From 20afa757f499688b56cbd6b8571014b75d6d4dce Mon Sep 17 00:00:00 2001
From: Jakob Odersky <jakob@odersky.com>
Date: Thu, 2 Aug 2018 15:24:52 -0700
Subject: Treat value classes transparently

---
 shared/src/main/scala/DerivedFormats.scala         | 24 ++++++++++++--------
 shared/src/test/scala/ProductTypeFormatTests.scala | 26 +++++++++++++++++++++-
 2 files changed, 40 insertions(+), 10 deletions(-)

diff --git a/shared/src/main/scala/DerivedFormats.scala b/shared/src/main/scala/DerivedFormats.scala
index f6a3f7b..ed39f46 100644
--- a/shared/src/main/scala/DerivedFormats.scala
+++ b/shared/src/main/scala/DerivedFormats.scala
@@ -24,18 +24,24 @@ trait DerivedFormats { self: BasicFormats =>
 
   def combine[T](ctx: CaseClass[JsonFormat, T]): JsonFormat[T] =
     new JsonFormat[T] {
-      override def write(value: T): JsValue = {
-        val fields: Seq[(String, JsValue)] = ctx.parameters.collect {
-          case param
-              if !param.option || param.dereference(value) != None || printNull =>
-            extractFieldName(param.label) -> param.typeclass.write(
-              param.dereference(value)
-            )
+      override def write(value: T): JsValue =
+        if (ctx.isValueClass) {
+          val param = ctx.parameters.head
+          param.typeclass.write(param.dereference(value))
+        } else {
+          val fields: Seq[(String, JsValue)] = ctx.parameters.collect {
+            case param
+                if !param.option || param.dereference(value) != None || printNull =>
+              extractFieldName(param.label) -> param.typeclass.write(
+                param.dereference(value)
+              )
+          }
+          JsObject(fields: _*)
         }
-        JsObject(fields: _*)
-      }
 
       override def read(value: JsValue): T = value match {
+        case js if ctx.isValueClass =>
+          ctx.construct(param => param.typeclass.read(value))
         case obj: JsObject =>
           if (ctx.isObject) {
             ctx.rawConstruct(Seq.empty)
diff --git a/shared/src/test/scala/ProductTypeFormatTests.scala b/shared/src/test/scala/ProductTypeFormatTests.scala
index a869ac5..d7721ba 100644
--- a/shared/src/test/scala/ProductTypeFormatTests.scala
+++ b/shared/src/test/scala/ProductTypeFormatTests.scala
@@ -90,11 +90,35 @@ class ProductTypeFormatTests
   case class Phantom[T](x: Int)
 
   // no json format required for T
-  implicit def phantom[T] = jsonFormat[Phantom[T]]
+  implicit def phantom[T]: RootJsonFormat[Phantom[T]] = jsonFormat[Phantom[T]]
 
   "Phantom types without a format" should behave like checkRoundtrip(
     Phantom[Int => String](42), // the given type parameter should not have a format
     """{"x":42}"""
   )
 
+  implicit val vc = jsonFormat[ProductTypeFormatTests.Wrapper]
+  "Value classes" should behave like checkRoundtrip(
+    ProductTypeFormatTests.Wrapper(42),
+    """42"""
+  )
+
+  implicit def vcg[T: JsonFormat] = jsonFormat[ProductTypeFormatTests.WrapperGeneric[T]]
+  "Value classes with generic parameters" should behave like checkRoundtrip(
+    ProductTypeFormatTests.WrapperGeneric[F](F(42)),
+    """{"y": 42}"""
+  )
+
+  implicit def vcp[T] = jsonFormat[ProductTypeFormatTests.WrapperPhantom[T]]
+  "value classes with unused generic parameters" should behave like checkRoundtrip(
+    ProductTypeFormatTests.WrapperPhantom[Int => String](42),
+    """42"""
+  )
+
+}
+
+object ProductTypeFormatTests {
+  case class Wrapper(x: Int) extends AnyVal
+  case class WrapperGeneric[A](x: A) extends AnyVal
+  case class WrapperPhantom[A](x: Int) extends AnyVal
 }
-- 
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