workaround for a possible scala bug in show for value class.

build.sbt

@@ -54,7 +54,14 @@ lazy val commonSettings = Seq(
   // workaround for https://github.com/scalastyle/scalastyle-sbt-plugin/issues/47
   scalastyleSources in Compile ++= (unmanagedSourceDirectories in Compile).value,
   ivyConfigurations += config("compile-time").hide,
-  unmanagedClasspath in Compile ++= update.value.select(configurationFilter("compile-time"))
+  unmanagedClasspath in Compile ++= update.value.select(configurationFilter("compile-time")),
+  unmanagedSourceDirectories in Test ++= {
+    val bd = baseDirectory.value
+    if (CrossVersion.partialVersion(scalaVersion.value) exists (_._2 >= 11))
+      CrossType.Pure.sharedSrcDir(bd, "test").toList map (f => file(f.getPath + "-2.11+"))
+    else
+      Nil
+  }
 ) ++ warnUnusedImport ++ update2_12 ++ xlint
 
 

core/src/main/scala/cats/Show.scala

@@ -1,6 +1,5 @@
 package cats
 
-import simulacrum.typeclass
 import cats.functor.Contravariant
 
 /**
@@ -10,16 +9,32 @@ import cats.functor.Contravariant
  * made a toString method, a Show instance will only exist if someone
  * explicitly provided one.
  */
-@typeclass trait Show[T] extends Show.ContravariantShow[T] {
-  // duplicated so simulacrum knows it requires an instance of this trait
-  def show(t: T): String
-}
+trait Show[T] extends Show.ContravariantShow[T]
 
+/**
+ * Hand rolling the type class boilerplate due to scala/bug#6260 and scala/bug#10458
+ */
 object Show {
-  trait ContravariantShow[-T] {
+
+  def apply[A](implicit instance: Show[A]): Show[A] = instance
+
+  trait ContravariantShow[-T] extends Serializable {
     def show(t: T): String
   }
 
+  trait Ops[A] {
+    def typeClassInstance: Show[A]
+    def self: A
+    def show: String = typeClassInstance.show(self)
+  }
+
+  trait ToShowOps {
+    implicit def toShow[A](target: A)(implicit tc: Show[A]): Ops[A] = new Ops[A] {
+      val self = target
+      val typeClassInstance = tc
+    }
+  }
+
   /** creates an instance of [[Show]] using the provided function */
   def show[A](f: A => String): Show[A] = new Show[A] {
     def show(a: A): String = f(a)

tests/src/test/scala-2.11+/cats/tests/RegressionTests.scala

@@ -0,0 +1,25 @@
+package cats
+package tests
+
+
+import ExtraRegressionTests._
+
+class ExtraRegressionTests extends CatsSuite {
+  /**
+   * Placed here to work around scala/bug#6260 on scala 2.10
+   */
+  test("#1802 duplicated method name") {
+    Show[First[String]]
+  }
+}
+
+
+object ExtraRegressionTests {
+  final case class First[A](getFirst: A) extends AnyVal
+  object First {
+    implicit def showInstance[A](implicit ev: Show[A]): Show[First[A]] = new Show[First[A]] {
+      override def show(f: First[A]): String = s"First(${ev.show(f.getFirst)})"
+    }
+  }
+
+}

tests/src/test/scala/cats/tests/RegressionTests.scala

@@ -4,7 +4,6 @@ package tests
 import cats.data.{Const, NonEmptyList}
 
 import scala.collection.mutable
-
 class RegressionTests extends CatsSuite {
 
   // toy state class
@@ -123,4 +122,5 @@ class RegressionTests extends CatsSuite {
     NonEmptyList.of(6,7,8).traverse_(validate) should === (Either.left("6 is greater than 5"))
     checkAndResetCount(1)
   }
+
 }