Test case and fix typos

Author: odersky

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

@@ -3024,7 +3024,7 @@ object Types {
             tp.superType.applyIfParameterized(args)
           case _ => defn.AnyType
         }
-        validSuper = ctx.period
+        if (canCache) validSuper = ctx.period
       }
       cachedSuper
     }

tests/pos/i2888.scala

@@ -0,0 +1,13 @@
+trait Foo[A, CC[X] <: Foo[X, CC, CC[X]], C <: CC[A]] {
+
+  def cc: CC[A]
+
+  def foo: Unit = ()
+
+  def bar: Unit = cc.foo
+
+}
+
+object Main {
+  def main(args: Array[String]): Unit = ()
+}