From 9f17b7e0d6dcc8829f5a139b1c97a63add4cb628 Mon Sep 17 00:00:00 2001
From: Georgi Krastev <joro.kr.21@gmail.com>
Date: Sun, 24 Dec 2017 14:52:16 +0100
Subject: [PATCH] Fix Lazy for tagged types

Fixes #584
---
 core/src/main/scala/shapeless/lazy.scala      |  2 +-
 .../scala/shapeless/labelledgeneric.scala     | 27 ++++++++++---------
 core/src/test/scala/shapeless/lazy.scala      | 17 ++++++++++++
 3 files changed, 33 insertions(+), 13 deletions(-)

diff --git a/core/src/main/scala/shapeless/lazy.scala b/core/src/main/scala/shapeless/lazy.scala
index 607e5e560..70b764d53 100644
--- a/core/src/main/scala/shapeless/lazy.scala
+++ b/core/src/main/scala/shapeless/lazy.scala
@@ -180,7 +180,7 @@ trait OpenImplicitMacros {
   def openImplicitTpeParam: Option[Type] =
     openImplicitTpe.map {
       case TypeRef(_, _, List(tpe)) =>
-        tpe.map(_.dealias)
+        tpe.dealias
       case other =>
         c.abort(c.enclosingPosition, s"Bad materialization: $other")
     }
diff --git a/core/src/test/scala/shapeless/labelledgeneric.scala b/core/src/test/scala/shapeless/labelledgeneric.scala
index 242a3be20..967d052c0 100644
--- a/core/src/test/scala/shapeless/labelledgeneric.scala
+++ b/core/src/test/scala/shapeless/labelledgeneric.scala
@@ -93,7 +93,7 @@ object ScalazTaggedAux {
     def apply(): String
   }
 
-  object TC {
+  object TC extends TCLowPriority {
     implicit val intTC: TC[Int] =
       new TC[Int] {
         def apply() = "Int"
@@ -114,22 +114,13 @@ object ScalazTaggedAux {
         def apply() = "HNil"
       }
 
-    implicit def hconsTCTagged[K <: Symbol, H, HT, T <: HList](implicit
-      key: Witness.Aux[K],
-      headTC: Lazy[TC[H @@ HT]],
-      tailTC: Lazy[TC[T]]
-    ): TC[FieldType[K, H @@ HT] :: T] =
-      new TC[FieldType[K, H @@ HT] :: T] {
-        def apply() = s"${key.value.name}: ${headTC.value()} :: ${tailTC.value()}"
-      }
-
     implicit def hconsTC[K <: Symbol, H, T <: HList](implicit
       key: Witness.Aux[K],
       headTC: Lazy[TC[H]],
-      tailTC: Lazy[TC[T]]
+      tailTC: TC[T]
     ): TC[FieldType[K, H] :: T] =
       new TC[FieldType[K, H] :: T] {
-        def apply() = s"${key.value.name}: ${headTC.value()} :: ${tailTC.value()}"
+        def apply() = s"${key.value.name}: ${headTC.value()} :: ${tailTC()}"
       }
 
     implicit def projectTC[F, G](implicit
@@ -140,6 +131,18 @@ object ScalazTaggedAux {
         def apply() = s"Proj(${tc.value()})"
       }
   }
+
+  abstract class TCLowPriority {
+    // FIXME: Workaround #309
+    implicit def hconsTCTagged[K <: Symbol, H, HT, T <: HList](implicit
+      key: Witness.Aux[K],
+      headTC: Lazy[TC[H @@ HT]],
+      tailTC: TC[T]
+    ): TC[FieldType[K, H @@ HT] :: T] =
+      new TC[FieldType[K, H @@ HT] :: T] {
+        def apply() = s"${key.value.name}: ${headTC.value()} :: ${tailTC()}"
+      }
+  }
 }
 
 class LabelledGenericTests {
diff --git a/core/src/test/scala/shapeless/lazy.scala b/core/src/test/scala/shapeless/lazy.scala
index 529b47594..9d4ad9f47 100644
--- a/core/src/test/scala/shapeless/lazy.scala
+++ b/core/src/test/scala/shapeless/lazy.scala
@@ -315,4 +315,21 @@ class LazyStrictTests {
       "lazily[W[String, Int]]", "No W\\[String, Int]"
     )
   }
+
+  @Test
+  def testInteractionWithTaggedTypes {
+    import tag._
+
+    class Readable[A]
+    trait IdTag
+    type Id = String @@ IdTag
+
+    implicit def taggedStringReadable[T, M[_, _]](
+      implicit ev: String @@ T =:= M[String, T]
+    ): Readable[M[String, T]] = new Readable
+
+    implicitly[Readable[Id]]
+    implicitly[Lazy[Readable[Id]]]
+    implicitly[Strict[Readable[Id]]]
+  }
 }