Effective Enums in Kotlin with reverse lookup?

Another option, that could be considered more "idiomatic", would be the following:

companion object {
    private val map = Type.values().associateBy(Type::value)
    operator fun get(value: Int) = map[value]
}

Which can then be used like Type[type].

It makes not much sense in this case, but here is a "logic extraction" for @JBNized's solution:

open class EnumCompanion<T, V>(private val valueMap: Map<T, V>) {
    fun fromInt(type: T) = valueMap[type]
}

enum class TT(val x: Int) {
    A(10),
    B(20),
    C(30);

    companion object : EnumCompanion<Int, TT>(TT.values().associateBy(TT::x))
}

//sorry I had to rename things for sanity

In general that's the thing about companion objects that they can be reused (unlike static members in a Java class)

we can use find which Returns the first element matching the given predicate, or null if no such element was found.

companion object {
   fun valueOf(value: Int): Type? = Type.values().find { it.value == value }
}

First of all, the argument of fromInt() should be an Int, not an Int?. Trying to get a Type using null will obviously lead to null, and a caller shouldn't even try doing that. The Map has also no reason to be mutable. The code can be reduced to:

companion object {
    private val map = Type.values().associateBy(Type::value)
    fun fromInt(type: Int) = map[type]
}

That code is so short that, frankly, I'm not sure it's worth trying to find a reusable solution.