Support @newtype in package objects with cross-file usage

Rewrite package objects to nested PackageDefs during the plugin phase,
avoiding Scala 3's "Trying to define package with same name as class"
conflict when other files declare the same package. Also add Scala 3.8.3
to the supported versions list.

Author: MateuszKubuszok

build.sbt

@@ -13,7 +13,7 @@ val scala3Versions = Seq(
   "3.5.0", "3.5.1", "3.5.2",
   "3.6.2", "3.6.3", "3.6.4",
   "3.7.0", "3.7.1", "3.7.2", "3.7.3", "3.7.4",
-  "3.8.0", "3.8.1", "3.8.2"
+  "3.8.0", "3.8.1", "3.8.2", "3.8.3"
 )
 
 val scala3Latest = scala3Versions.last

plugin/src/main/scala/io/estatico/newtype/compat/NewTypeTreeTransformer.scala

@@ -264,7 +264,16 @@ class NewTypeTreeTransformer extends UntypedTreeMap:
 
   override def transform(tree: Tree)(using Context): Tree = tree match
     case PackageDef(pid, stats) =>
-      cpy.PackageDef(tree)(transform(pid).asInstanceOf[RefTree], transformStats(stats))
+      val transformedPid = transform(pid).asInstanceOf[RefTree]
+      stats.partition { case md: ModuleDef => md.mods.is(Package); case _ => false } match
+        case (pkgObj :: Nil, otherStats) =>
+          val md = pkgObj.asInstanceOf[ModuleDef]
+          val innerBody = transformStats(md.impl.body)
+          val allInnerStats = otherStats.map(transform) ++ innerBody
+          val newPid = Select(transformedPid, md.name)
+          untpd.PackageDef(newPid, allInnerStats)
+        case _ =>
+          cpy.PackageDef(tree)(transformedPid, transformStats(stats))
     case Block(stats, expr) =>
       cpy.Block(tree)(transformStats(stats), transform(expr))
     case td @ TypeDef(name, rhs: Template) if !findNewTypeAnnotation(td).isDefined =>

tests/src/test/scala/io/estatico/newtype/pkgobject/PackageObjectTest.scala

@@ -0,0 +1,36 @@
+package io.estatico.newtype.pkgobject
+
+import io.estatico.newtype.ops._
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import java.util.UUID
+
+class PackageObjectTest extends AnyFlatSpec with Matchers {
+
+  behavior of "@newtype in package object"
+
+  it should "create a newtype defined in a package object" in {
+    val uuid = UUID.randomUUID()
+    val x: ClayTableId = ClayTableId(uuid)
+    x.value shouldBe uuid
+    x.coerce[UUID] shouldBe uuid
+  }
+
+  it should "support wrapping via coerce" in {
+    val uuid = UUID.randomUUID()
+    val x = uuid.coerce[ClayTableId]
+    x.value shouldBe uuid
+  }
+
+  it should "work with multiple newtypes in the same package object" in {
+    val x: ClayFieldName = ClayFieldName("test")
+    x.value shouldBe "test"
+  }
+
+  it should "make types accessible from other files in the same package" in {
+    val uuid = UUID.randomUUID()
+    val u = UsesPackageObjectTypes(ClayTableId(uuid), ClayFieldName("field"))
+    u.tableId.value shouldBe uuid
+    u.fieldName.value shouldBe "field"
+  }
+}

tests/src/test/scala/io/estatico/newtype/pkgobject/UsesPackageObjectTypes.scala

@@ -0,0 +1,8 @@
+package io.estatico.newtype.pkgobject
+
+import java.util.UUID
+
+final case class UsesPackageObjectTypes(
+  tableId: ClayTableId,
+  fieldName: ClayFieldName
+)

tests/src/test/scala/io/estatico/newtype/pkgobject/package.scala

@@ -0,0 +1,22 @@
+package io.estatico.newtype
+
+import cats.{Hash, Order, Show}
+import io.estatico.newtype.macros.newtype
+import java.util.UUID
+
+package object pkgobject {
+
+  @newtype final case class ClayTableId(value: UUID)
+  object ClayTableId {
+    implicit lazy val showForClayTableId: Show[ClayTableId] = Show.show(_.value.toString())
+    implicit lazy val orderForClayTableId: Order[ClayTableId] = Order.by(_.value)
+    implicit lazy val hashForClayTableId: Hash[ClayTableId] = Hash.by(_.value)
+  }
+
+  @newtype final case class ClayFieldName(value: String)
+  object ClayFieldName {
+    implicit lazy val showForClayFieldName: Show[ClayFieldName] = Show.show(_.value)
+    implicit lazy val orderForClayFieldName: Order[ClayFieldName] = Order.by(_.value)
+    implicit lazy val hashForClayFieldName: Hash[ClayFieldName] = Hash.by(_.value)
+  }
+}