@file:UseContextualSerialization doesn't work with actual typealias on Kotlin 2.0.0

[rjaros]

When using a @file:UseContextualSerialization annotation with an expected type declared in common target and defined as actual typealias on other targets, the compiler reports an error about missing serializer.

To, Reproduce
common code:

expect class LocalDateTime

common code, different file:

@file:UseContextualSerialization(LocalDateTime::class)

@Serializable
data class Person(
    val id: Int? = 0,
    val firstName: String? = null,
    val lastName: String? = null,
    val createdAt: LocalDateTime? = null,
)

jvm code:

actual typealias LocalDateTime = java.time.LocalDateTime

The compiler fails with an error message:

Serializer has not been found for type 'java.time.LocalDateTime?'. To use context serializer as fallback, explicitly annotate type or property with @Contextual

Expected behavior
No error. The same code works fine with Kotlin 1.9.24.
Using @Contextual annotation directly on a property works as well.

Environment

• Kotlin version: 2.0.0
• Library version: 1.7.0-RC
• Kotlin platforms: JS, JVM

[sandwwraith]

Quick question: does it work with 2.0 when typealias LocalDateTime = java.time.LocalDateTime is not actual, just a regular one?

[rjaros]

Apparently it doesn't work with regular typealias as well. This is all you need to have a compile error with 2.0.0:

@file:UseContextualSerialization(MyDate::class)
package com.example

import kotlinx.serialization.Serializable
import kotlinx.serialization.UseContextualSerialization

typealias MyDate = java.time.LocalDate

@Serializable
data class MyClass(val d: MyDate?)

fun main() {
}

The same file compiles fine with 1.9.24.

[sandwwraith]

Thanks, I'll take a look on it.

[sandwwraith]

By the way, your original reproducer (with @file:UseContextualSerialization(LocalDateTime::class) and expect class LocalDateTime in common code) works fine on 2.0.0. So you probably had UseContextualSerialization in the platform sources.

[rjaros]

You are right. I've simplified my example too much without checking. In my project, the expect class and typealias are defined in a multiplatform library. But the rest of the code is the same and the UseContextualSerialization is used in the common sources. When using a library, it doesn't work on 2.0.0.