UndeclaredThrowableException when using "suspend" modifier

Retrofit crashes with UndeclaredThrowableException when using suspend modifier and making multiple requests which are supposed to throw IOException.

Retrofit version: 2.6.0, 2.6.1-SNAPSHOT (20190612)

class RetrofitUndeclaredThrowableExceptionBug {

   interface Service {
        @GET("/")
        suspend fun download()
    }

    @Test
    fun test() = runBlocking {
        val retrofit = Retrofit.Builder()
            .baseUrl("https://unresolved-host.com/") // An unresolved host to simulate IOException
            .build()

        val service = retrofit.create<Service>(Service::class.java)

        // First attempt works fine
        try {
            service.download()
        } catch (e: IOException) {
            println("Catched 1: $e")
        }

        // Second attempt sometimes crashes
        try {
            service.download()
        } catch (e: IOException) {
            println("Catched 2: $e")
        }

        // And this will crash for sure (sometimes sooner, sometimes later)
        repeat(Integer.MAX_VALUE) {
            try {
                service.download()
            } catch (e: IOException) {
                println("Catched 3: $e")
            }
        }
    }
}

[PaulWoitaschek]

The same happens when throwing an exception from an interceptor

[PaulWoitaschek]

There should be a fat warning on the release notes, that using suspend modifier for retrofit is not production ready.

Network calls can throw checked IOExceptions so you need to declare @Throws(IOException::class) on every single function on your interface or it will blow up at runtime.