Fix #2391: Make child annotation lazy

compiler/src/dotty/tools/dotc/core/Annotations.scala

@@ -141,9 +141,15 @@ object Annotations {
       apply(defn.AliasAnnot, List(
         ref(TermRef.withSigAndDenot(sym.owner.thisType, sym.name, sym.signature, sym))))
 
-    def makeChild(sym: Symbol)(implicit ctx: Context) =
-      deferred(defn.ChildAnnot,
-        implicit ctx => New(defn.ChildAnnotType.appliedTo(sym.owner.thisType.select(sym.name, sym)), Nil))
+    def makeChild(delayedSym: Context => Symbol)(implicit ctx: Context): Annotation = {
+      def makeChildLater(implicit ctx: Context) = {
+        val sym = delayedSym(ctx)
+        New(defn.ChildAnnotType.appliedTo(sym.owner.thisType.select(sym.name, sym)), Nil)
+      }
+      deferred(defn.ChildAnnot, implicit ctx => makeChildLater(ctx))
+    }
+
+    def makeChild(sym: Symbol)(implicit ctx: Context): Annotation = makeChild(_ => sym)
 
     def makeSourceFile(path: String)(implicit ctx: Context) =
       apply(defn.SourceFileAnnot, Literal(Constant(path)))

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -829,8 +829,13 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
     assert(tag == CHILDREN)
     val end = readNat() + readIndex
     val target = readSymbolRef()
-    while (readIndex != end)
-      target.addAnnotation(Annotation.makeChild(readSymbolRef()))
+    while (readIndex != end) {
+      val start = readIndex
+      readNat() // skip reference for now
+      target.addAnnotation(
+          Annotation.makeChild(implicit ctx =>
+              atReadPos(start, () => readSymbolRef())))
+    }
   }
 
   /* Read a reference to a pickled item */

tests/new/projection.scala

@@ -0,0 +1,4 @@
+class C { type T }
+object test {
+  def x: C#T = ???
+}

tests/pos/i2391/Containers.scala

@@ -0,0 +1,10 @@
+// Containers.scala
+package foo
+
+trait ParentContainer {
+  sealed trait Entry
+}
+
+class ChildContainer extends ParentContainer {
+  trait LazyEntry extends Entry
+}

tests/pos/i2391/User.scala

@@ -0,0 +1,6 @@
+// User.scala
+package foo
+
+trait User {
+  type Entry <: ChildContainer#Entry
+}