Ycheck error in FirstTransform

[abeln]

class Test {
  def foo(): Unit = {
    val x: String|Null = ???
    if (x != null) {
      val y = x.length
    } else {
      val y = x
    }
  }
}

dotc -Ycheck:firstTransform kk.scala

Expected: the code should typecheck.
Got: exception

} of class class dotty.tools.dotc.ast.Trees$PackageDef # 1147
*** error while checking kk.scala after phase completeJavaEnums ***
exception occurred while compiling kk.scala
java.lang.AssertionError: assertion failed: non member selection of method length in class String from (String | Null)(x) in x.length while compiling kk.scala
Exception in thread "main" java.lang.AssertionError: assertion failed: non member selection of method length in class String from (String | Null)(x) in x.length
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:16)
	at dotty.tools.dotc.transform.FirstTransform.checkPostCondition(FirstTransform.scala:53)
	at dotty.tools.dotc.transform.TreeChecker.dotty$tools$dotc$transform$TreeChecker$Checker$$_$typedUnadapted$$anonfun$1(TreeChecker.scala:284)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:392)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typedUnadapted(TreeChecker.scala:284)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2239)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2251)
	at dotty.tools.dotc.transform.TreeChecker$Checker.typed(TreeChecker.scala:255)

If we look at the failed assertion, it's

  override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = {
    tree match {
      case Select(qual, name) if !name.is(OuterSelectName) && tree.symbol.exists =>
        assert(
          qual.tpe.derivesFrom(tree.symbol.owner) ||
            tree.symbol.is(JavaStatic) && qual.tpe.derivesFrom(tree.symbol.enclosingClass),
          i"non member selection of ${tree.symbol.showLocated} from ${qual.tpe} in $tree")

Here qual.tpe is String|Null while tree.symbol.owner is String.
I think the problem is that probably String|Null <: String, but Null does not deriveFrom String.
Indeed, looking at SymDenotations it seems like Null is special-cased in isSubClass but not in derivesFrom. This seems to be by design

    /** Is this a subclass of the given class `base`? */
    def isSubClass(base: Symbol)(implicit ctx: Context): Boolean = false

    /** Is this a subclass of `base`,
     *  and is the denoting symbol also different from `Null` or `Nothing`?
     *  @note  erroneous classes are assumed to derive from all other classes
     *         and all classes derive from them.
     */
    def derivesFrom(base: Symbol)(implicit ctx: Context): Boolean = fals

So maybe we should just switch to using isSubClass?

[liufengyun]

As it's in Ycheck, I think it's fine to create a special case for nullable types.

[nicolasstucki]

It works with -Yexplicit-nulls

[griggt]

Fixed by 2b52368 in #10014