Added support for null values for nullable enums in lanient mode

formats/json-tests/commonTest/src/kotlinx/serialization/json/JsonCoerceInputValuesTest.kt

@@ -5,7 +5,6 @@
 package kotlinx.serialization.json
 
 import kotlinx.serialization.*
-import kotlinx.serialization.json.internal.*
 import kotlinx.serialization.test.assertFailsWithSerial
 import kotlin.test.*
 
@@ -25,6 +24,11 @@ class JsonCoerceInputValuesTest : JsonTestBase() {
         val foo: String
     )
 
+    @Serializable
+    data class NullableEnumHolder(
+        val enum: SampleEnum?
+    )
+
     val json = Json {
         coerceInputValues = true
         isLenient = true
@@ -99,4 +103,13 @@ class JsonCoerceInputValuesTest : JsonTestBase() {
             assertEquals(expected, json.decodeFromString(MultipleValues.serializer(), input), "Failed on input: $input")
         }
     }
+
+    @Test
+    fun testNullSupportForEnums() = parametrizedTest(json) {
+        var decoded = decodeFromString<NullableEnumHolder>("""{"enum": null}""")
+        assertNull(decoded.enum)
+
+        decoded = decodeFromString<NullableEnumHolder>("""{"enum": OptionA}""")
+        assertEquals(SampleEnum.OptionA, decoded.enum)
+    }
 }

formats/json/commonMain/src/kotlinx/serialization/json/internal/JsonNamesMap.kt

@@ -99,11 +99,16 @@ internal fun SerialDescriptor.getJsonNameIndexOrThrow(json: Json, name: String,
 internal inline fun Json.tryCoerceValue(
     elementDescriptor: SerialDescriptor,
     peekNull: () -> Boolean,
+    tryPeekNull: () -> Boolean,
     peekString: () -> String?,
     onEnumCoercing: () -> Unit = {}
 ): Boolean {
     if (!elementDescriptor.isNullable && peekNull()) return true
     if (elementDescriptor.kind == SerialKind.ENUM) {
+        if (elementDescriptor.isNullable && tryPeekNull()) {
+            return false
+        }
+
         val enumValue = peekString()
             ?: return false // if value is not a string, decodeEnum() will throw correct exception
         val enumIndex = elementDescriptor.getJsonNameIndex(this, enumValue)

formats/json/commonMain/src/kotlinx/serialization/json/internal/StreamingJsonDecoder.kt

@@ -209,6 +209,7 @@ internal open class StreamingJsonDecoder(
     private fun coerceInputValue(descriptor: SerialDescriptor, index: Int): Boolean = json.tryCoerceValue(
         descriptor.getElementDescriptor(index),
         { !lexer.tryConsumeNotNull() },
+        { !lexer.tryConsumeNotNull(false) },
         { lexer.peekString(configuration.isLenient) },
         { lexer.consumeString() /* skip unknown enum string*/ }
     )

formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt

@@ -196,6 +196,7 @@ private open class JsonTreeDecoder(
         json.tryCoerceValue(
             descriptor.getElementDescriptor(index),
             { currentElement(tag) is JsonNull },
+            { currentElement(tag) is JsonNull },
             { (currentElement(tag) as? JsonPrimitive)?.contentOrNull }
         )
 

formats/json/commonMain/src/kotlinx/serialization/json/internal/lexer/AbstractJsonLexer.kt

@@ -246,8 +246,11 @@ internal abstract class AbstractJsonLexer {
      * Tries to consume `null` token from input.
      * Returns `true` if the next 4 chars in input are not `null`,
      * `false` otherwise and consumes it.
+     *
+     * If [moveForward] is true and if the next 4 chars in input are `null` then
+     * current position in lexer move forward on these 4 chars.
      */
-    fun tryConsumeNotNull(): Boolean {
+    fun tryConsumeNotNull(moveForward: Boolean = true): Boolean {
         var current = skipWhitespaces()
         current = prefetchOrEof(current)
         // Cannot consume null due to EOF, maybe something else
@@ -261,7 +264,10 @@ internal abstract class AbstractJsonLexer {
          * distinguish it from 'null'
          */
         if (len > 4 && charToTokenClass(source[current + 4]) == TC_OTHER) return true
-        currentPosition = current + 4
+
+        if (moveForward) {
+            currentPosition = current + 4
+        }
         return false
     }
 

formats/json/jsMain/src/kotlinx/serialization/json/internal/DynamicDecoders.kt

@@ -75,6 +75,7 @@ private open class DynamicInput(
         json.tryCoerceValue(
             descriptor.getElementDescriptor(index),
             { getByTag(tag) == null },
+            { getByTag(tag) == null },
             { getByTag(tag) as? String }
         )