From 9753b344df3c75e71799ea666d2018ee5f19747f Mon Sep 17 00:00:00 2001
From: Tim Nielens <11885535+tnielens@users.noreply.github.com>
Date: Tue, 6 Apr 2021 23:53:04 +0200
Subject: [PATCH] fix show interpolation ambiguity with Seq subtypes (#3852)

Co-authored-by: Tim Nielens <tim.nielens@restore.energy>
---
 core/src/main/scala/cats/Show.scala             |  7 +++++--
 tests/src/test/scala/cats/tests/ShowSuite.scala | 12 ++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/core/src/main/scala/cats/Show.scala b/core/src/main/scala/cats/Show.scala
index 9b1e03c6f6..de7b208a71 100644
--- a/core/src/main/scala/cats/Show.scala
+++ b/core/src/main/scala/cats/Show.scala
@@ -89,7 +89,6 @@ object Show extends ScalaVersionSpecificShowInstances with ShowInstances {
   implicit def catsShowForOption[A: Show]: Show[Option[A]] = cats.instances.option.catsStdShowForOption[A]
   implicit def catsShowForTry[A: Show]: Show[Try[A]] = cats.instances.try_.catsStdShowForTry[A]
   implicit def catsShowForList[A: Show]: Show[List[A]] = cats.instances.list.catsStdShowForList[A]
-  implicit def catsShowForSeq[A: Show]: Show[Seq[A]] = cats.instances.seq.catsStdShowForSeq[A]
   implicit def catsShowForVector[A: Show]: Show[Vector[A]] = cats.instances.vector.catsStdShowForVector[A]
   implicit def catsShowForQueue[A: Show]: Show[Queue[A]] = cats.instances.queue.catsStdShowForQueue[A]
   implicit def catsShowForEither[A: Show, B: Show]: Show[Either[A, B]] =
@@ -104,7 +103,11 @@ object Show extends ScalaVersionSpecificShowInstances with ShowInstances {
   def catsShowForTuple2[A: Show, B: Show]: Show[(A, B)] = cats.instances.tuple.catsStdShowForTuple2[A, B]
 }
 
-private[cats] trait ShowInstances extends cats.instances.NTupleShowInstances {
+private[cats] trait ShowInstances extends cats.instances.NTupleShowInstances with ShowInstances0 {
   implicit def catsShowForFiniteDuration: Show[FiniteDuration] =
     cats.instances.finiteDuration.catsStdShowForFiniteDurationUnambiguous
 }
+
+private[cats] trait ShowInstances0 {
+  implicit def catsShowForSeq[A: Show]: Show[Seq[A]] = cats.instances.seq.catsStdShowForSeq[A]
+}
diff --git a/tests/src/test/scala/cats/tests/ShowSuite.scala b/tests/src/test/scala/cats/tests/ShowSuite.scala
index ff56379880..a501a1a8ad 100644
--- a/tests/src/test/scala/cats/tests/ShowSuite.scala
+++ b/tests/src/test/scala/cats/tests/ShowSuite.scala
@@ -8,6 +8,7 @@ import cats.laws.discipline.{ContravariantTests, MiniInt, SerializableTests}
 import cats.laws.discipline.arbitrary._
 import cats.laws.discipline.eq._
 import java.util.concurrent.TimeUnit
+import scala.collection.immutable.Seq
 import scala.concurrent.duration.{Duration, FiniteDuration}
 
 class ShowSuite extends CatsSuite {
@@ -48,6 +49,17 @@ class ShowSuite extends CatsSuite {
     show"instance resolution is not ambiguous for ${FiniteDuration(3L, TimeUnit.SECONDS)}"
     show"instance resolution is not ambiguous for ${Duration(3L, TimeUnit.SECONDS)}"
   }
+
+  test("show interpolation with Seq subtypes isn't ambiguous") {
+    implicitly[ContravariantShow[Seq[Int]]]
+    implicitly[ContravariantShow[List[Int]]]
+    implicitly[ContravariantShow[Vector[Int]]]
+
+    val goodmornings = Seq("guten Tag", "good morning", "bonjour")
+    assertEquals(show"$goodmornings", "List(guten Tag, good morning, bonjour)")
+    assertEquals(show"${goodmornings.toList}", "List(guten Tag, good morning, bonjour)")
+    assertEquals(show"${goodmornings.toVector}", "Vector(guten Tag, good morning, bonjour)")
+  }
 }
 
 final class ShowSuite2 extends munit.FunSuite {