Wrong inference for reified type parameter in Kotlin

Question

Given the following code in Kotlin:

import com.fasterxml.jackson.module.kotlin.*

data class MyReply<R> (
        val results : Array<R>? = null
)

class ErrorClient() {
    val JSON = jacksonObjectMapper()

    inline fun<reified R> request(): Array<R>? {
        val json_in = """{"results": [2]}"""
        val res: MyReply<R> = JSON.readValue(json_in)
        return res.results
    }

    fun read(): Array<Int>? {
        val res: Array<Int>? = request()
        return res
    }
}

and the following tests:

import org.junit.Test

class ErrorTest {
    val client = ErrorClient()

    @Test
    fun `direct`() {
        val res: Array<Int>? = client.request()
        println(res)
    }

    @Test
    fun `indirect`() {
        val res : Array<Int>? = client.read()
        println(res)
    }
}

Short story: The first test passes and the second fails. Why?

Long story: I am experiencing a wrong type inference of the reified parameter R when calling the inline function via the read() class method, but the direct call to request() works. In the indirect case the type is erronously inferred to be java.lang.Object and thus the test fails with

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;
    at ErrorClient.read(Error.kt:17)
    at ErrorTest.indirect(ErrorTest.kt:14)

Joshua Joshua · Accepted Answer · 2017-09-29T07:27:37

This is not about reified. I test the code with

val res: MyReply<R> = MyReply()

It does not throw any errors. This problem is your JSON.readValue return an Object instead of Integer. Kotlin try to cast it to Integer but it fails.

Wrong inference for reified type parameter in Kotlin

1 Answers