Index members of a class before evaluating its parents

Avoids missing member in tangledCompanion.scala, which is a minimization
of intermittent failures in CollectionStrawMan6. Intermittent, because it
depended on order of compilation. CollectionTests have to be compiled together with
but before CollectionStrawMan6 (this was _sometimes_ the case because partest did not
honor indicated compilation order so far). I.e.

    dotc CollectionTests_2.scala CollectionStrawMan6_1.scala

would trigger the error. tangledCompanion.scala captures the dependencies
in a single file.

Author: odersky

src/dotty/tools/dotc/typer/Namer.scala

@@ -698,13 +698,14 @@ class Namer { typer: Typer =>
       // the parent types are elaborated.
       index(constr)
       symbolOfTree(constr).ensureCompleted()
+      
+      index(rest)(inClassContext(selfInfo))
 
       val tparamAccessors = decls.filter(_ is TypeParamAccessor).toList
       val parentTypes = ensureFirstIsClass(parents.map(checkedParentType(_, tparamAccessors)))
       val parentRefs = ctx.normalizeToClassRefs(parentTypes, cls, decls)
       typr.println(s"completing $denot, parents = $parents, parentTypes = $parentTypes, parentRefs = $parentRefs")
 
-      index(rest)(inClassContext(selfInfo))
       tempInfo.finalize(denot, parentRefs)
 
       Checking.checkWellFormed(cls)

tests/pos/tangledCompanion.scala

@@ -0,0 +1,26 @@
+object Test {
+
+  import Coll._
+  import LazyList.#::
+
+  val xs = LazyList.Empty
+
+}
+
+object Coll {
+
+  trait IterableFactory[+C[X]]
+
+  class LazyList[+A](expr: => LazyList.Evaluated[A])
+
+  object LazyList extends IterableFactory[LazyList] {
+
+    type Evaluated[+A] = Option[(A, LazyList[A])]
+
+    object Empty extends LazyList[Nothing](None)
+
+    object #:: {
+      def unapply[A](s: LazyList[A]): Evaluated[A] = ???
+    }
+  }
+}