Variadic template compilation issue on MS Visual Studio 2017 - c++

Question

I need a template to find out the order of types in which the class inherits from its bases and their index. The code works fine with clang and gcc but in Visual Studio, which is the target environment, I'm getting an internal compiler error "fatal error C1001: An internal error has occurred in the compiler.". I'm looking for some workaround or maybe an error in my code. Yes, I have already tried google.
Thanks, in advance.
#include <type_traits>
#include <iostream>
struct BaseA
{
};
struct BaseB
{
};
struct BaseC
{
};
template <class... Types>
class type_list {};
template<typename Type, typename TypeList>
struct get_idx_for_type;
template<typename Type, template<typename...> typename TypeList, typename ...Types>
struct get_idx_for_type<Type, TypeList<Types...>>
{
template<int I, typename T, typename ...Rest>
struct find_type;
template<int I, typename T, typename U, typename ...Rest>
struct find_type< I, T, U, Rest... >
{
// problematic line for compiler, problem is somewhere in find_type recursion
static constexpr int value = std::is_same<T, U>::value ? I : find_type<I + 1, T, Rest...>::value;
};
template<int I, typename T, typename U>
struct find_type< I, T, U >
{
static constexpr int value = std::is_same<T, U>::value ? I : -1;
};
static constexpr int value = find_type<0, Type, Types...>::value;
};
template<typename ...Bases>
struct Foo : public Bases...
{
using base_types_list = type_list<Bases...>;
};
int main()
{
using T = Foo<BaseA, BaseB, BaseC>;
Foo<BaseA, BaseB, BaseC> q;
int a = get_idx_for_type<BaseA, T::base_types_list>::value;
std::cout << a << std::endl;
return 0;
}

Answer 1

An internal compiler error is always a bug in the compiler, whether or not there
is anything wrong with your code. To work around this one, you can replace:
template<int I, typename T, typename U, typename ...Rest>
struct find_type< I, T, U, Rest... >
{
// problematic line for compiler, problem is somewhere in find_type recursion
static constexpr int value = std::is_same<T, U>::value ? I : find_type<I + 1, T, Rest...>::value;
};
with:
template<int I, typename T, typename U, typename V, typename ...Rest>
struct find_type< I, T, U, V, Rest... >
{
static constexpr int value = std::is_same<T, U>::value ? I : find_type<I + 1, T, V, Rest...>::value;
};
to assist VC++ in disambiguating it from:
template<int I, typename T, typename U>
struct find_type< I, T, U >
{
static constexpr int value = std::is_same<T, U>::value ? I : -1;
};
when ...Rest is empty.
Live demo

Answer 2

I'm getting an internal compiler error "fatal error C1001: An internal error has occurred in the compiler.".
So, I suppose, your code is correct but the compiler is bugged.
I'm looking for some workaround or maybe an error in my code.
I don't have your compiler, so it's a shoot in the dark, but I propose to rewrite your find_type struct using another specialization (a specialization for U and T are the same type; a specialization for U and T are different) and inherit the value from std::integral_constant.
So I propose the following
template <int, typename ...>
struct find_type;
template <int I, typename T>
struct find_type<I, T> : public std::integral_constant<int, -1>
{ };
template <int I, typename T, typename ... Rest>
struct find_type<I, T, T, Rest...> : public std::integral_constant<int, I>
{ };
template <int I, typename T, typename U, typename ... Rest>
struct find_type<I, T, U, Rest...> : public find_type<I+1, T, Rest...>
{ };

Answer 3

Another possible alternative is pass through a template function, instead of a find_type struct.
By example... if you define a static constexpr method find_type_func() as follows (C++17, but I can easily adapt it to C++14 if you want)
template <typename T, typename ... List>
constexpr static int find_type_func ()
{
int ret { -1 };
int i { 0 };
((ret = (ret == -1) && std::is_same<T, List>{} ? i : ret, ++i), ...);
return ret;
}
you can delete the struct find_type and initialize value as follows
static constexpr int value = find_type_func<Type, Types...>();
-- EDIT --
As asked, a C++14 version
template <typename T, typename ... List>
constexpr static int find_type_func ()
{
using unused = int[];
int ret { -1 };
int i { 0 };
(void)unused { 0, (std::is_same<T, List>::value ? ret = i : ++i)... };
return ret;
}