Why is it necessary to define constexpr function before using it in constexpr expression?

Question

The following code compiles fine and runs successfully.

#include <iostream>

constexpr int f();

int main()
{
    int a = f();
    std::cout << a << "\n";

    // error: ‘constexpr int f()’ used before its definition
    // constexpr int b = f();
    // std::cout << b << "\n";
}

constexpr int f()
{
    return 10;
}

Here is the output.

$ g++ -std=c++11 foo.cpp && ./a.out 
10

But when I uncomment the two lines of code in comments, I get this error.

$ g++ -std=c++11 foo.cpp && ./a.out 
foo.cpp: In function ‘int main()’:
foo.cpp:11:25: error: ‘constexpr int f()’ used before its definition
     constexpr int b = f();
                     ^

I know that I can get rid of this error quite easily by moving the definition of constexpr int f() before main().

Here are my questions:

1. Why must a constexpr function be defined before using it in a constexpr expression?
2. Why is declaring a constexpr function before using it in a constexpr not sufficient?
3. Why can't the compiler figure out where the constexpr function is defined just like it can when a constexpr function or a non-constexpr function is used in a non-constexpr expression?
4. Why is it okay to define a constexpr function after using it in a non-constexpr expression?