Why does <= can be applied to Int and Long, but == can't?

Question

I am curious why I can apply <= to both Int and Long, but == doesn't work. See playground:

fun main() { 
    val a = 0L
    
    if (a == 0) { 
        // Compile error (Operator '==' cannot be applied to 'Long' and 'Int')
    }
    
    if (a <= 0) { 
        // No compile error
    }
}

Answer 1

<= works simply because there is a compareTo overload defined in Long that takes Int:

class Long {
    // ...
    operator fun compareTo(other: Int): Int
    // ...
}

<= is simply translated to a call to this (plus a comparison of the returned Int).

While == also translates to a call to equals(Any?) (with additional null checks), there is this bit in the spec that disallows comparing the quality of things of completely different type:

If type of A and type of B are definitely distinct and not related by subtyping, A == B is an invalid expression and should result in a compile-time error.

Answer 2

When comparing Int and Long values using == in Kotlin, the compiler will perform a type check and will determine that the types are not compatible. Therefore, it is not possible to compare Int and Long values using ==. There is no implicit widening conversions for numbers in Kotlin due to its documentation.

However, the <= operator is used for comparing numeric values, and it is overloaded for both Int and Long types. When comparing Int and Long values using <=, Long#compareTo() method is being called. As you can see in documentation, it can accept both Int and Long types as Parameters.