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

@@ -98,12 +98,16 @@ internal fun SerialDescriptor.getJsonNameIndexOrThrow(json: Json, name: String,
 @OptIn(ExperimentalSerializationApi::class)
 internal inline fun Json.tryCoerceValue(
     elementDescriptor: SerialDescriptor,
-    peekNull: () -> Boolean,
+    peekNull: (consume: Boolean) -> Boolean,
     peekString: () -> String?,
     onEnumCoercing: () -> Unit = {}
 ): Boolean {
-    if (!elementDescriptor.isNullable && peekNull()) return true
+    if (!elementDescriptor.isNullable && peekNull(true)) return true
     if (elementDescriptor.kind == SerialKind.ENUM) {
+        if (elementDescriptor.isNullable && peekNull(false)) {
+            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

@@ -133,7 +133,7 @@ internal open class StreamingJsonDecoder(
     }
 
     override fun decodeNotNullMark(): Boolean {
-        return !(elementMarker?.isUnmarkedNull ?: false) && lexer.tryConsumeNotNull()
+        return !(elementMarker?.isUnmarkedNull ?: false) && !lexer.tryConsumeNull()
     }
 
     override fun decodeNull(): Nothing? {
@@ -208,7 +208,7 @@ internal open class StreamingJsonDecoder(
      */
     private fun coerceInputValue(descriptor: SerialDescriptor, index: Int): Boolean = json.tryCoerceValue(
         descriptor.getElementDescriptor(index),
-        { !lexer.tryConsumeNotNull() },
+        { lexer.tryConsumeNull(it) },
         { lexer.peekString(configuration.isLenient) },
         { lexer.consumeString() /* skip unknown enum string*/ }
     )

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

@@ -244,25 +244,28 @@ 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.
+     * Returns `false` if the next 4 chars in input are not `null`,
+     * `true` otherwise and consumes it if [doConsume] is `true`.
      */
-    fun tryConsumeNotNull(): Boolean {
+    fun tryConsumeNull(doConsume: Boolean = true): Boolean {
         var current = skipWhitespaces()
         current = prefetchOrEof(current)
         // Cannot consume null due to EOF, maybe something else
         val len = source.length - current
-        if (len < 4 || current == -1) return true
+        if (len < 4 || current == -1) return false
         for (i in 0..3) {
-            if (NULL[i] != source[current + i]) return true
+            if (NULL[i] != source[current + i]) return false
         }
         /*
          * If we're in lenient mode, this might be the string with 'null' prefix,
          * distinguish it from 'null'
          */
-        if (len > 4 && charToTokenClass(source[current + 4]) == TC_OTHER) return true
-        currentPosition = current + 4
-        return false
+        if (len > 4 && charToTokenClass(source[current + 4]) == TC_OTHER) return false
+
+        if (doConsume) {
+            currentPosition = current + 4
+        }
+        return true
     }
 
     open fun skipWhitespaces(): Int {