Support Option of sealed trait with 1 subtype (sksamuel#888)

Author: pjanczyk

avro4s-core/src/main/scala/com/sksamuel/avro4s/decoders/magnolia.scala

@@ -14,6 +14,10 @@ trait MagnoliaDerivedDecoder extends AutoDerivation[Decoder] :
 
   override def split[T](ctx: SealedTrait[Decoder, T]): Decoder[T] =
     DatatypeShape.of[T](ctx) match {
-      case SealedTraitShape.TypeUnion => TypeUnions.decoder(ctx)
+      case SealedTraitShape.TypeUnion =>
+        ctx.subtypes match {
+          case IArray(single) => single.typeclass.asInstanceOf[Decoder[T]]
+          case multiple => TypeUnions.decoder(ctx)
+        }
       case SealedTraitShape.Enum => SealedTraits.decoder(ctx)
     }

avro4s-core/src/main/scala/com/sksamuel/avro4s/encoders/magnolia.scala

@@ -12,6 +12,10 @@ trait MagnoliaDerivedEncoder extends AutoDerivation[Encoder] :
 
   override def split[T](ctx: SealedTrait[Encoder, T]): Encoder[T] =
     DatatypeShape.of[T](ctx) match {
-      case SealedTraitShape.TypeUnion => TypeUnions.encoder(ctx)
+      case SealedTraitShape.TypeUnion =>
+        ctx.subtypes match {
+          case IArray(single) => single.typeclass.asInstanceOf[Encoder[T]]
+          case multiple => TypeUnions.encoder(ctx)
+        }
       case SealedTraitShape.Enum => SealedTraits.encoder(ctx)
     }

avro4s-core/src/main/scala/com/sksamuel/avro4s/schemas/magnolia.scala

@@ -17,6 +17,10 @@ trait MagnoliaDerivedSchemas extends AutoDerivation[SchemaFor] :
 
   override def split[T](ctx: SealedTrait[SchemaFor, T]): SchemaFor[T] =
     DatatypeShape.of[T](ctx) match {
-      case SealedTraitShape.TypeUnion => TypeUnions.schema(ctx)
+      case SealedTraitShape.TypeUnion =>
+        ctx.subtypes match {
+          case IArray(single) => single.typeclass.forType[T]
+          case multiple => TypeUnions.schema(ctx)
+        }
       case SealedTraitShape.Enum => SchemaFor[T](SealedTraits.schema(ctx))
     }

avro4s-core/src/test/scala/com/sksamuel/avro4s/github/GithubIssue885.scala

@@ -0,0 +1,19 @@
+package com.sksamuel.avro4s.github
+
+import com.sksamuel.avro4s.{AvroSchema, FromRecord, SchemaFor, ToRecord}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class GithubIssue885 extends AnyFunSuite with Matchers {
+
+  test("support Option of sealed trait with 1 subtype") {
+    sealed trait Example
+    case class A(v: String) extends Example
+
+    val value = Some(A("a"))
+
+    val schema = AvroSchema[Option[Example]]
+    val record = ToRecord[Option[Example]](schema).to(value)
+    FromRecord[Option[Example]](schema).from(record) shouldBe value
+  }
+}