diff --git a/src/dotty/tools/dotc/ast/tpd.scala b/src/dotty/tools/dotc/ast/tpd.scala
index fecfefd37ac4..37671d537159 100644
--- a/src/dotty/tools/dotc/ast/tpd.scala
+++ b/src/dotty/tools/dotc/ast/tpd.scala
@@ -8,6 +8,7 @@ import SymDenotations._, Symbols._, StdNames._, Annotations._, Trees._, Symbols.
 import CheckTrees._, Denotations._, Decorators._
 import config.Printers._
 import typer.ErrorReporting._
+import scala.annotation.tailrec
 
 /** Some creators for typed trees */
 object tpd extends Trees.Instance[Type] with TypedTreeInfo {
@@ -413,6 +414,92 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     def tpes: List[Type] = xs map (_.tpe)
   }
 
+  /** A tree map that retypes some nodes if their element types have changed,
+   *  instead of simply copying the original type. The potential retyped nodes
+   *  are those nodes where the element type may be part of the parent type.
+   */
+  class RetypingTreeMap extends TreeMap {
+    def retypeSelect(tree: Select, qualifier: Tree, name: Name)(implicit ctx: Context) = {
+      val tree1 = cpy.Select(tree, qualifier, name)
+      if ((tree1 eq tree) || (qualifier.tpe eq tree.qualifier.tpe)) tree1
+      else (tree1.tpe match {
+        case tpe: NamedType => tree1.withType(tpe.derivedSelect(qualifier.tpe))
+        case _ => tree1
+      })
+    }
+    def retypePair(tree: Pair, left: Tree, right: Tree)(implicit ctx: Context) = {
+      val tree1 = cpy.Pair(tree, left, right)
+      if ((tree1 eq tree) || ((left.tpe eq tree.left.tpe) && (right.tpe eq tree.right.tpe))) tree1
+      else ta.assignType(tree1, left, right)
+    }
+    def retypeBlock(tree: Block, stats: List[Tree], expr: Tree)(implicit ctx: Context) = {
+      val tree1 = cpy.Block(tree, stats, expr)
+      if ((tree1 eq tree) || (expr.tpe eq tree.expr.tpe)) tree1
+      else ta.assignType(tree1, stats, expr)
+    }
+    def retypeIf(tree: If, cond: Tree, thenp: Tree, elsep: Tree)(implicit ctx: Context) = {
+      val tree1 = cpy.If(tree, cond, thenp, elsep)
+      if ((tree1 eq tree) || (thenp.tpe eq tree.thenp.tpe) && (elsep.tpe eq tree.elsep.tpe)) tree1
+      else ta.assignType(tree1, thenp, elsep)
+    }
+    def retypeMatch(tree: Match, selector: Tree, cases: List[CaseDef])(implicit ctx: Context) = {
+      val tree1 = cpy.Match(tree, selector, cases)
+      if ((tree1 eq tree) || sameTypes(cases, tree.cases)) tree1
+      else ta.assignType(tree1, cases)
+    }
+    def retypeCaseDef(tree: CaseDef, pat: Tree, guard: Tree, body: Tree)(implicit ctx: Context) = {
+      val tree1 = cpy.CaseDef(tree, pat, guard, body)
+      if ((tree eq tree1) || (body.tpe eq tree.body.tpe)) tree1
+      else ta.assignType(tree1, body)
+    }
+    def retypeTry(tree: Try, expr: Tree, handler: Tree, finalizer: Tree)(implicit ctx: Context) = {
+      val tree1 = cpy.Try(tree, expr, handler, finalizer)
+      if ((tree1 eq tree) || ((expr.tpe eq tree.expr.tpe) && (handler.tpe eq tree.handler.tpe))) tree
+      else ta.assignType(tree1, expr, handler)
+    }
+    def retypeSeqLiteral(tree: SeqLiteral, elems: List[Tree])(implicit ctx: Context) = {
+      val tree1 = cpy.SeqLiteral(tree, elems)
+      if ((tree1 eq tree) || sameTypes(elems, tree.elems)) tree1
+      else ta.assignType(tree1, elems)
+    }
+    def retypeAnnotated(tree: Annotated, annot: Tree, arg: Tree)(implicit ctx: Context) = {
+      val tree1 = cpy.Annotated(tree, annot, arg)
+      if ((tree1 eq tree) || (arg.tpe eq tree.arg.tpe) && (annot eq tree.annot)) tree1
+      else ta.assignType(tree1, annot, arg)
+    }
+    override def transform(tree: Tree)(implicit ctx: Context): Tree = tree match {
+      case tree: Ident => // left here for performance
+        super.transform(tree)
+      case tree @ Select(qualifier, name) =>
+        retypeSelect(tree, transform(qualifier), name)
+      case tree @ Pair(left, right) =>
+        retypePair(tree, transform(left), transform(right))
+      case tree @ Block(stats, expr) =>
+        retypeBlock(tree, transformStats(stats), transform(expr))
+      case tree @ If(cond, thenp, elsep) =>
+        retypeIf(tree, transform(cond), transform(thenp), transform(elsep))
+      case tree @ Match(selector, cases) =>
+        retypeMatch(tree, transform(selector), transformSub(cases))
+      case tree @ CaseDef(pat, guard, body) =>
+        retypeCaseDef(tree, transform(pat), transform(guard), transform(body))
+      case tree @ Try(block, handler, finalizer) =>
+        retypeTry(tree, transform(block), transform(handler), transform(finalizer))
+      case tree @ SeqLiteral(elems) =>
+        retypeSeqLiteral(tree, transform(elems))
+      case tree @ Annotated(annot, arg) =>
+        retypeAnnotated(tree, transform(annot), transform(arg))
+      case _ =>
+        super.transform(tree)
+    }
+
+    @tailrec
+    final def sameTypes(trees: List[tpd.Tree], trees1: List[tpd.Tree]): Boolean = {
+      if (trees.isEmpty) trees.isEmpty
+      else if (trees1.isEmpty) trees.isEmpty
+      else (trees.head.tpe eq trees1.head.tpe) && sameTypes(trees.tail, trees1.tail)
+    }
+  }
+
   /** A map that applies three functions together to a tree and makes sure
    *  they are coordinated so that the result is well-typed. The functions are
    *  @param  typeMap  A function from Type to type that gets applied to the
@@ -425,7 +512,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   final class TreeTypeMap(
       val typeMap: Type => Type = IdentityTypeMap,
       val ownerMap: Symbol => Symbol = identity _,
-      val treeMap: Tree => Tree = identity _)(implicit ctx: Context) extends TreeMap {
+      val treeMap: Tree => Tree = identity _)(implicit ctx: Context) extends RetypingTreeMap {
 
