Fix #6159: Fix opaque alias

Author: odersky

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

@@ -1084,8 +1084,7 @@ object SymDenotations {
           case RefinedType(_, _, TypeBounds(lo, _)) =>
             def extractAlias(tp: Type): Type = tp match {
               case OrType(alias, _) => alias
-              case HKTypeLambda(tparams, tp) =>
-                HKTypeLambda(tparams.map(_.paramInfo), extractAlias(tp))
+              case tp: HKTypeLambda => tp.derivedLambdaType(resType = extractAlias(tp.resType))
             }
             extractAlias(lo)
         }

tests/pos/i6159.scala

@@ -0,0 +1,14 @@
+trait A
+trait B extends A
+
+object O {
+  opaque type T[X <: A] = X
+  type U = T.U
+  object T{
+    type U[X <: A] = X
+    def t(a: T[B]): T[B] = a
+    def u(a: U[B]): U[B] = a
+  }
+  def t(a: T[B]): T[B] = a
+  def u(a: T.U[B]): T.U[B] = a
+}