Allow Tuple.Head and Tuple.Tail to work with EmptyTuple

Author: Decel

library/src/scala/Tuple.scala

@@ -89,7 +89,8 @@ object Tuple {
   }
 
   /** Type of the head of a tuple */
-  type Head[X <: NonEmptyTuple] = X match {
+  type Head[X <: Tuple] = X match {
+    case EmptyTuple => EmptyTuple
     case x *: _ => x
   }
 
@@ -101,7 +102,8 @@ object Tuple {
   }
 
   /** Type of the tail of a tuple */
-  type Tail[X <: NonEmptyTuple] <: Tuple = X match {
+  type Tail[X <: Tuple] <: Tuple = X match {
+    case EmptyTuple => EmptyTuple
     case _ *: xs => xs
   }