Fix #1501 - Check trait inheritance condition

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -260,7 +260,7 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def AnonClass(parents: List[Type], fns: List[TermSymbol], methNames: List[TermName])(implicit ctx: Context): Block = {
     val owner = fns.head.owner
     val parents1 =
-      if (parents.head.classSymbol.is(Trait)) defn.ObjectType :: parents
+      if (parents.head.classSymbol.is(Trait)) parents.head.parents.head :: parents
       else parents
     val cls = ctx.newNormalizedClassSymbol(owner, tpnme.ANON_FUN, Synthetic, parents1,
         coord = fns.map(_.pos).reduceLeft(_ union _))

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

@@ -476,7 +476,7 @@ class Definitions {
   lazy val BoxedNumberClass          = ctx.requiredClass("java.lang.Number")
   lazy val ThrowableClass            = ctx.requiredClass("java.lang.Throwable")
   lazy val ClassCastExceptionClass   = ctx.requiredClass("java.lang.ClassCastException")
-  lazy val JavaSerializableClass     = ctx.requiredClass("java.lang.Serializable")
+  lazy val JavaSerializableClass     = ctx.requiredClass("java.io.Serializable")
   lazy val ComparableClass           = ctx.requiredClass("java.lang.Comparable")
 
   // in scalac modified to have Any as parent

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -448,7 +448,6 @@ object Checking {
       }
       stats.foreach(checkValueClassMember)
     }
-
   }
 }
 
@@ -601,6 +600,31 @@ trait Checking {
   /** Verify classes extending AnyVal meet the requirements */
   def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context) =
     Checking.checkDerivedValueClass(clazz, stats)
+
+  /** Given a parent `parent` of a class `cls`, if `parent` is a trait check that
+   *  the superclass of `cls` derived from the superclass of `parent`.
+   *
+   *  An exception is made if `cls` extends `Any`, and `parent` is `java.io.Serializable`
+   *  or `java.lang.Comparable`. These two classes are treated by Scala as universal
+   *  traits. E.g. the following is OK:
+   *
+   *      ... extends Any with java.io.Serializable
+   *
+   *  The standard library relies on this idiom.
+   */
+  def checkTraitInheritance(parent: Symbol, cls: ClassSymbol, pos: Position)(implicit ctx: Context): Unit = {
+    parent match {
+      case parent: ClassSymbol if parent is Trait =>
+        val psuper = parent.superClass
+        val csuper = cls.superClass
+        val ok = csuper.derivesFrom(psuper) ||
+          parent.is(JavaDefined) && csuper == defn.AnyClass &&
+          (parent == defn.JavaSerializableClass || parent == defn.ComparableClass)
+        if (!ok)
+          ctx.error(em"illegal trait inheritance: super$csuper does not derive from $parent's super$psuper", pos)
+      case _ =>
+    }
+  }
 }
 
 trait NoChecking extends Checking {
@@ -617,4 +641,5 @@ trait NoChecking extends Checking {
   override def checkSimpleKinded(tpt: Tree)(implicit ctx: Context): Tree = tpt
   override def checkNotSingleton(tpt: Tree, where: String)(implicit ctx: Context): Tree = tpt
   override def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context) = ()
+  override def checkTraitInheritance(parentSym: Symbol, cls: ClassSymbol, pos: Position)(implicit ctx: Context) = ()
 }

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1288,13 +1288,15 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
       if (tree.isType) {
         val result = typedType(tree)(superCtx)
         val psym = result.tpe.typeSymbol
+        checkTraitInheritance(psym, cls, tree.pos)
         if (psym.is(Trait) && !cls.is(Trait) && !cls.superClass.isSubClass(psym))
           maybeCall(result, psym, psym.primaryConstructor.info)
         else
           result
       }
       else {
         val result = typedExpr(tree)(superCtx)
+        checkTraitInheritance(result.symbol, cls, tree.pos)
         checkParentCall(result, cls)
         result
       }

tests/neg/i1501.scala

@@ -0,0 +1,28 @@
+class A {
+  def foo: Int = 1
+}
+
+trait B extends A
+
+abstract class D {
+  def foo: Int
+}
+
+class C extends D with B // error: illegal trait inheritance
+trait E extends D with B // error: illegal trait inheritance
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(new C().foo)
+  }
+}
+
+object Test2 {
+  class A
+  class SubA(x: Int) extends A
+  trait TA extends A
+  trait TSubA extends SubA(2) // error: trait TSubA may not call constructor of class SubA
+
+
+  class Foo extends TA with TSubA // error: missing argument for parameter x of constructor SubA:
+}

tests/neg/i1653.scala

@@ -1,3 +1,3 @@
 trait Foo {
-  def foo() = new Unit with Foo  // error
+  def foo() = new Unit with Foo  // error: cannot extend final class Unit  // error: illegal trait inheritance
 }