diff --git a/src/dotty/tools/dotc/config/Printers.scala b/src/dotty/tools/dotc/config/Printers.scala
index 84157f3a9212..e236e34e0608 100644
--- a/src/dotty/tools/dotc/config/Printers.scala
+++ b/src/dotty/tools/dotc/config/Printers.scala
@@ -14,6 +14,7 @@ object Printers {
   val core: Printer = noPrinter
   val typr: Printer = noPrinter
   val constr: Printer = noPrinter
+  val checks: Printer = noPrinter
   val overload: Printer = noPrinter
   val implicits: Printer = noPrinter
   val implicitsDetailed: Printer = noPrinter
diff --git a/src/dotty/tools/dotc/core/Flags.scala b/src/dotty/tools/dotc/core/Flags.scala
index 0678b79beb06..d150bd6ab980 100644
--- a/src/dotty/tools/dotc/core/Flags.scala
+++ b/src/dotty/tools/dotc/core/Flags.scala
@@ -422,7 +422,7 @@ object Flags {
   final val RetainedTypeArgFlags = VarianceFlags | ExpandedName | Protected | Local
 
   /** Modules always have these flags set */
-  final val ModuleCreationFlags = ModuleVal
+  final val ModuleCreationFlags = ModuleVal | Final | Stable
 
   /** Module classes always have these flags set */
   final val ModuleClassCreationFlags = ModuleClass | Final
diff --git a/src/dotty/tools/dotc/core/SymDenotations.scala b/src/dotty/tools/dotc/core/SymDenotations.scala
index 5fbf6c2c423b..b9eb2f945471 100644
--- a/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -355,7 +355,7 @@ object SymDenotations {
     final def isStable(implicit ctx: Context) = {
       val isUnstable =
         (this is UnstableValue) ||
-        info.isVolatile && !hasAnnotation(defn.uncheckedStableClass)
+        ctx.isVolatile(info) && !hasAnnotation(defn.uncheckedStableClass)
       (this is Stable) || isType || {
         if (isUnstable) false
         else { setFlag(Stable); true }
diff --git a/src/dotty/tools/dotc/core/TypeOps.scala b/src/dotty/tools/dotc/core/TypeOps.scala
index ca69ab6156ff..152f8d1b7a12 100644
--- a/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/src/dotty/tools/dotc/core/TypeOps.scala
@@ -3,6 +3,8 @@ package core
 
 import Contexts._, Types._, Symbols._, Names._, Flags._, Scopes._
 import SymDenotations._
+import config.Printers._
+import Decorators._
 import util.SimpleMap
 
 trait TypeOps { this: Context =>
@@ -74,8 +76,29 @@ trait TypeOps { this: Context =>
     def apply(tp: Type) = simplify(tp, this)
   }
 
