semantic$WorkList.work$$anonfun$1(Semantic.scala:989)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:333)
	at dotty.tools.dotc.transform.init.Semantic$WorkList.work(Semantic.scala:989)
	at dotty.tools.dotc.transform.init.Semantic$.check(Semantic.scala:1033)
	at dotty.tools.dotc.transform.init.Checker.runOn$$anonfun$1(Checker.scala:38)
	at dotty.tools.dotc.transform.init.Semantic$.withInitialState(Semantic.scala:1044)
	at dotty.tools.dotc.transform.init.Checker.runOn(Checker.scala:40)
	at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:225)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1328)
	at dotty.tools.dotc.Run.runPhases$1(Run.scala:236)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:244)
	at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:68)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:253)
	at dotty.tools.dotc.Run.compileSources(Run.scala:186)
	at dotty.tools.dotc.Run.compile(Run.scala:170)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:35)
	at dotty.tools.dotc.Driver.process(Driver.scala:195)
	at dotty.tools.dotc.Driver.process(Driver.scala:163)
	at dotty.tools.dotc.Driver.process(Driver.scala:175)
	at dotty.tools.dotc.Driver.main(Driver.scala:205)
	at dotty.tools.dotc.Main.main(Main.scala)

Expectation

This code should not cause the compiler to throw an exception.

The Comparable trait is defined as the Comparable<T> interface in Java, with its only method being public int compareTo(T o);, so the code above would be expected to be a correct implementation of the trait (assuming we were to provide an actual implementation).

@olhotak @liufengyun