diff --git a/src/dotty/tools/dotc/core/TypeErasure.scala b/src/dotty/tools/dotc/core/TypeErasure.scala
index 9b41eb982a5e..0ef31015c2de 100644
--- a/src/dotty/tools/dotc/core/TypeErasure.scala
+++ b/src/dotty/tools/dotc/core/TypeErasure.scala
@@ -439,6 +439,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
     case ErasedValueType(_, underlying) =>
       sigName(underlying)
     case tp: TypeRef =>
+      if (!tp.denot.exists) throw new MissingType(tp.prefix, tp.name)
       val sym = tp.symbol
       if (!sym.isClass) sigName(tp.info)
       else if (isDerivedValueClass(sym)) sigName(eraseDerivedValueClassRef(tp))
diff --git a/src/dotty/tools/dotc/core/Types.scala b/src/dotty/tools/dotc/core/Types.scala
index d6bb9c3c574e..f4dbfb0e32c0 100644
--- a/src/dotty/tools/dotc/core/Types.scala
+++ b/src/dotty/tools/dotc/core/Types.scala
@@ -936,6 +936,7 @@ object Types {
     /** the self type of the underlying classtype */
     def givenSelfType(implicit ctx: Context): Type = this match {
       case tp @ RefinedType(parent, name) => tp.wrapIfMember(parent.givenSelfType)
+      case tp: ThisType => tp.tref.givenSelfType
       case tp: TypeProxy => tp.underlying.givenSelfType
       case _ => NoType
     }
@@ -3181,14 +3182,23 @@ object Types {
   // ----- Exceptions -------------------------------------------------------------
 
   class TypeError(msg: String) extends Exception(msg)
-  class FatalTypeError(msg: String) extends TypeError(msg)
 
   class MalformedType(pre: Type, denot: Denotation, absMembers: Set[Name])
-    extends FatalTypeError(
-      s"""malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}""")
+    extends TypeError(
+      s"malformed type: $pre is not a legal prefix for $denot because it contains abstract type member${if (absMembers.size == 1) "" else "s"} ${absMembers.mkString(", ")}")
+
+  class MissingType(pre: Type, name: Name)(implicit ctx: Context) extends TypeError(
+    i"""cannot resolve reference to type $pre.$name
+       |the classfile defining the type might be missing from the classpath${otherReason(pre)}""".stripMargin)
+
+  private def otherReason(pre: Type)(implicit ctx: Context): String = pre match {
+    case pre: ThisType if pre.givenSelfType.exists =>
+      i"\nor the self type of $pre might not contain all transitive dependencies"
+    case _ => ""
+  }
 
   class CyclicReference private (val denot: SymDenotation)
-    extends FatalTypeError(s"cyclic reference involving $denot") {
+    extends TypeError(s"cyclic reference involving $denot") {
     def show(implicit ctx: Context) = s"cyclic reference involving ${denot.show}"
   }
 
@@ -3204,7 +3214,7 @@ object Types {
     }
   }
 
-  class MergeError(msg: String) extends FatalTypeError(msg)
+  class MergeError(msg: String) extends TypeError(msg)
 
   // ----- Debug ---------------------------------------------------------
 
diff --git a/src/dotty/tools/dotc/printing/PlainPrinter.scala b/src/dotty/tools/dotc/printing/PlainPrinter.scala
index 5d2309e95158..45928af4b7bd 100644
--- a/src/dotty/tools/dotc/printing/PlainPrinter.scala
+++ b/src/dotty/tools/dotc/printing/PlainPrinter.scala
@@ -120,7 +120,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: TermRef if tp.denot.isOverloaded =>
         "<overloaded " ~ toTextRef(tp) ~ ">"
       case tp: SingletonType =>
-        toText(tp.underlying) ~ "(" ~ toTextRef(tp) ~ ")"
+        toTextLocal(tp.underlying) ~ "(" ~ toTextRef(tp) ~ ")"
       case tp: TypeRef =>
         toTextPrefix(tp.prefix) ~ selectionString(tp)
       case tp: RefinedType =>
diff --git a/src/dotty/tools/dotc/typer/RefChecks.scala b/src/dotty/tools/dotc/typer/RefChecks.scala
index bb22e20452af..71fba158871c 100644
--- a/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -73,19 +73,25 @@ object RefChecks {
 
