example for https://www.notion.so/essentialbooks/Visibility-of-function-prototypes-and-declarations-6c8eabb8f9f2428187331c87a55a57cb (made with https://codeeval.dev)

main.cc

int main()
{
    // error: foo is called, but has not yet been declared
    foo(2); 
}

// this definition is not known in main
void foo(int x)
{
}