Disallow extending a trait twice in the same class

Author: odersky

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

@@ -1460,9 +1460,13 @@ class Typer extends Namer
         ref
     }
 
+    val seenParents = mutable.Set[Symbol]()
+
     def typedParent(tree: untpd.Tree): Tree = {
       var result = if (tree.isType) typedType(tree)(superCtx) else typedExpr(tree)(superCtx)
       val psym = result.tpe.typeSymbol
+      if (seenParents.contains(psym)) ctx.error(i"$psym is extended twice", tree.pos)
+      seenParents += psym
       if (tree.isType) {
         if (psym.is(Trait) && !cls.is(Trait) && !cls.superClass.isSubClass(psym))
           result = maybeCall(result, psym, psym.primaryConstructor.info)

tests/neg/i3989e.scala

@@ -0,0 +1,11 @@
+object Test extends App {
+  trait A[+X](val x: X)
+  class B extends A(5) with A("hello") // error: A is extended twice
+
+  def f(a: A[Int]): Int = a match {
+    case b: B => b.x
+    case _ => 0
+  }
+
+  f(new B)
+}