Disallow 'discriminatorFieldName' to be 'None' when 'alwaysEmitDiscriminator' is 'true'

Author: plokhotnyuk

jsoniter-scala-macros/shared/src/main/scala-2/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMaker.scala

@@ -582,6 +582,8 @@ object JsonCodecMaker {
           fail("'requireDefaultFields' and 'transientDefault' cannot be 'true' simultaneously")
         if (cfg.circeLikeObjectEncoding && cfg.discriminatorFieldName.nonEmpty)
           fail("'discriminatorFieldName' should be 'None' when 'circeLikeObjectEncoding' is 'true'")
+        if (cfg.alwaysEmitDiscriminator && cfg.discriminatorFieldName.isEmpty)
+          fail("'discriminatorFieldName' should not be 'None' when 'alwaysEmitDiscriminator' is 'true'")
         if (cfg.decodingOnly && cfg.encodingOnly)
           fail("'decodingOnly' and 'encodingOnly' cannot be 'true' simultaneously")
         cfg

jsoniter-scala-macros/shared/src/main/scala-3/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMaker.scala

@@ -728,6 +728,8 @@ object JsonCodecMaker {
               report.errorAndAbort("'requireDefaultFields' and 'transientDefault' cannot be 'true' simultaneously")
             if (cfg.circeLikeObjectEncoding && cfg.discriminatorFieldName.nonEmpty)
               report.errorAndAbort("'discriminatorFieldName' should be 'None' when 'circeLikeObjectEncoding' is 'true'")
+            if (cfg.alwaysEmitDiscriminator && cfg.discriminatorFieldName.isEmpty)
+              report.errorAndAbort("'discriminatorFieldName' should not be 'None' when 'alwaysEmitDiscriminator' is 'true'")
             if (cfg.decodingOnly && cfg.encodingOnly)
               report.errorAndAbort("'decodingOnly' and 'encodingOnly' cannot be 'true' simultaneously")
             cfg

jsoniter-scala-macros/shared/src/test/scala/com/github/plokhotnyuk/jsoniter_scala/macros/JsonCodecMakerSpec.scala

@@ -2249,6 +2249,15 @@ class JsonCodecMakerSpec extends VerifyingSpec {
         """{"type":"CCC","a":1,"b":"VVV"}""")
       verifySerDeser(make[DDD.type](CodecMakerConfig.withAlwaysEmitDiscriminator(true)), DDD, """{"type":"DDD"}""")
     }
+    "don't generate codecs for CodecMakerConfig.withDiscriminatorFieldName(_root_.scala.None).withAlwaysEmitDiscriminator(true) compile-time configuration" in {
+      assert(intercept[TestFailedException](assertCompiles {
+        """sealed trait A
+          |case class B(y: Int) extends A
+          |JsonCodecMaker.make[B](CodecMakerConfig.withDiscriminatorFieldName(_root_.scala.None).withAlwaysEmitDiscriminator(true))""".stripMargin
+      }).getMessage.contains {
+        "'discriminatorFieldName' should not be 'None' when 'alwaysEmitDiscriminator' is 'true'"
+      })
+    }
     "deserialize ADTs with extra fields" in {
       sealed trait Base