Fix #2088: Test for simple cycle in type aliases

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

@@ -208,7 +208,9 @@ 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 =>
+              (!sym.is(Private) && prefix.derivesFrom(sym.owner)) ||
+              (!prefix.symbol.isStaticOwner && 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 +1,23 @@
+class Base {
+  type A = { // error: illegal cyclic reference: alias Object{m: Foo.B} of type B refers back to the type itself
+    val m: Foo.A
+  }
+
+  protected type B = { // error: illegal cyclic reference: alias Object{m: Foo.B} of type B refers back to the type itself
+    val m: Foo.B
+  }
+
+  private type C = {
+    val m: Foo.C // error: type `C` is not a member of Foo.type
+  }
+
+  type D = { // error: illegal cyclic reference: alias Object{m: Foo.E} of type D refers back to the type itself
+    val m: Foo.E
+  }
+
+  type E = {
+    val m: Foo.D
+  }
+}
+
+object Foo extends Base