identifier is undefined but code compiles

I have a very simple program.

main.cpp:

#include "io.h"

int main() {
    int firstNumber {readNumber()};
    int secondNumber {readNumber()};

    writeAnswer(firstNumber + secondNumber);
}

io.cpp:

#include "io.h"
#include <iostream>

int readNumber()
{
    int returnValue{};
    std::cout << "Please enter an integer: ";
    std::cin >> returnValue;

    return returnValue;
}

void writeAnswer(int sum)
{
    std::cout << "The result is: " << sum;
}

io.h:

#pragma once

int readNumber();
void writeAnswer(int sum);

First of all, the code compiles fine but Visual Studio complains in my main.cpp:

Funny thing, when I include my header file twice … the “error” is gone:

I’m so confused why Visual Studio complains about that? What do I wrong?