failed-inference-cutdown.cpp

#include <tuple>
#include <utility>
#include <type_traits>
#include <iostream>

template<typename... T> class typelist {};

template<typename F, std::size_t I, typename... Tuples>
using apply_result_type = decltype(std::declval<F>()(std::get<I>(std::declval<Tuples>())...));

template<typename F, std::size_t... Indices, typename... Tuples>
typelist<apply_result_type<F, Indices, Tuples...>...> compute_result_types(typelist<F, Tuples...>, std::index_sequence<Indices...>);

struct functor
{
	int operator()(int a, int b) { return a + b; }
	double operator()(double a, double b) { return a + b; }
	void operator()(char, char) { return; }
};

int main()
{
	auto t1 = std::make_tuple(1, 2.0, 'a');
	auto t2 = std::make_tuple(2, 4.0, 'b');
	apply_result_type<functor, 0, decltype(t1), decltype(t2)> x; // works
	compute_result_types(typelist<functor, decltype(t1), decltype(t2)>{}, std::index_sequence<0, 1, 2>{}); // inference fails:
	// error C2893: Failed to specialize function template 'typelist<unknown-type...> compute_result_types(typelist<F,Tuples...>,std::integer_sequence<size_t,_Indexes2...>)'
	// seems like the non-type parameters don't get captured into size_t Indices... properly.
	return 0;
}