type_tokens.cpp

// We create a global char for each type
// But we actually care about its address to be used as a constexpr token
template <class T> char token;
using type_token = const void *;

// A utility to get a list of tokens for a list of types
template <class ... Elems> constexpr type_token tokens[sizeof...(Elems)] = {&token<Elems>...};

// A utility template to pass around a list of types as a value
template <class ... T> struct list{};

// A constexpr function that finds the index of a type in a list of types
template <class T, class ... Ts> constexpr int find(list<Ts...>) {
    int result = -1;
    for(int i=0; i<sizeof...(Ts); ++i) if(tokens<Ts...>[i] == &token<T>) return i;
    return result;
}

template <int N> int f() {return N;}

int phony = f<find<int>(list<double, int>{})>();