Check that a self type T is closed.

What is checked: A self type T is a subtype of all
selftypes of classes refernced by T. That is, a self type
has to subsume all self types of its required type. Ot,
otherwise said, requirements must be closed; you cannot
discover new ones in following them.

src/dotty/tools/dotc/typer/RefChecks.scala

@@ -71,14 +71,20 @@ object RefChecks {
     }
   }
 
-  /** Check that self type of this class conforms to self types of parents */
-  private def checkSelfType(clazz: Symbol)(implicit ctx: Context): Unit = clazz.info match {
+  /** Check that self type of this class conforms to self types of parents
+   *  and required classes.
+   */
+  private def checkSelfType(cls: Symbol)(implicit ctx: Context): Unit = cls.info match {
     case cinfo: ClassInfo =>
-      for (parent <- cinfo.classParents) {
-        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 ${parent.classSymbol}", clazz.pos)
+      def checkSelfConforms(other: TypeRef, category: String, relation: String) = {
+        val otherSelf = other.givenSelfType.asSeenFrom(cls.thisType, other.classSymbol)
+        if (otherSelf.exists && !(cinfo.selfType <:< otherSelf))
+          ctx.error(d"$category: self type ${cinfo.selfType} of $cls does not conform to self type $otherSelf of $relation ${other.classSymbol}", cls.pos)
       }
+      for (parent <- cinfo.classParents)
+        checkSelfConforms(parent, "illegal inheritance", "parent")
+      for (reqd <- cinfo.givenSelfType.classSymbols)
+        checkSelfConforms(reqd.typeRef, "missing requirement", "required")
     case _ =>
   }
 

test/dotc/tests.scala

@@ -136,7 +136,7 @@ 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)

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