Fix #95 by failing a compilation in case of no leaf subclasses was found for the ADT base

Author: plokhotnyuk

macros/src/main/scala/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMaker.scala

@@ -143,12 +143,20 @@ object JsonCodecMaker {
             s"of a sealed definition for '$tpe' or using a custom implicitly accessible codec for the ADT base."))
       }
 
-      def adtLeafClasses(tpe: Type): Set[Type] = tpe.typeSymbol.asClass.knownDirectSubclasses.flatMap { s =>
-        val subTpe = s.asClass.toType
-        if (isSealedAdtBase(subTpe)) adtLeafClasses(subTpe)
-        else if (s.asClass.isCaseClass) Set(subTpe)
-        else fail("Only case classes & case objects are supported for ADT leaf classes. Please consider using " +
-          s"of them for ADT with base '$tpe' or using a custom implicitly accessible codec for the ADT base.")
+      def adtLeafClasses(tpe: Type): Seq[Type] = {
+        def collectRecursively(tpe: Type): Set[Type] =
+          tpe.typeSymbol.asClass.knownDirectSubclasses.flatMap { s =>
+            val subTpe = s.asClass.toType
+            if (isSealedAdtBase(subTpe)) collectRecursively(subTpe)
+            else if (s.asClass.isCaseClass) Set(subTpe)
+            else fail("Only case classes & case objects are supported for ADT leaf classes. Please consider using " +
+              s"of them for ADT with base '$tpe' or using a custom implicitly accessible codec for the ADT base.")
+          }
+
+        val classes = collectRecursively(tpe).toSeq
+        if (classes.isEmpty) fail(s"Cannot find leaf classes for ADT base '$tpe'. Please consider adding them or " +
+          "using a custom implicitly accessible codec for the ADT base.")
+        classes
       }
 
       def companion(tpe: Type): Tree = Ident(tpe.typeSymbol.companion)
@@ -702,7 +710,7 @@ object JsonCodecMaker {
             JsonReader.toHashCode(cs, cs.length)
           }
 
-          val leafClasses = adtLeafClasses(tpe).toSeq
+          val leafClasses = adtLeafClasses(tpe)
           val discrName = codecConfig.discriminatorFieldName
           checkDiscriminatorValueCollisions(discrName, leafClasses.map(discriminatorValue))
           val discriminatorValueError = q"in.discriminatorValueError($discrName)"

macros/src/test/scala/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMakerSpec.scala

@@ -931,6 +931,22 @@ class JsonCodecMakerSpec extends WordSpec with Matchers {
           .stripMargin.replace('\n', ' ')
       })
     }
+    "don't generate codec for ADT base without leaf classes" in {
+      assert(intercept[TestFailedException](assertCompiles {
+        """sealed trait X
+          |JsonCodecMaker.make[X](CodecMakerConfig())""".stripMargin
+      }).getMessage.contains {
+        """Cannot find leaf classes for ADT base 'X'. Please consider adding them or using a custom implicitly
+          |accessible codec for the ADT base.""".stripMargin.replace('\n', ' ')
+      })
+      assert(intercept[TestFailedException](assertCompiles {
+        """sealed abstract class X
+          |JsonCodecMaker.make[X](CodecMakerConfig())""".stripMargin
+      }).getMessage.contains {
+        """Cannot find leaf classes for ADT base 'X'. Please consider adding them or using a custom implicitly
+          |accessible codec for the ADT base.""".stripMargin.replace('\n', ' ')
+      })
+    }
     "don't generate codec for case objects which are mapped to the same discriminator value" in {
       assert(intercept[TestFailedException](assertCompiles {
         """sealed trait X