Issue 2657 - Ambiguous duration instances

core/src/main/scala/cats/implicits.scala

@@ -11,3 +11,4 @@ object implicits
     with instances.AllInstancesBinCompat0
     with instances.AllInstancesBinCompat1
     with instances.AllInstancesBinCompat2
+    with instances.AllInstancesBinCompat3

core/src/main/scala/cats/instances/all.scala

@@ -44,3 +44,5 @@ trait AllInstancesBinCompat1
     with SortedMapInstancesBinCompat0
 
 trait AllInstancesBinCompat2 extends DurationInstances with FiniteDurationInstances
+
+trait AllInstancesBinCompat3 extends AllCoreDurationInstances

core/src/main/scala/cats/instances/duration.scala

@@ -1,9 +1,26 @@
 package cats
 package instances
 
-import scala.concurrent.duration.Duration
+import scala.concurrent.duration.{Duration, FiniteDuration}
 
 trait DurationInstances extends cats.kernel.instances.DurationInstances {
-  implicit val catsStdShowForDuration: Show[Duration] =
+
+  @deprecated("Left to keep binary compatibility. Use CoreDurationInstances.catsStdShowForDurationUnambiguous instead.",
+              "1.5.0")
+  val catsStdShowForDuration: Show[Duration] =
+    AllCoreDurationInstances.catsStdShowForDurationUnambiguous
+}
+
+private[instances] trait AllCoreDurationInstances extends CoreFiniteDurationInstances
+
+private[instances] trait CoreFiniteDurationInstances extends CoreDurationInstances {
+  implicit final val catsStdShowForFiniteDurationUnambiguous: Show[FiniteDuration] =
+    Show.fromToString[FiniteDuration]
+}
+
+private[instances] trait CoreDurationInstances {
+  implicit final val catsStdShowForDurationUnambiguous: Show[Duration] =
     Show.fromToString[Duration]
 }
+
+private[instances] object AllCoreDurationInstances extends AllCoreDurationInstances

core/src/main/scala/cats/instances/finiteDuration.scala

@@ -4,6 +4,11 @@ package instances
 import scala.concurrent.duration.FiniteDuration
 
 trait FiniteDurationInstances extends cats.kernel.instances.FiniteDurationInstances {
-  implicit val catsStdShowForFiniteDuration: Show[FiniteDuration] =
-    Show.fromToString[FiniteDuration]
+
+  @deprecated(
+    "Left to keep binary compatibility. Use CoreFiniteDurationInstances.catsStdShowForFiniteDurationUnambiguous instead.",
+    "1.5.0"
+  )
+  val catsStdShowForFiniteDuration: Show[FiniteDuration] =
+    AllCoreDurationInstances.catsStdShowForFiniteDurationUnambiguous
 }

core/src/main/scala/cats/instances/package.scala

@@ -9,12 +9,12 @@ package object instances {
   object byte extends ByteInstances
   object char extends CharInstances
   object double extends DoubleInstances
-  object duration extends DurationInstances
+  object duration extends CoreDurationInstances with DurationInstances
   object either extends EitherInstances
   object eq extends EqInstances
   object equiv extends EquivInstances
   object float extends FloatInstances
-  object finiteDuration extends FiniteDurationInstances
+  object finiteDuration extends CoreFiniteDurationInstances with FiniteDurationInstances
   object function extends FunctionInstances with FunctionInstancesBinCompat0
   object future extends FutureInstances
   object int extends IntInstances

testkit/src/main/scala/cats/tests/CatsSuite.scala

@@ -3,7 +3,13 @@ package tests
 
 import catalysts.Platform
 
-import cats.instances.{AllInstances, AllInstancesBinCompat0, AllInstancesBinCompat1, AllInstancesBinCompat2}
+import cats.instances.{
+  AllInstances,
+  AllInstancesBinCompat0,
+  AllInstancesBinCompat1,
+  AllInstancesBinCompat2,
+  AllInstancesBinCompat3
+}
 import cats.syntax.{
   AllSyntax,
   AllSyntaxBinCompat0,
@@ -47,6 +53,7 @@ trait CatsSuite
     with AllInstancesBinCompat0
     with AllInstancesBinCompat1
     with AllInstancesBinCompat2
+    with AllInstancesBinCompat3
     with AllSyntax
     with AllSyntaxBinCompat0
     with AllSyntaxBinCompat1

tests/src/test/scala/cats/tests/ShowSuite.scala

@@ -1,10 +1,15 @@
 package cats
 package tests
 
-import cats.Contravariant
+import java.util.concurrent.TimeUnit
+
+import cats.Show.ContravariantShow
 import cats.laws.discipline.arbitrary._
 import cats.laws.discipline.{ContravariantTests, SerializableTests}
 import cats.laws.discipline.eq._
+import org.scalatest.FunSuite
+
+import scala.concurrent.duration.{Duration, FiniteDuration}
 
 class ShowSuite extends CatsSuite {
   checkAll("Contravariant[Show]", ContravariantTests[Show].contravariant[Int, Int, Int])
@@ -34,4 +39,41 @@ class ShowSuite extends CatsSuite {
 
     assertResult("Good morning")(show"Good $tod")
   }
+
+  test("contravariant show is not ambiguous when both FiniteDuration's and Duration's instances are in scope") {
+    implicitly[ContravariantShow[FiniteDuration]]
+    implicitly[ContravariantShow[Duration]]
+  }
+
+  test("show interpolation works when both FiniteDuration's and Duration's instances are in scope") {
+    show"instance resolution is not ambiguous for ${FiniteDuration(3L, TimeUnit.SECONDS)}"
+    show"instance resolution is not ambiguous for ${Duration(3L, TimeUnit.SECONDS)}"
+  }
+}
+
+final class ShowSuite2 extends FunSuite {
+
+  test(
+    "contravariant show for FiniteDuration can be inferred when importing both duration's and finiteDuration's instances"
+  ) {
+
+    import cats.instances.duration._
+    import cats.instances.finiteDuration._
+
+    implicitly[Order[Duration]]
+    implicitly[Order[FiniteDuration]]
+
+    implicitly[ContravariantShow[FiniteDuration]]
+  }
+
+  test("all the Duration's and FiniteDuration's instances can be correctly inferred when importing implicits") {
+
+    import cats.implicits._
+
+    implicitly[Order[Duration]]
+    implicitly[Order[FiniteDuration]]
+
+    implicitly[ContravariantShow[Duration]]
+    implicitly[ContravariantShow[FiniteDuration]]
+  }
 }