Useful C++ template metaprograms

Question

C++ implements a Turing-complete functional programming language which is evaluated at compile time.

Hence, we can use a C++ compiler as an interpreter to run our metaprogram without ever generating code. Unfortunately, static_assert won't allow us to emit our result at compile-time, so we do need* to generate an executable to display the result.

* we can display via a compile-time error in template instantiation, but the result is less readable.

Challenge

Implement a useful or impressive program purely via template metaprogramming. This is a popularity contest, so the code with the most votes wins.

Rules:

1. Your program must compile against the C++14 standard. This means that the actual compiler you use doesn't matter to anyone else.

   -std=c++14

2. "External" input is passed via preprocessor definitions.

   -Dparam=42

3. Output is via a single cout/printf line which does no run-time calculations besides those necessary for stringifying the result, or is via a compile-time error.

   int main(int argc, char **argv) { std::cout << MyMetaProgram<param>::result << std::endl; }

---

Example: Factorial

bang.h

template <unsigned long long N>
struct Bang
{
    static constexpr unsigned long long value = N * Bang<N-1>::value;
};

template <>
struct Bang<0>
{
    static constexpr unsigned long long value = 1;
};

bang.cpp

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

int main(int argc, char **argv)
{
    std::cout << Bang<X>::value << std::endl;
}

execute

$ g++ -std=c++14 -DX=6 bang.cpp -o bang && ./bang
120