Correctly support registering both default polymorphic serializer & deserializer (#1849)

Fixes #1837

Author: sandwwraith

core/commonMain/src/kotlinx/serialization/modules/SerializersModuleBuilders.kt

@@ -153,9 +153,9 @@ public class SerializersModuleBuilder @PublishedApi internal constructor() : Ser
         defaultSerializerProvider: (value: Base) -> SerializationStrategy<Base>?,
         allowOverwrite: Boolean
     ) {
-        val previous = polyBase2DefaultDeserializerProvider[baseClass]
+        val previous = polyBase2DefaultSerializerProvider[baseClass]
         if (previous != null && previous != defaultSerializerProvider && !allowOverwrite) {
-            throw IllegalArgumentException("Default serializers provider for class $baseClass is already registered: $previous")
+            throw IllegalArgumentException("Default serializers provider for $baseClass is already registered: $previous")
         }
         polyBase2DefaultSerializerProvider[baseClass] = defaultSerializerProvider
     }
@@ -168,7 +168,7 @@ public class SerializersModuleBuilder @PublishedApi internal constructor() : Ser
     ) {
         val previous = polyBase2DefaultDeserializerProvider[baseClass]
         if (previous != null && previous != defaultDeserializerProvider && !allowOverwrite) {
-            throw IllegalArgumentException("Default deserializers provider for class $baseClass is already registered: $previous")
+            throw IllegalArgumentException("Default deserializers provider for $baseClass is already registered: $previous")
         }
         polyBase2DefaultDeserializerProvider[baseClass] = defaultDeserializerProvider
     }

core/commonTest/src/kotlinx/serialization/modules/ModuleBuildersTest.kt

@@ -9,6 +9,7 @@ package kotlinx.serialization.modules
 import kotlinx.serialization.*
 import kotlinx.serialization.builtins.*
 import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
 import kotlin.reflect.*
 import kotlin.test.*
 
@@ -305,6 +306,21 @@ class ModuleBuildersTest {
         assertEquals<Any?>(Unit.serializer(), m2.getPolymorphic(Any::class, serializedClassName = "foo"))
     }
 
+    @Test
+    fun testBothPolymorphicDefaults() {
+        val anySerializer = object : KSerializer<Any> {
+            override val descriptor: SerialDescriptor get() = error("descriptor")
+            override fun serialize(encoder: Encoder, value: Any): Unit = error("serialize")
+            override fun deserialize(decoder: Decoder): Any = error("deserialize")
+        }
+        val module = SerializersModule {
+            polymorphicDefaultDeserializer(Any::class) { _ -> anySerializer }
+            polymorphicDefaultSerializer(Any::class) { _ -> anySerializer }
+        }
+        assertEquals(anySerializer, module.getPolymorphic(Any::class, 42))
+        assertEquals(anySerializer, module.getPolymorphic(Any::class, serializedClassName = "42"))
+    }
+
     @Test
     fun testPolymorphicForStandardSubtypesOfAny() {
         val serializer = object : KSerializer<Int> by Int.serializer() {}