undefind vs. deleted vs. undeclared function

I am using the default constructor as an example here, but the same question applies to all the special member functions.

Also, I am using https://en.cppreference.com instead of the standard because that is what I am reading and that is what I am having trouble understanding. If using the standard would change the question in any way, please let me know.

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As stated in https://en.cppreference.com/w/cpp/language/default_constructor, if a class has no constructor then the compiler will always declare a default one. Then, if certain conditions aren't met, it will be undefined (pre c++11) or defined as deleted (post c++11).

All this seems to imply that there is a difference between a function being not declared, declared but not defined, or declared and deleted. However, all three options won't compile/link, e.g.

class MyClass {
public:
    void foo();
    void bar() = delete;
};

int main() {
    MyClass c;
    //c.foo();  // won't link
    //c.bar();  // won't compile
    //c.baz();  // won't compile
}

So why is it so important to change the definition from "declared and undefined" to "declared and deleted", and why not just leave it as "undeclared" in the first place?