From 091788d0ad6f94c91cc20d6074c3403934702325 Mon Sep 17 00:00:00 2001
From: Jon Pretty <jon.pretty@propensive.com>
Date: Wed, 6 Jun 2018 21:27:28 +0100
Subject: Permit derivations of products when `dispatch` not defined

---
 core/shared/src/main/scala/magnolia.scala |  3 +--
 examples/shared/src/main/scala/hash.scala | 37 +++++++++++++++++++++++++++++++
 tests/src/main/scala/tests.scala          | 15 ++++++++++++-
 3 files changed, 52 insertions(+), 3 deletions(-)
 create mode 100644 examples/shared/src/main/scala/hash.scala

diff --git a/core/shared/src/main/scala/magnolia.scala b/core/shared/src/main/scala/magnolia.scala
index 6001977..dd1e0e8 100644
--- a/core/shared/src/main/scala/magnolia.scala
+++ b/core/shared/src/main/scala/magnolia.scala
@@ -127,9 +127,7 @@ object Magnolia {
         error(s"magnolia: the method `combine` should take a single parameter of type $expected")
     }
 
-    // FIXME: Only run these methods if they're used, particularly `dispatch`
     checkMethod("combine", "case classes", "CaseClass[Typeclass, _]")
-    checkMethod("dispatch", "sealed traits", "SealedTrait[Typeclass, _]")
 
     val expandDeferred = new Transformer {
       override def transform(tree: Tree) = tree match {
@@ -333,6 +331,7 @@ object Magnolia {
         }})}))
           }""")
       } else if (isSealedTrait) {
+        checkMethod("dispatch", "sealed traits", "SealedTrait[Typeclass, _]")
         val genericSubtypes = knownSubclasses(classType.get)
         val subtypes = genericSubtypes.map { sub =>
           val subType = sub.asType.toType // FIXME: Broken for path dependent types
diff --git a/examples/shared/src/main/scala/hash.scala b/examples/shared/src/main/scala/hash.scala
new file mode 100644
index 0000000..2c284c5
--- /dev/null
+++ b/examples/shared/src/main/scala/hash.scala
@@ -0,0 +1,37 @@
+/* Magnolia, version 0.7.1. Copyright 2018 Jon Pretty, Propensive Ltd.
+ *
+ * The primary distribution site is: http://co.ntextu.al/
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+ * except in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the
+ * License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+ * either express or implied. See the License for the specific language governing permissions
+ * and limitations under the License.
+ */
+package magnolia.examples
+
+import magnolia._
+import scala.language.experimental.macros
+
+trait WeakHash[T] { def hash(value: T): Int }
+
+object WeakHash {
+
+  type Typeclass[T] = WeakHash[T]
+
+  def combine[T](ctx: CaseClass[WeakHash, T]): WeakHash[T] = new WeakHash[T] {
+    def hash(value: T): Int = ctx.parameters.map { param =>
+      param.typeclass.hash(param.dereference(value))
+    }.foldLeft(0)(_ ^ _)
+  }
+
+  implicit val string: WeakHash[String] = _.map(_.toInt).sum
+  implicit val int: WeakHash[Int] = identity
+  implicit val double: WeakHash[Double] = _.toInt
+
+  implicit def gen[T]: WeakHash[T] = macro Magnolia.gen[T]
+}
diff --git a/tests/src/main/scala/tests.scala b/tests/src/main/scala/tests.scala
index c3b90cc..f408253 100644
--- a/tests/src/main/scala/tests.scala
+++ b/tests/src/main/scala/tests.scala
@@ -55,7 +55,6 @@ case object Red extends Color
 case object Green extends Color
 case object Blue extends Color
 
-
 case class MyAnnotation(order: Int) extends StaticAnnotation
 
 @MyAnnotation(0) case class Attributed(
@@ -382,5 +381,19 @@ object Tests extends TestApp {
       Show.gen[Attributed].show(Attributed("xyz", 100))
     }.assert(_ == "Attributed{MyAnnotation(0)}(p1{MyAnnotation(1)}=xyz,p2{MyAnnotation(2)}=100)")
 
+    test("allow no-coproduct derivation definitions") {
+      scalac"""
+        WeakHash.gen[Person]
+      """
+    }.assert(_ == Returns(fqt"magnolia.examples.WeakHash.Typeclass[magnolia.tests.Person]"))
+    
+    test("disallow coproduct derivations without dispatch method") {
+      scalac"""
+        WeakHash.gen[Entity]
+      """
+    }.assert(_ == TypecheckError("magnolia: the method `dispatch` must be defined on the derivation object WeakHash to derive typeclasses for sealed traits"))
+
+
+
   }
 }
-- 
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