Fix #2088: Test for simple cycle in type aliases

Author: nicolasstucki

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

@@ -206,7 +206,9 @@ object Checking {
           def isInteresting(prefix: Type): Boolean = prefix.stripTypeVar match {
             case NoPrefix => true
             case prefix: ThisType => sym.owner.isClass && prefix.cls.isContainedIn(sym.owner)
-            case prefix: NamedType => !prefix.symbol.isStaticOwner && isInteresting(prefix.prefix)
+            case prefix: NamedType =>
+              !prefix.symbol.isStaticOwner &&
+              (pre.derivesFrom(sym.owner) || isInteresting(prefix.prefix))
             case SuperType(thistp, _) => isInteresting(thistp)
             case AndType(tp1, tp2) => isInteresting(tp1) || isInteresting(tp2)
             case OrType(tp1, tp2) => isInteresting(tp1) && isInteresting(tp2)

tests/neg/cycles.scala

@@ -23,7 +23,7 @@ class C {
 
 class E {
   class F {
-    type T <: x.type // old-error: not stable
+    type T <: x.type // error
     val z: x.type = ??? // old-error: not stable
   }
   lazy val x: F#T = ???

tests/neg/i2088.scala

@@ -0,0 +1,7 @@
+class Base {
+  type A = { // error
+    val m: Foo.A
+  }
+}
+
+object Foo extends Base