   /** Check that final and sealed restrictions on class parents
    *  and that self type of this class conforms to self types of parents.
+   *  and required classes.
    */
-  private def checkParents(clazz: Symbol)(implicit ctx: Context): Unit = clazz.info match {
+  private def checkParents(cls: Symbol)(implicit ctx: Context): Unit = cls.info match {
     case cinfo: ClassInfo =>
+      def checkSelfConforms(other: TypeRef, category: String, relation: String) = {
+        val otherSelf = other.givenSelfType.asSeenFrom(cls.thisType, other.classSymbol)
+        if (otherSelf.exists && !(cinfo.selfType <:< otherSelf))
+          ctx.error(d"$category: self type ${cinfo.selfType} of $cls does not conform to self type $otherSelf of $relation ${other.classSymbol}", cls.pos)
+      }
       for (parent <- cinfo.classParents) {
         val pclazz = parent.classSymbol
         if (pclazz.is(Final))
-          ctx.error(d"cannot extend final $pclazz", clazz.pos)
-        if (pclazz.is(Sealed) && pclazz.associatedFile != clazz.associatedFile)
-          ctx.error(d"cannot extend sealed $pclazz in different compilation unit", clazz.pos)
-        val pself = parent.givenSelfType.asSeenFrom(clazz.thisType, parent.classSymbol)
-        if (pself.exists && !(cinfo.selfType <:< pself))
-          ctx.error(d"illegal inheritance: self type ${cinfo.selfType} of $clazz does not conform to self type $pself of parent ${parent.classSymbol}", clazz.pos)
+          ctx.error(d"cannot extend final $pclazz", cls.pos)
+        if (pclazz.is(Sealed) && pclazz.associatedFile != cls.associatedFile)
+          ctx.error(d"cannot extend sealed $pclazz in different compilation unit", cls.pos)
+        checkSelfConforms(parent, "illegal inheritance", "parent")
       }
+      for (reqd <- cinfo.givenSelfType.classSymbols)
+        checkSelfConforms(reqd.typeRef, "missing requirement", "required")
     case _ =>
   }
 
diff --git a/src/dotty/tools/dotc/typer/Typer.scala b/src/dotty/tools/dotc/typer/Typer.scala
index 479eedd380a4..33ec156a1d72 100644
--- a/src/dotty/tools/dotc/typer/Typer.scala
+++ b/src/dotty/tools/dotc/typer/Typer.scala
@@ -1087,7 +1087,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
     try adapt(typedUnadapted(tree, pt), pt, tree)
     catch {
       case ex: CyclicReference => errorTree(tree, cyclicErrorMsg(ex))
-      case ex: FatalTypeError => errorTree(tree, ex.getMessage)
+      case ex: TypeError => errorTree(tree, ex.getMessage)
     }
   }
 
diff --git a/test/dotc/tests.scala b/test/dotc/tests.scala
index f222daca4e49..3bce26253025 100644
--- a/test/dotc/tests.scala
+++ b/test/dotc/tests.scala
@@ -138,7 +138,8 @@ class tests extends CompilerTest {
   @Test def neg_moduleSubtyping = compileFile(negDir, "moduleSubtyping", xerrors = 4)
   @Test def neg_escapingRefs = compileFile(negDir, "escapingRefs", xerrors = 2)
   @Test def neg_instantiateAbstract = compileFile(negDir, "instantiateAbstract", xerrors = 8)
-  @Test def neg_selfInheritance = compileFile(negDir, "selfInheritance", xerrors = 5)
+  @Test def neg_selfInheritance = compileFile(negDir, "selfInheritance", xerrors = 6)
+  @Test def neg_selfreq = compileFile(negDir, "selfreq", xerrors = 4)
   @Test def neg_shadowedImplicits = compileFile(negDir, "arrayclone-new", xerrors = 2)
   @Test def neg_traitParamsTyper = compileFile(negDir, "traitParamsTyper", xerrors = 5)
   @Test def neg_traitParamsMixin = compileFile(negDir, "traitParamsMixin", xerrors = 2)
diff --git a/tests/neg/selfInheritance.scala b/tests/neg/selfInheritance.scala
index 5f61c5bbbdb8..993765817dfa 100644
--- a/tests/neg/selfInheritance.scala
+++ b/tests/neg/selfInheritance.scala
@@ -26,3 +26,7 @@ object Test {
   object M extends C // error
 
 }
+
+trait X { self: Y => }
+trait Y { self: Z => }
+trait Z
diff --git a/tests/neg/selfreq.scala b/tests/neg/selfreq.scala
new file mode 100644
index 000000000000..e75e03c16442
--- /dev/null
+++ b/tests/neg/selfreq.scala
@@ -0,0 +1,37 @@
+trait X { self: Y =>
+
+  type T <: self.U
+
+  def foo(x: T): T
+  def foo(x: String): String
+
+}
+
+trait Y { self: Z =>
+
+  type U <: self.V
+
+}
+
+trait Z {
+
+  class V
+
+}
+
+object O {
+  val x: X = ???
+  x.foo("a")
+}
+
+import scala.tools.nsc.interpreter.IMain
+
+object Test extends dotty.runtime.LegacyApp {
+  val engine = new IMain.Factory getScriptEngine()
+  engine.asInstanceOf[IMain].settings.usejavacp.value = true
+  val res2 = engine.asInstanceOf[javax.script.Compilable]
+  res2 compile "8" eval()
+  val res5 = res2 compile """println("hello") ; 8"""
+  res5 eval()
+  res5 eval()
+}