+  /** A type is volatile if its DNF contains an alternative of the form
+   *  {P1, ..., Pn}, {N1, ..., Nk}, where the Pi are parent typerefs and the
+   *  Nj are refinement names, and one the 4 following conditions is met:
+   *
+   *  1. At least two of the parents Pi are abstract types.
+   *  2. One of the parents Pi is an abstract type, and one other type Pj,
+   *     j != i has an abstract member which has the same name as an
+   *     abstract member of the whole type.
+   *  3. One of the parents Pi is an abstract type, and one of the refinement
+   *     names Nj refers to an abstract member of the whole type.
+   *  4. One of the parents Pi is an an alias type with a volatile alias
+   *     or an abstract type with a volatile upper bound.
+   *
+   *  Lazy values are not allowed to have volatile type, as otherwise
+   *  unsoundness can result.
+   */
   final def isVolatile(tp: Type): Boolean = {
-    /** Pre-filter to avoid expensive DNF computation */
+
+    /** Pre-filter to avoid expensive DNF computation
+     *  If needsChecking returns false it is guaranteed that
+     *  DNF does not contain intersections, or abstract types with upper
+     *  bounds that themselves need checking.
+     */
     def needsChecking(tp: Type, isPart: Boolean): Boolean = tp match {
       case tp: TypeRef =>
         tp.info match {
@@ -88,31 +111,62 @@ trait TypeOps { this: Context =>
         needsChecking(tp.parent, true)
       case tp: TypeProxy =>
         needsChecking(tp.underlying, isPart)
-      case AndType(l, r) =>
-        needsChecking(l, true) || needsChecking(r, true)
-      case OrType(l, r) =>
-        isPart || needsChecking(l, isPart) && needsChecking(r, isPart)
+      case tp: AndType =>
+        true
+      case tp: OrType =>
+        isPart || needsChecking(tp.tp1, isPart) && needsChecking(tp.tp2, isPart)
       case _ =>
         false
     }
+
     needsChecking(tp, false) && {
-      tp.DNF forall { case (parents, refinedNames) =>
+      DNF(tp) forall { case (parents, refinedNames) =>
         val absParents = parents filter (_.symbol is Deferred)
-        absParents.size >= 2 || {
-          val ap = absParents.head
-          ((parents exists (p =>
-                (p ne ap)
-             || p.memberNames(abstractTypeNameFilter, tp).nonEmpty
-             || p.memberNames(abstractTermNameFilter, tp).nonEmpty))
-          || (refinedNames & tp.memberNames(abstractTypeNameFilter, tp)).nonEmpty
-          || (refinedNames & tp.memberNames(abstractTermNameFilter, tp)).nonEmpty
-          || isVolatile(ap)
-          )
+        absParents.nonEmpty && {
+          absParents.lengthCompare(2) >= 0 || {
+            val ap = absParents.head
+            ((parents exists (p =>
+              (p ne ap)
+              || p.memberNames(abstractTypeNameFilter, tp).nonEmpty
+              || p.memberNames(abstractTermNameFilter, tp).nonEmpty))
+            || (refinedNames & tp.memberNames(abstractTypeNameFilter, tp)).nonEmpty
+            || (refinedNames & tp.memberNames(abstractTermNameFilter, tp)).nonEmpty
+            || isVolatile(ap))
+          }
         }
       }
     }
   }
 
+  /** The disjunctive normal form of this type.
+   *  This collects a set of alternatives, each alternative consisting
+   *  of a set of typerefs and a set of refinement names. Both sets are represented
+   *  as lists, to obtain a deterministic order. Collected are
+   *  all type refs reachable by following aliases and type proxies, and
+   *  collecting the elements of conjunctions (&) and disjunctions (|).
+   *  The set of refinement names in each alternative
+   *  are the set of names in refinement types encountered during the collection.
+   */
+  final def DNF(tp: Type): List[(List[TypeRef], Set[Name])] = ctx.traceIndented(s"DNF($this)", checks) {
+    tp.dealias match {
+      case tp: TypeRef =>
+        (tp :: Nil, Set[Name]()) :: Nil
+      case RefinedType(parent, name) =>
+        for ((ps, rs) <- DNF(parent)) yield (ps, rs + name)
+      case tp: TypeProxy =>
+        DNF(tp.underlying)
+      case AndType(l, r) =>
+        for ((lps, lrs) <- DNF(l); (rps, rrs) <- DNF(r))
+          yield (lps | rps, lrs | rrs)
+      case OrType(l, r) =>
+        DNF(l) | DNF(r)
+      case tp =>
+        TypeOps.emptyDNF
+    }
+  }
+
+
+
   private def enterArgBinding(formal: Symbol, info: Type, cls: ClassSymbol, decls: Scope) = {
     val lazyInfo = new LazyType { // needed so we do not force `formal`.
       def complete(denot: SymDenotation)(implicit ctx: Context): Unit = {
@@ -215,6 +269,6 @@ trait TypeOps { this: Context =>
 }
 
 object TypeOps {
-
+  val emptyDNF = (Nil, Set[Name]()) :: Nil
   var track = false // !!!DEBUG
 }
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index bb30d9a9c7a6..67dc13775c3d 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -106,7 +106,8 @@ object Types {
       classSymbol.derivesFrom(cls)
 
    /** A type T is a legal prefix in a type selection T#A if
-     *  T is stable or T contains no uninstantiated type variables.
+     *  T is stable or T contains no abstract types
+     *  !!! Todo: What about non-final vals that contain abstract types?
      */
     final def isLegalPrefix(implicit ctx: Context): Boolean =
       isStable || memberNames(abstractTypeNameFilter).isEmpty
@@ -127,25 +128,6 @@ object Types {
     /** Is some part of this type produced as a repair for an error? */
     final def isErroneous(implicit ctx: Context): Boolean = existsPart(_.isError)
 
