Transform infos of SkolemTypes in TypeMap

A SkolemType is not a symbol whose info is transformed separately. That's why mapping
a SkolemType must involve mapping its info.

A test case is the new version of pos_valueclasses/t7818.scala.

Author: odersky

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

@@ -4378,6 +4378,8 @@ object Types {
       tp.derivedAnnotatedType(underlying, annot)
     protected def derivedWildcardType(tp: WildcardType, bounds: Type): Type =
       tp.derivedWildcardType(bounds)
+    protected def derivedSkolemType(tp: SkolemType, info: Type): Type =
+      tp.derivedSkolemType(info)
     protected def derivedClassInfo(tp: ClassInfo, pre: Type): Type =
       tp.derivedClassInfo(pre)
     protected def derivedJavaArrayType(tp: JavaArrayType, elemtp: Type): Type =
@@ -4475,7 +4477,7 @@ object Types {
           derivedMatchType(tp, this(tp.bound), this(tp.scrutinee), tp.cases.mapConserve(this))
 
         case tp: SkolemType =>
-          tp
+          derivedSkolemType(tp, this(tp.info))
 
         case tp @ AnnotatedType(underlying, annot) =>
           val underlying1 = this(underlying)
@@ -4756,6 +4758,13 @@ object Types {
       tp.derivedWildcardType(rangeToBounds(bounds))
     }
 
+    override protected def derivedSkolemType(tp: SkolemType, info: Type): Type = info match {
+      case Range(lo, hi) =>
+        range(tp.derivedSkolemType(lo), tp.derivedSkolemType(hi))
+      case _ =>
+        tp.derivedSkolemType(info)
+    }
+
     override protected def derivedClassInfo(tp: ClassInfo, pre: Type): Type = {
       assert(!isRange(pre))
         // we don't know what to do here; this case has to be handled in subclasses

tests/pos/pos_valueclasses/t7818.scala

@@ -7,6 +7,11 @@ class Observable1[+T](val asJava: JObservable[_ <: T]) extends AnyVal {
   // to the typer parameter of the extension method into which the RHS is
   // transplanted.
   def synchronize: Observable1[T] = new Observable1(foo(asJava))
+
+  // Was generating a Ycheck error after ExtensionMethods.
+  // Fixed by having TypeMap go over info of SkolemTypes
+  private[this] def id(x: JObservable[_ <: T]) = x
+  def synchronize2: Observable1[T] = new Observable1(foo(id(asJava)))
 }
 
 class JObservable[T]