Fix lookup of primitive array serializers by Java type token

core/jvmMain/src/kotlinx/serialization/SerializersJvm.kt

@@ -124,13 +124,13 @@ private fun SerializersModule.serializerByJavaTypeImpl(type: Type, failOnMissing
 
 @OptIn(ExperimentalSerializationApi::class)
 private fun SerializersModule.typeSerializer(type: Class<*>, failOnMissingTypeArgSerializer: Boolean): KSerializer<Any>? {
-    return if (!type.isArray) {
-        reflectiveOrContextual(type.kotlin as KClass<Any>, emptyList())
-    } else {
+    return if (type.isArray && !type.componentType.isPrimitive) {
         val eType: Class<*> = type.componentType
         val s = if (failOnMissingTypeArgSerializer) serializer(eType) else (serializerOrNull(eType) ?: return null)
         val arraySerializer = ArraySerializer(eType.kotlin as KClass<Any>, s)
         arraySerializer as KSerializer<Any>
+    } else {
+        reflectiveOrContextual(type.kotlin as KClass<Any>, emptyList())
     }
 }
 

formats/json/jvmTest/src/kotlinx/serialization/features/SerializerByTypeTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2020 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ * Copyright 2017-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 
 package kotlinx.serialization.features
@@ -93,6 +93,15 @@ class SerializerByTypeTest {
         assertSerializedWithType(token, """["a","b","c"]""", myArr)
     }
 
+    @Test
+    fun testPrimitiveArrayResolving() {
+        val myArr = intArrayOf(1, 2, 3)
+        val token = IntArray::class.java
+        val name = serializer(token).descriptor.serialName
+        assertTrue(name.contains("IntArray"))
+        assertSerializedWithType(token, """[1,2,3]""", myArr)
+    }
+
     @Test
     fun testReifiedListResolving() {
         val myList = listOf("a", "b", "c")