Replace scala-collection-compat with ScalaCollectionCompat

build.sbt

@@ -124,16 +124,20 @@ lazy val `play-json` = crossProject(JVMPlatform, JSPlatform).crossType(CrossType
       "org.typelevel" %% "macro-compat" % "1.1.1",
       "org.scala-lang" % "scala-compiler" % scalaVersion.value % "provided"
     ),
-    libraryDependencies ++=
-      (CrossVersion.partialVersion(scalaVersion.value) match {
-        case Some((2, 13)) => Seq("org.scala-lang.modules" %%% "scala-collection-compat" % "0.2.0")
-        case _ => Seq("org.scala-lang.modules" %%% "scala-collection-compat" % "0.1.1")
-      }),
     libraryDependencies ++=
       (CrossVersion.partialVersion(scalaVersion.value) match {
         case Some((2, 13)) => Seq()
         case _ => Seq(compilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full))
       }),
+    unmanagedSourceDirectories in Compile += {
+    //val sourceDir = (sourceDirectory in Compile).value
+      // ^ gives jvm/src/main, for some reason
+      val sourceDir = baseDirectory.value / "../shared/src/main"
+      CrossVersion.partialVersion(scalaVersion.value) match {
+        case Some((2, n)) if n >= 13 => sourceDir / "scala-2.13+"
+        case _                       => sourceDir / "scala-2.13-"
+      }
+    },
     sourceGenerators in Compile += Def.task{
       val dir = (sourceManaged in Compile).value
 

play-json/shared/src/main/scala-2.13+/play/api/libs/json/ScalaCollectionCompat.scala

@@ -0,0 +1,10 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+/** Copied from scala-collection-compat, because its binary API isn't stable. */
+private[json] object ScalaCollectionCompat {
+  type Factory[-A, +C] = scala.collection.Factory[A, C]
+}

play-json/shared/src/main/scala-2.13-/play/api/libs/json/ScalaCollectionCompat.scala

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2009-2019 Lightbend Inc. <https://www.lightbend.com>
+ */
+
+package play.api.libs.json
+
+import scala.language.implicitConversions
+import scala.language.higherKinds
+
+import scala.collection.generic.CanBuildFrom
+import scala.collection.mutable
+
+/** Copied from scala-collection-compat, because its binary API isn't stable. */
+private[json] object ScalaCollectionCompat {
+
+  /**
+   * A factory that builds a collection of type `C` with elements of type `A`.
+   *
+   * Type param `A` is the type of elements (e.g. `Int`, `Boolean`, etc.).
+   * Type param `C` is the type of collection (e.g. `List[Int]`, `TreeMap[Int, String]`, etc.).
+   */
+  type Factory[-A, +C] = CanBuildFrom[Nothing, A, C]
+
+  final implicit class FactoryOps[-A, +C](private val factory: Factory[A, C]) {
+    def newBuilder: mutable.Builder[A, C] = factory()
+  }
+
+}

play-json/shared/src/main/scala/play/api/libs/json/Reads.scala

@@ -4,18 +4,16 @@
 
 package play.api.libs.json
 
-import scala.annotation.implicitNotFound
-
-import scala.util.control
+import scala.language.higherKinds
 
+import scala.annotation.implicitNotFound
 import scala.collection.Seq
-import scala.collection.compat._
 import scala.collection.immutable.Map
 import scala.collection.mutable.Builder
+import scala.reflect.ClassTag
+import scala.util.control
 
-import scala.language.higherKinds
-
-import reflect.ClassTag
+import ScalaCollectionCompat._
 
 /**
  * A `Reads` object describes how to decode JSON into a value.