Fix #9717: Fix typed tree copier for Selects over Nothing

Typed tree copier would assign a `a.b` where `a` has type `Nothing`
the type `Nothing`. This is wrong, for instance when `b` is $asInstanceOf$
over a value of type `Nothing`, which is the case in i9717.scala

Author: odersky

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -584,7 +584,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
           tree1.withTypeUnchecked(tree.tpe)
         case _ =>
           val tree2 = tree.tpe match {
-            case tpe: NamedType => tree1.withType(tpe.derivedSelect(qualifier.tpe.widenIfUnstable))
+            case tpe: NamedType =>
+              val qualType = qualifier.tpe.widenIfUnstable
+              if qualType.isBottomType then tree1.withTypeUnchecked(tree.tpe)
+              else tree1.withType(tpe.derivedSelect(qualType))
             case _ => tree1.withTypeUnchecked(tree.tpe)
           }
           ConstFold(tree2)

tests/pos/i9717/Test.scala

@@ -0,0 +1,4 @@
+// minimized facade
+object Object {
+  def assign[T, U](target: T, source: U): T with U = js.native
+}

tests/pos/i9717/js.scala

@@ -0,0 +1,4 @@
+// minimized scala.js
+package object js {
+  def native: Nothing = ???
+}