-    /** A type is volatile if its DNF contains an alternative of the form
-     *  {P1, ..., Pn}, {N1, ..., Nk}, where the Pi are parent typerefs and the
-     *  Nj are refinement names, and one the 4 following conditions is met:
-     *
-     *  1. At least two of the parents Pi are abstract types.
-     *  2. One of the parents Pi is an abstract type, and one other type Pj,
-     *     j != i has an abstract member which has the same name as an
-     *     abstract member of the whole type.
-     *  3. One of the parents Pi is an abstract type, and one of the refinement
-     *     names Nj refers to an abstract member of the whole type.
-     *  4. One of the parents Pi is an abstract type with a volatile upper bound.
-     *
-     *  Lazy values are not allowed to have volatile type, as otherwise
-     *  unsoundness can result.
-     */
-    final def isVolatile(implicit ctx: Context): Boolean = track("isVolatile") {
-      ctx.isVolatile(this)
-    }
-
     /** Does the type carry an annotation that is an instance of `cls`? */
     final def hasAnnotation(cls: ClassSymbol)(implicit ctx: Context): Boolean = stripTypeVar match {
       case AnnotatedType(annot, tp) => (annot matches cls) || (tp hasAnnotation cls)
@@ -741,31 +723,6 @@ object Types {
     def typeParamNamed(name: TypeName)(implicit ctx: Context): Symbol =
       classSymbol.decls.lookup(name) orElse member(name).symbol
 
-    /** The disjunctive normal form of this type.
-     *  This collects a set of alternatives, each alternative consisting
-     *  of a set of typerefs and a set of refinement names. Both sets are represented
-     *  as lists, to obtain a deterministic order. Collected are
-     *  all type refs reachable by following aliases and type proxies, and
-     *  collecting the elements of conjunctions (&) and disjunctions (|).
-     *  The set of refinement names in each alternative
-     *  are the set of names in refinement types encountered during the collection.
-     */
-    final def DNF(implicit ctx: Context): List[(List[TypeRef], Set[Name])] = dealias match {
-      case tp: TypeRef =>
-        (tp :: Nil, Set[Name]()) :: Nil
-      case RefinedType(parent, name) =>
-        for ((ps, rs) <- parent.DNF) yield (ps, rs + name)
-      case tp: TypeProxy =>
-        tp.underlying.DNF
-      case AndType(l, r) =>
-        for ((lps, lrs) <- l.DNF; (rps, rrs) <- r.DNF)
-        yield (lps | rps, lrs | rrs)
-      case OrType(l, r) =>
-        l.DNF | r.DNF
-      case tp =>
-        emptyDNF
-    }
-
 // ----- Substitutions -----------------------------------------------------
 
     /** Substitute all types that refer in their symbol attribute to
diff --git a/src/dotty/tools/dotc/typer/Checking.scala b/src/dotty/tools/dotc/typer/Checking.scala
index 25e6a7aa7182..ecf3eb9bca0c 100644
--- a/src/dotty/tools/dotc/typer/Checking.scala
+++ b/src/dotty/tools/dotc/typer/Checking.scala
@@ -23,6 +23,7 @@ trait NoChecking {
   def checkValue(tree: Tree, proto: Type)(implicit ctx: Context): tree.type = tree
   def checkBounds(args: List[tpd.Tree], poly: PolyType, pos: Position)(implicit ctx: Context): Unit = ()
   def checkStable(tp: Type, pos: Position)(implicit ctx: Context): Unit = ()
+  def checkLegalPrefix(tp: Type, pos: Position)(implicit ctx: Context): Unit = ()
   def checkClassTypeWithStablePrefix(tp: Type, pos: Position, traitReq: Boolean)(implicit ctx: Context): Type = tp
   def checkImplicitTptNonEmpty(defTree: untpd.ValOrDefDef)(implicit ctx: Context): Unit = ()
   def checkImplicitParamsNotSingletons(vparamss: List[List[ValDef]])(implicit ctx: Context): Unit = ()
@@ -53,13 +54,17 @@ trait Checking extends NoChecking {
       if (!(bounds.lo <:< arg.tpe)) notConforms("lower", bounds.lo)
     }
 
