Fix scala#9069: Complete children of sealed classes early

Complete children of sealed classes or traits when the parent class is completed.

Author: odersky

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

@@ -1280,6 +1280,24 @@ class Namer { typer: Typer =>
         }
       }
 
+      def completeChildren() =
+
+        def completeChildrenIn(owner: Symbol) =
+          def maybeChild(c: Symbol) =
+            !owner.is(Package)
+            || c.infoOrCompleter.isInstanceOf[ClassCompleter]
+              && c.associatedFile == cls.associatedFile
+          for c <- owner.info.decls.toList if c.isClass && maybeChild(c) do
+            //println(i"possible child of $cls: $c")
+            c.ensureCompleted()
+
+        if cls.is(Sealed) then
+          //println(i"completing children of $cls in ${cls.owner}")
+          completeChildrenIn(cls.owner)
+          completeChildrenIn(cls.companionClass)
+          //println(i"completed children of $cls")
+      end completeChildren
+
       completeConstructor(denot)
       denot.info = tempInfo
 
@@ -1307,6 +1325,7 @@ class Namer { typer: Typer =>
       cls.setNoInitsFlags(parentsKind(parents), untpd.bodyKind(rest))
       if (cls.isNoInitsClass) cls.primaryConstructor.setFlag(StableRealizable)
       processExports(localCtx)
+      completeChildren()
     }
   }
 

tests/pos/i9069/Test.scala

@@ -0,0 +1,2 @@
+def Test =
+  summon[deriving.Mirror.SumOf[Foo]]

tests/pos/i9069/Wrapper.scala

@@ -0,0 +1,2 @@
+sealed trait Foo
+case class Bar(x: Int) extends Foo