     override def transform(tree: tpd.Tree)(implicit ctx: Context): tpd.Tree = {
       val tree1 = treeMap(tree)
@@ -436,10 +523,10 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
           cpy.DefDef(ddef, mods, name, tparams1, vparamss1, tmap2.transform(tpt), tmap2.transform(rhs))
         case blk @ Block(stats, expr) =>
           val (tmap1, stats1) = transformDefs(stats)
-          cpy.Block(blk, stats1, tmap1.transform(expr))
+          retypeBlock(blk, stats1, tmap1.transform(expr))
         case cdef @ CaseDef(pat, guard, rhs) =>
           val tmap = withMappedSyms(patVars(pat))
-          cpy.CaseDef(cdef, tmap.transform(pat), tmap.transform(guard), tmap.transform(rhs))
+          retypeCaseDef(cdef, tmap.transform(pat), tmap.transform(guard), tmap.transform(rhs))
         case tree1 =>
           super.transform(tree1)
       }
diff --git a/src/dotty/tools/dotc/transform/FullParameterization.scala b/src/dotty/tools/dotc/transform/FullParameterization.scala
index e21a10492f4f..3a40f9f3ef53 100644
--- a/src/dotty/tools/dotc/transform/FullParameterization.scala
+++ b/src/dotty/tools/dotc/transform/FullParameterization.scala
@@ -142,7 +142,7 @@ trait FullParameterization {
    *  followed by the class parameters of its enclosing class.
    */
   private def allInstanceTypeParams(originalDef: DefDef)(implicit ctx: Context): List[Symbol] =
-    originalDef.tparams.map(_.symbol) ::: originalDef.symbol.owner.typeParams
+    originalDef.tparams.map(_.symbol) ::: originalDef.symbol.enclosingClass.typeParams
 
   /** Given an instance method definition `originalDef`, return a
    *  fully parameterized method definition derived from `originalDef`, which
@@ -152,7 +152,7 @@ trait FullParameterization {
   def fullyParameterizedDef(derived: TermSymbol, originalDef: DefDef)(implicit ctx: Context): Tree =
     polyDefDef(derived, trefs => vrefss => {
       val origMeth = originalDef.symbol
-      val origClass = origMeth.owner.asClass
+      val origClass = origMeth.enclosingClass.asClass
       val origTParams = allInstanceTypeParams(originalDef)
       val origVParams = originalDef.vparamss.flatten map (_.symbol)
       val thisRef :: argRefs = vrefss.flatten
@@ -219,7 +219,7 @@ trait FullParameterization {
   def forwarder(derived: TermSymbol, originalDef: DefDef)(implicit ctx: Context): Tree =
     ref(derived.termRef)
       .appliedToTypes(allInstanceTypeParams(originalDef).map(_.typeRef))
-      .appliedTo(This(originalDef.symbol.owner.asClass))
+      .appliedTo(This(originalDef.symbol.enclosingClass.asClass))
       .appliedToArgss(originalDef.vparamss.nestedMap(vparam => ref(vparam.symbol)))
       .withPos(originalDef.rhs.pos)
 }
\ No newline at end of file
diff --git a/src/dotty/tools/dotc/transform/TreeChecker.scala b/src/dotty/tools/dotc/transform/TreeChecker.scala
index 87e84e169a10..57a2b17075f5 100644
--- a/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -94,7 +94,7 @@ class TreeChecker {
           ownerMatches(symOwner, ctxOwner.owner)
       for (tree <- trees if tree.isDef)
         assert(ownerMatches(tree.symbol.owner, ctx.owner),
-               i"bad owner; $tree has owner ${tree.symbol.owner}, expected was ${ctx.owner}")
+               i"bad owner; $tree has owner ${tree.symbol.owner.showLocated}, expected was ${ctx.owner.showLocated}")
       super.index(trees)
     }
   }