Implement strict decoding

derivation/src/main/scala/io/circe/magnolia/MagnoliaDecoder.scala

@@ -25,7 +25,7 @@ private[magnolia] object MagnoliaDecoder {
       throw new DerivationError("Duplicate key detected after applying transformation function for case class parameters")
     }
 
-    if (configuration.useDefaults) {
+    val nonStrictDecoder = if (configuration.useDefaults) {
       new Decoder[T] {
         override def apply(c: HCursor): Result[T] = {
           caseClass.constructEither { p =>
@@ -52,6 +52,26 @@ private[magnolia] object MagnoliaDecoder {
         }.leftMap(_.head)
       }
     }
+
+    if (configuration.strictDecoding) {
+      val expectedFields = paramJsonKeyLookup.values
+      val strictDecoder = nonStrictDecoder.validate { cursor: HCursor =>
+        val maybeUnexpectedErrors = for {
+          json <- cursor.focus
+          jsonKeys <- json.hcursor.keys
+          unexpected = jsonKeys.toSet -- expectedFields
+        } yield {
+          unexpected.toList map { unexpectedField =>
+            s"Unexpected field: [$unexpectedField]. Valid fields: ${expectedFields.mkString(",")}"
+          }
+        }
+
+        maybeUnexpectedErrors.getOrElse(List("Couldn't determine decoded fields."))
+      }
+      (c: HCursor) => strictDecoder(c)
+    } else {
+      nonStrictDecoder
+    }
   }
 
   private[magnolia] def dispatch[T](

derivation/src/main/scala/io/circe/magnolia/configured/Configuration.scala

@@ -19,12 +19,15 @@ import java.util.regex.Pattern
   *                                  formatting or case changes.
   *                                  If there are collisions in transformed constructor names, an exception will be thrown
   *                                  during derivation (runtime)
+  * @param strictDecoding When true, raises a decoding error when there are any extraneous fields in the given JSON
+  *                       that aren't present in the case class.
   */
 final case class Configuration(
   transformMemberNames: String => String,
   transformConstructorNames: String => String,
   useDefaults: Boolean,
-  discriminator: Option[String]
+  discriminator: Option[String],
+  strictDecoding: Boolean = false
 ) {
   def withSnakeCaseMemberNames: Configuration = copy(
     transformMemberNames = Configuration.snakeCaseTransformation
@@ -44,6 +47,7 @@ final case class Configuration(
 
   def withDefaults: Configuration = copy(useDefaults = true)
   def withDiscriminator(discriminator: String): Configuration = copy(discriminator = Some(discriminator))
+  def withStrictDecoding: Configuration = copy(strictDecoding = true)
 }
 
 final object Configuration {

tests/src/test/scala/io/circe/magnolia/configured/ConfiguredAutoDerivedEquivalenceSuite.scala

@@ -134,6 +134,7 @@ class ConfiguredAutoDerivedEquivalenceSuite extends CirceSuite {
   testWithConfiguration("with snake case configuration", Configuration.default.withSnakeCaseConstructorNames.withSnakeCaseMemberNames)
   testWithConfiguration("with useDefault = true", Configuration.default.copy(useDefaults = true))
   testWithConfiguration("with discriminator", Configuration.default.copy(discriminator = Some("type")))
+  testWithConfiguration("with strict", Configuration.default.copy(strictDecoding = true))
 
   "If a sealed trait subtype has explicit Encoder instance that doesn't encode to a JsonObject, the derived encoder" should
     s"wrap it with type constructor even when discriminator is specified by the configuration" in {

tests/src/test/scala/io/circe/magnolia/configured/ConfiguredSemiautoDerivedEquivalenceSuite.scala

@@ -126,6 +126,7 @@ class ConfiguredSemiautoDerivedEquivalenceSuite extends CirceSuite {
   testWithConfiguration("with snake case configuration", Configuration.default.withSnakeCaseConstructorNames.withSnakeCaseMemberNames)
   testWithConfiguration("with useDefault = true", Configuration.default.copy(useDefaults = true))
   testWithConfiguration("with discriminator", Configuration.default.copy(discriminator = Some("type")))
+  testWithConfiguration("with strict", Configuration.default.copy(strictDecoding = true))
 
   "If a sealed trait subtype has explicit Encoder instance that doesn't encode to a JsonObject, the derived encoder" should
     s"wrap it with type constructor even when discriminator is specified by the configuration" in {

tests/src/test/scala/io/circe/magnolia/configured/ConfiguredSemiautoDerivedSuite.scala

@@ -2,12 +2,7 @@ package io.circe.magnolia.configured
 
 import io.circe.magnolia.DerivationError
 import io.circe._
-import io.circe.magnolia.configured.ConfiguredSemiautoDerivedSuite.{
-  DefaultConfig,
-  KebabCase,
-  SnakeCaseAndDiscriminator,
-  WithDefaultValue
-}
+import io.circe.magnolia.configured.ConfiguredSemiautoDerivedSuite.{DefaultConfig, KebabCase, Lenient, SnakeCaseAndDiscriminator, Strict, WithDefaultValue}
 import io.circe.tests.CirceSuite
 import io.circe.tests.examples.{Bar, ClassWithDefaults, ClassWithJsonKey, NonProfit, Organization, Public}
 import org.scalatest.Inside
@@ -192,6 +187,38 @@ class ConfiguredSemiautoDerivedSuite extends CirceSuite with Inside {
     )
   }
 
+  "Configuration#strictDecoding" should "Raise error when strict decoding enabled and extraneous key is found in JSON" in {
+    val input = parse("""
+      {
+        "NonProfit": {
+          "orgName": "RSPCA",
+          "extraneous": true
+        }
+      }
+    """)
+    inside(input.flatMap(i => Strict.decoder(i.hcursor))) {
+      case Left(e: DecodingFailure) => {
+        assert(e.message.contains("Unexpected field"))
+        assert(e.message.contains("extraneous"))
+        assert(e.message.contains("orgName"))
+      }
+      case x => fail(x.toString)
+    }
+  }
+
+  "Configuration#strictDecoding" should "Should not raise error when strict decoding is disabled and extraneous key is found in JSON" in {
+    val input = parse("""
+      {
+        "NonProfit": {
+          "orgName": "RSPCA",
+          "extraneous": true
+        }
+      }
+    """)
+    val expected = NonProfit("RSPCA")
+    assert(input.flatMap(i => Lenient.decoder(i.hcursor)) == Right(expected))
+  }
+
   "Encoder derivation" should "fail if transforming parameter names has collisions" in {
     implicit val config: Configuration = Configuration.default.copy(transformMemberNames = _ => "sameKey")
 
@@ -279,4 +306,18 @@ object ConfiguredSemiautoDerivedSuite {
     val decoder: Decoder[Organization] = deriveConfiguredMagnoliaDecoder[Organization]
   }
 
+  object Strict {
+    implicit val configuration: Configuration = Configuration.default.withStrictDecoding
+
+    val encoder: Encoder[Organization] = deriveConfiguredMagnoliaEncoder[Organization]
+    val decoder: Decoder[Organization] = deriveConfiguredMagnoliaDecoder[Organization]
+  }
+
+  object Lenient {
+    implicit val configuration: Configuration = Configuration.default
+
+    val encoder: Encoder[Organization] = deriveConfiguredMagnoliaEncoder[Organization]
+    val decoder: Decoder[Organization] = deriveConfiguredMagnoliaDecoder[Organization]
+  }
+
 }