Code for blog post.

Makefile

CXX=clang++
CXXFLAGS=-O3 -Werror -Weverything -pedantic -Wno-c++98-compat -std=c++14

all: test_mult

clean:
	rm -f test_mult

mult.h

#ifndef MULT_H
#define MULT_H

struct A { using type = A; };
struct B { using type = B; };

template <typename... Args>
struct Mult;

template <typename LHS, typename... RHS>
struct Mult<LHS, RHS...> {
  using type =
      typename Mult<typename LHS::type, typename Mult<RHS...>::type>::type;
};

template <typename LHS, typename RHS>
struct Mult<LHS, RHS> {
  using type = typename Mult<typename LHS::type, typename RHS::type>::type;
};

template <>
struct Mult<A, A> {
  using type = A;
};
template <>
struct Mult<B, B> {
  using type = A;
};
template <>
struct Mult<A, B> {
  using type = B;
};
template <>
struct Mult<B, A> {
  using type = B;
};

#endif  // MULT_H

test_mult.cc

#include "mult.h"

#include <iostream>
#include <typeinfo>

int main() {
  std::cout << typeid(Mult<B, A, B, B, B, A>::type).name() << '\n';
}