QueryReceiver

internal val customClientKey = ContextPropertyKey<CustomClient>()

val ContextReceiver.customClient: CustomClient?
    get() = get(customClientKey)

var ContextReceiverBuilder.customClient: CustomClient
    get() = error("You cannot retrieve a builder property directly.")
    set(value) = set(customClientKey, value)

inline fun <T> buildCustomQueryKey(
    id: QueryId<T>,
    crossinline fetch: suspend CustomClient.() -> T
): QueryKey<T> = buildQueryKey(
    id = id,
    fetch = {
        val client = checkNotNull(customClient) { "customClient isn't available. Did you forget to set it up?" }
        with(client) { fetch() }
    }
)

QueryReceiver {
    customClient = newCustomClient()
}

class GetPostKey(private val postId: Int) : QueryKey<Post> by buildCustomQueryKey(
    id = ..,
    fetch = { // CustomClient.() -> Post
        doSomething()
    }
)