-  /** Check that type `tp` is stable.
+  /** Check that type `tp` is stable. */
+  override def checkStable(tp: Type, pos: Position)(implicit ctx: Context): Unit =
+    if (!tp.isStable) ctx.error(i"$tp is not stable", pos)
+
+  /** Check that type `tp` is a legal prefix for '#'.
    *  @return The type itself
    */
-  override def checkStable(tp: Type, pos: Position)(implicit ctx: Context): Unit =
-    if (!tp.isStable) ctx.error(i"Prefix of type ${tp.widenIfUnstable} is not stable", pos)
+  override def checkLegalPrefix(tp: Type, pos: Position)(implicit ctx: Context): Unit =
+    if (!tp.isLegalPrefix) ctx.error(i"$tp is not a valid prefix for '#'", pos)
 
-  /** Check that `tp` is a class type with a stable prefix. Also, if `isFirst` is
+ /** Check that `tp` is a class type with a stable prefix. Also, if `isFirst` is
    *  false check that `tp` is a trait.
    *  @return  `tp` itself if it is a class or trait ref, ObjectClass.typeRef if not.
    */
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 51eba3b02564..7bef0268e04b 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -265,11 +265,13 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
   def typedSelect(tree: untpd.Select, pt: Type)(implicit ctx: Context): Tree = track("typedSelect") {
     val qual1 = typedExpr(tree.qualifier, selectionProto(tree.name, pt, this))
+    if (tree.name.isTypeName) checkStable(qual1.tpe, qual1.pos)
     checkValue(assignType(cpy.Select(tree, qual1, tree.name), qual1), pt)
   }
 
   def typedSelectFromTypeTree(tree: untpd.SelectFromTypeTree, pt: Type)(implicit ctx: Context): SelectFromTypeTree = track("typedSelectFromTypeTree") {
     val qual1 = typedType(tree.qualifier, selectionProto(tree.name, pt, this))
+    checkLegalPrefix(qual1.tpe, qual1.pos)
     assignType(cpy.SelectFromTypeTree(tree, qual1, tree.name), qual1)
   }
 
diff --git a/src/dotty/tools/dotc/util/common.scala b/src/dotty/tools/dotc/util/common.scala
index 5f133b079a04..d7596c28e8ec 100644
--- a/src/dotty/tools/dotc/util/common.scala
+++ b/src/dotty/tools/dotc/util/common.scala
@@ -7,8 +7,6 @@ import core.Types.WildcardType
 /** Common values hoisted out for performance */
 object common {
 
-  val emptyDNF = (Nil, Set[Name]()) :: Nil
-
   val alwaysTrue = Function.const(true) _
   val alwaysZero = Function.const(0) _
   val alwaysWildcardType = Function.const(WildcardType) _
diff --git a/test/dotc/tests.scala b/test/dotc/tests.scala
index 9914485ac3cb..e5360bc0833b 100644
--- a/test/dotc/tests.scala
+++ b/test/dotc/tests.scala
@@ -52,6 +52,7 @@ class tests extends CompilerTest {
   @Test def neg_rootImports = compileFile(negDir, "rootImplicits", xerrors = 2)
   @Test def neg_templateParents() = compileFile(negDir, "templateParents", xerrors = 3)
   @Test def neg_i39 = compileFile(negDir, "i39", xerrors = 1)
+  @Test def neg_i50_volatile = compileFile(negDir, "i50-volatile", xerrors = 4)
 
   @Test def dotc = compileDir(dotcDir + "tools/dotc")
   @Test def dotc_ast = compileDir(dotcDir + "tools/dotc/ast")
diff --git a/tests/neg/i50-volatile.scala b/tests/neg/i50-volatile.scala
new file mode 100644
index 000000000000..ae31a764c354
--- /dev/null
+++ b/tests/neg/i50-volatile.scala
@@ -0,0 +1,25 @@
+object Test {
+  class Base {
+    class Inner
+  }
+  type A <: Base {
+    type X = String
+  }
+  type B <: {
+    type X = Int
+  }
+  lazy val o: A & B = ???
+
+  class Client extends o.Inner
+
+  def xToString(x: o.X): String = x
+
+  def intToString(i: Int): String = xToString(i)
+}
+object Test2 {
+
+  import Test.o._
+
+  def xToString(x: X): String = x
+
+}