I don't understand the algorithm for `serializer(f...

# serialization

[CLOVIS]

I don't understand the algorithm for serializer(foo: KType) .

I have this test code:

test("Access a serializer that doesn't exist on all platforms") {
		check(serializerOrNull(typeOf<Foo>()) == null) { "The class Foo is not serializable, this code shouldn't find a serializer" }
	}

	test("Access a serializer that exists on all platforms") {
		check(serializerOrNull(typeOf<Profile>()) != null) { "The class Profile is serializable, this code should find a serializer" }
	}

	test("Access a serializer for ObjectId on all platforms") {
		check(serializerOrNull(typeOf<ObjectId>()) != null) { "The class ObjectId is serializable, this code should find a serializer" }
	}

	test("Access a serializer for Foo2 on all platforms") {
		check(serializerOrNull(typeOf<Foo2>()) != null) { "The class Foo2 is serializable, this code should find a serializer" }
	}

with the types:

@Serializable
data class Profile(
	val name: String,
	val age: Int,
)

class Foo(val a: Int)

and in another module, that has the Serialization plugin:

@Serializable(with = ObjectId.Serializer::class)
class ObjectId : Comparable<ObjectId> {

    class Serializer : KSerializer<ObjectId> { … }
}

@Serializable
class Foo2(val a: Int)

and I get the result:

• Serializer of `Foo`: OK on all platforms (returns null)
• Serializer of Profile : OK on all platforms (returns non-null)
• Serializer of ObjectId : Fails on non-JVM platforms (returns null)
• Serializer of `Foo2`: OK on all platforms (returns non-null)
So I guess that means serializer(KType) cannot find classes with a custom serializer? That's very unfortunate.

[CLOVIS]

I just checked and yeah, if @ObjectId uses a plain old @Serializable without a with , then serializer(KType) finds it 🤦‍♂️

[CLOVIS]

Reproduced on the Kotlin Playground but… it seems to affect the JVM too, even though it doesn't in my tests?

https://pl.kotl.in/x8DIrVEvA

However the Kotlin Playground doesn't have KxS on non-JVM platforms so it's not easy to verify

[CLOVIS]

Reported: https://github.com/Kotlin/kotlinx.serialization/issues/3120

[CLOVIS]

Update: it's https://github.com/Kotlin/kotlinx.serialization/issues/2382