[avro] Add Either typeclass support

Support scala Either as a avro type union

avro/src/main/scala/magnolify/avro/AvroType.scala

@@ -27,7 +27,7 @@ import org.joda.{time => joda}
 import java.nio.{ByteBuffer, ByteOrder}
 import java.time._
 import java.{util => ju}
-import scala.annotation.implicitNotFound
+import scala.annotation.{implicitNotFound, nowarn}
 import scala.collection.concurrent
 import scala.collection.compat._
 import scala.jdk.CollectionConverters._
@@ -253,6 +253,31 @@ object AvroField {
       }
     }
 
+  implicit def afEither[A, B](implicit
+    fa: AvroField[A],
+    fb: AvroField[B]
+  ): AvroField[Either[A, B]] =
+    new Aux[Either[A, B], Any, Any] {
+      override protected def buildSchema(cm: CaseMapper): Schema =
+        Schema.createUnion(fa.schema(cm), fb.schema(cm))
+      // `Either[A, B]` is a `UNION` of `[A, B]` and must default to first type `A`
+      override def makeDefault(d: Either[A, B])(cm: CaseMapper): fa.ToT = {
+        require(d.isLeft, "Either[A, B] can only default to Left[A]")
+        fa.to(d.left.get)(cm): @nowarn
+      }
+      override def from(v: Any)(cm: CaseMapper): Either[A, B] = {
+        GenericData.get().resolveUnion(schema(cm), v) match {
+          case 0 => Left(fa.from(v.asInstanceOf[fa.FromT])(cm))
+          case 1 => Right(fb.from(v.asInstanceOf[fb.FromT])(cm))
+        }
+      }
+
+      override def to(v: Either[A, B])(cm: CaseMapper): Any = v match {
+        case Left(a)    => fa.to(a)(cm)
+        case Right(b) => fb.to(b)(cm)
+      }
+    }
+
   implicit def afIterable[T, C[_]](implicit
     f: AvroField[T],
     ti: C[T] => Iterable[T],

avro/src/test/scala/magnolify/avro/AvroTypeSuite.scala

@@ -317,6 +317,7 @@ class AvroTypeSuite extends MagnolifySuite {
       val inner = DefaultInner(
         2,
         Some(2),
+        Right("2"),
         List(2, 2),
         Map("b" -> 2),
         JavaEnums.Color.GREEN,
@@ -337,6 +338,7 @@ class AvroTypeSuite extends MagnolifySuite {
       val inner = DefaultInner(
         3,
         Some(3),
+        Right("3"),
         List(3, 3),
         Map("c" -> 3),
         JavaEnums.Color.BLUE,
@@ -359,6 +361,7 @@ class AvroTypeSuite extends MagnolifySuite {
   }
 
   testFail(AvroType[DefaultSome])("Option[T] can only default to None")
+  testFail(AvroType[DefaultEither])("Either[A, B] can only default to Left[A]")
 
   {
     implicit val at: AvroType[LowerCamel] = AvroType[LowerCamel](CaseMapper(_.toUpperCase))
@@ -493,6 +496,7 @@ case class DoubleFieldDoc(@doc("doc1") @doc("doc2") i: Int)
 case class DefaultInner(
   i: Int = 1,
   o: Option[Int] = None,
+  e: Either[Int, String] = Left(1),
   l: List[Int] = List(1, 1),
   m: Map[String, Int] = Map("a" -> 1),
   je: JavaEnums.Color = JavaEnums.Color.RED,
@@ -508,6 +512,7 @@ case class DefaultOuter(
   i: DefaultInner = DefaultInner(
     2,
     None,
+    Left(2),
     List(2, 2),
     Map("b" -> 2),
     JavaEnums.Color.GREEN,
@@ -521,7 +526,9 @@ case class DefaultOuter(
   ),
   o: Option[DefaultInner] = None
 )
+
 case class DefaultSome(o: Option[Int] = Some(1))
+case class DefaultEither(e: Either[Int, String] = Right("1"))
 
 case class DefaultBytes(
   a: Array[Byte] = Array(2, 2)