Support retreiving serializer for sealed interface (#1968)

Fixes #1965

Author: sandwwraith

core/commonTest/src/kotlinx/serialization/SerializersLookupInterfaceTest.kt

@@ -0,0 +1,52 @@
+package kotlinx.serialization
+
+import kotlinx.serialization.test.*
+import kotlin.test.*
+
+class SerializersLookupInterfaceTest {
+
+    interface I
+
+    @Polymorphic
+    interface I2
+
+    @Suppress("SERIALIZER_TYPE_INCOMPATIBLE")
+    @Serializable(PolymorphicSerializer::class)
+    interface I3
+
+    @Serializable
+    @SerialName("S")
+    sealed interface S
+
+    // TODO: not working because (see #1207, plugin does not produce companion object for interfaces)
+    // We even have #1853 with tests for that
+    // @Serializable(ExternalSerializer::class)
+    // interface External
+
+
+    @Test
+    fun testSealedInterfaceLookup() {
+        if (currentPlatform == Platform.JS_LEGACY) return
+        val serializer = serializer<S>()
+        assertTrue(serializer is SealedClassSerializer)
+        assertEquals("S", serializer.descriptor.serialName)
+    }
+
+    @Test
+    fun testInterfaceLookup() {
+        // Native does not have KClass.isInterface
+        if (currentPlatform == Platform.NATIVE || currentPlatform == Platform.JS_LEGACY) return
+
+        val serializer1 = serializer<I>()
+        assertTrue(serializer1 is PolymorphicSerializer)
+        assertEquals("kotlinx.serialization.Polymorphic<I>", serializer1.descriptor.serialName)
+
+        val serializer2 = serializer<I2>()
+        assertTrue(serializer2 is PolymorphicSerializer)
+        assertEquals("kotlinx.serialization.Polymorphic<I2>", serializer2.descriptor.serialName)
+
+        val serializer3 = serializer<I3>()
+        assertTrue(serializer3 is PolymorphicSerializer)
+        assertEquals("kotlinx.serialization.Polymorphic<I3>", serializer3.descriptor.serialName)
+    }
+}

core/jvmMain/src/kotlinx/serialization/internal/Platform.kt

@@ -44,9 +44,8 @@ internal fun <T: Any> Class<T>.constructSerializerForGivenTypeArgs(vararg args:
     if (isEnum && isNotAnnotated()) {
         return createEnumSerializer()
     }
-    if (isInterface) {
-        return interfaceSerializer()
-    }
+    // Fall-through if the serializer is not found -- lookup on companions (for sealed interfaces) or fallback to polymorphic if applicable
+    if (isInterface) interfaceSerializer()?.let { return it }
     // Search for serializer defined on companion object.
     val serializer = invokeSerializerOnCompanion<T>(this, *args)
     if (serializer != null) return serializer