Allocate only one Label per tag type, and only one default Label.

This comes at the cost of a self type error when attempting to use a non-case
object as a tag.

Author: Fulvio Valente

src/main/scala/Label.scala

@@ -6,22 +6,23 @@ trait Label[A] {
 }
 
 object Label {
-  private[id] def default[A] = new Label[A] {
-    def label = ""
+  private val defaultLabel: Label[Nothing] = new Label[Nothing] {
+    val label = ""
   }
 
+  private[id] def default[A]: Label[A] = defaultLabel.asInstanceOf[Label[A]]
+
   private[id] sealed trait LabelDefinitionConflict
 
-  trait MakeLabel { self =>
+  trait MakeLabel { self: Product =>
     // See eidos.id.Format.UUID for an explanation of this
     // format: off
     final def `"In Eidos, you can only extend one of MakeLabel or CustomLabel"`
         : LabelDefinitionConflict = null
 
-    implicit final def l(implicit ev: self.type <:< Product): Label[this.type] =
-      new Label[this.type] {
-        def label = self.productPrefix
-      }
+    implicit final val l: Label[this.type] = new Label[this.type] {
+      val label = self.productPrefix
+    }
   }
 
   trait CustomLabel {
@@ -30,10 +31,10 @@ object Label {
     // format: on
     def label: String
 
-    private def customLabel = label
+    private val customLabel = label
 
-    implicit final def l: Label[this.type] = new Label[this.type] {
-      def label = customLabel
+    implicit final val l: Label[this.type] = new Label[this.type] {
+      val label = customLabel
     }
   }
 }

src/test/scala/EidosSpec.scala

@@ -83,9 +83,8 @@ class EidosSpec extends Specification with TypecheckMatchers with ScalaCheck {
   "Tag" should {
     "be case objects" in {
       trait Trait
-
-      // MakeLabel is the reason why "case" is required
-      object Object extends MakeLabel
+      class Class
+      object Object
       type Object = Object.type
 
       case object CaseObject
@@ -96,8 +95,15 @@ class EidosSpec extends Specification with TypecheckMatchers with ScalaCheck {
 
       // format: off
       { typecheck("""Id.of[Trait]("")""") must failWith(errorMessage("Trait")) }
+      { typecheck("""Id.of[Class]("")""") must failWith(errorMessage("Class")) }
       { typecheck("""Id.of[Object]("")""") must failWith(errorMessage("Object")) }
       { typecheck("""Id.of[CaseObject]("")""") must succeed }
+
+      // Non-case objects will fail before they are used in an Id.of call, but
+      // with a more vague error because MakeLabel needs access to productPrefix
+      // sooner in order to avoid allocating duplicate labels
+      { typecheck("""object Object extends MakeLabel""") must failWith(
+        "self-type Object.type does not conform to eidos.MakeLabel's selftype") }
       // format: on
     }
   }