Treat Collection as ArrayList in serializer by type lookups

core/commonMain/src/kotlinx/serialization/Serializers.kt

@@ -79,7 +79,7 @@ private fun SerializersModule.builtinSerializerOrNull(
         .map(::serializer)
     // Array is not supported, see KT-32839
     return when (rootClass) {
-        List::class, MutableList::class, ArrayList::class -> ArrayListSerializer(serializers[0])
+        Collection::class, List::class, MutableList::class, ArrayList::class -> ArrayListSerializer(serializers[0])
         HashSet::class -> HashSetSerializer(serializers[0])
         Set::class, MutableSet::class, LinkedHashSet::class -> LinkedHashSetSerializer(serializers[0])
         HashMap::class -> HashMapSerializer(serializers[0], serializers[1])

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

@@ -47,8 +47,8 @@ public fun SerializersModule.serializer(type: Type): KSerializer<Any> = when (ty
         val rootClass = (type.rawType as Class<*>)
         val args = (type.actualTypeArguments)
         when {
-            List::class.java.isAssignableFrom(rootClass) -> ListSerializer(serializer(args[0])) as KSerializer<Any>
             Set::class.java.isAssignableFrom(rootClass) -> SetSerializer(serializer(args[0])) as KSerializer<Any>
+            List::class.java.isAssignableFrom(rootClass) || Collection::class.java.isAssignableFrom(rootClass) -> ListSerializer(serializer(args[0])) as KSerializer<Any>
             Map::class.java.isAssignableFrom(rootClass) -> MapSerializer(
                 serializer(args[0]),
                 serializer(args[1])

formats/json/commonTest/src/kotlinx/serialization/SerializersLookupTest.kt

@@ -6,9 +6,7 @@ package kotlinx.serialization
 
 import kotlinx.serialization.builtins.*
 import kotlinx.serialization.descriptors.*
-import kotlinx.serialization.descriptors.PrimitiveSerialDescriptor
 import kotlinx.serialization.encoding.*
-import kotlinx.serialization.internal.*
 import kotlinx.serialization.json.*
 import kotlinx.serialization.modules.*
 import kotlinx.serialization.test.*
@@ -45,6 +43,13 @@ class SerializersLookupTest : JsonTestBase() {
         assertSerializedWithType("""["a","b","c"]""", myArr)
     }
 
+    @Test
+    fun testListAsCollection() {
+        val myArr: Collection<String> = listOf("a", "b", "c")
+        assertSerializedWithType("""["a","b","c"]""", myArr)
+    }
+
+
     @Test
     fun testPrimitiveSet() {
         val mySet = setOf("a", "b", "c", "c")

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

@@ -78,6 +78,19 @@ class SerializerByTypeTest {
         assertEquals("""["a","b","c"]""", s)
     }
 
+    @Test
+    fun testListAsCollection() {
+        val myArr: Collection<String> = listOf("a", "b", "c")
+        val token = object : ParameterizedType {
+            override fun getRawType(): Type = Collection::class.java
+            override fun getOwnerType(): Type? = null
+            override fun getActualTypeArguments(): Array<Type> = arrayOf(String::class.java)
+        }
+        val serial = serializer(token)
+        val s = json.encodeToString(serial, myArr)
+        assertEquals("""["a","b","c"]""", s)
+    }
+
 
     @Test
     fun testReifiedArrayResolving() {