I have a question about access modifiers override.

First, let’s look at the following code.

fun main() {
    val b: AAA = BBB()

    b.testA()
}

open class AAA() {
    fun testA() {
        println("This is testA()")
    }
}

open class BBB() : AAA()

result:
This is testA()

No problem with this code, b has printed testA() of class AAA.

Let’s see the following code.

fun main() {
    val b: AAA = BBB()

    b.testA()
}

open class AAA() {
    open fun testA() {
        println("This is testA()")
    }
}

class BBB() : AAA() {
    override fun testA() {
        println("This is overrided testA() ")
    }
}

result:
This is overridden testA()

This code is also no problem.

Now, here’s the code I’m going to ask.

fun main() {
    val b: AAA = BBB()

    b.testA()
}

open class AAA() {
    protected open fun testA() {
        println("This is testA()")
    }
}

class BBB() : AAA() {
    public override fun testA() {
        println("This is overridden testA()")
    }
}

This code will not be executed. The IDE complains Cannot access 'testA': it is protected in 'AAA'.

Although testA() of AAA is protected, it was overridden as public in BBB, so I thought that naturally the overridden testA() of BBB would be called like the second code.

What’s the problem?
Am I missing something?
What part of inheritance and access modifiers am I misunderstanding?