Merge pull request #9120 from dotty-staging/fix-#9052

fix #9052: throw CyclicReference if LazyRef is already set

Author: odersky

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

@@ -490,7 +490,10 @@ object SymDenotations {
         def recur(self: Type): Unit = self match
           case RefinedType(parent, name, rinfo) => rinfo match
             case TypeAlias(lzy: LazyRef) if name == this.name =>
-              lzy.update(tp)
+              if !lzy.completed then
+                lzy.update(tp)
+              else
+                throw CyclicReference(this)
             case _ =>
               recur(parent)
         recur(owner.asClass.givenSelfType)

tests/neg/i9052/A.scala

@@ -0,0 +1,6 @@
+object impl:
+  case object UNone
+
+import impl._
+
+opaque type UOption[+A] = (A | UNone.type) // error: Cyclic Reference involving UOption

tests/neg/i9052/B.scala

@@ -0,0 +1 @@
+val UNone: UOption[Nothing] = ??? // error: Cyclic Reference involving UOption