would get return, arguments types from lambda.

ok, look at the following code.<3

code

lambda_traits.hpp

# pragma once
# include <tuple>
# include <type_traits>

namespace stx
{

    namespace lambda_detail
    {
        template<class Ret, class Cls, class IsMutable, class... Args>
        struct types
        {
            using is_mutable = IsMutable;

            enum { arity = sizeof...(Args) };

            using return_type = Ret;

            template<size_t i>
            struct arg
            {
                typedef typename std::tuple_element<i, std::tuple<Args...>>::type type;
            };
        };
    }

    template<class Ld>
    struct lambda_type
        : lambda_type<decltype(&Ld::operator())>
    {};

    template<class Ret, class Cls, class... Args>
    struct lambda_type<Ret(Cls::*)(Args...)>
        : lambda_detail::types<Ret,Cls,std::true_type,Args...>
    {};

    template<class Ret, class Cls, class... Args>
    struct lambda_type<Ret(Cls::*)(Args...) const>
        : lambda_detail::types<Ret,Cls,std::false_type,Args...>
    {};

};

main.cpp

# include <iostream>
# include <typeinfo>
# include "lambda_traits"

int main(void){
    std::cout << "[is mutable lambda]" << std::endl;
    {
        auto test = [](int a) mutable->long{return static_cast<long>(a); };
        std::cout << "ret type : " << std::is_same<stx::lambda_type<decltype(test)>::return_type,long>::value << std::endl;
        std::cout << "arg size : " << stx::lambda_type<decltype(test)>::arity << std::endl;
        std::cout << "arg 0 type : " << std::is_same<stx::lambda_type<decltype(test)>::arg<0>::type,int>::value << std::endl;
        std::cout << "is mutable : " << std::is_same<stx::lambda_type<decltype(test)>::is_mutable,std::true_type>::value << std::endl;
    }
    std::cout << "[is normal lambda]" << std::endl;
    {
        auto test = [](int a, int b)->long{return static_cast<long>(a); };
        std::cout << "ret type : " << std::is_same<stx::lambda_type<decltype(test)>::return_type,long>::value << std::endl;
        std::cout << "arg size : " << stx::lambda_type<decltype(test)>::arity << std::endl;
        std::cout << "arg 0 type : " << std::is_same<stx::lambda_type<decltype(test)>::arg<0>::type,int>::value << std::endl;
        std::cout << "is mutable : " << std::is_same<stx::lambda_type<decltype(test)>::is_mutable,std::true_type>::value << std::endl;
    }
}

output

[is mutable lambda]
ret type : 1
arg size : 1
arg 0 type : 1
is mutable : 1
[is normal lambda]
ret type : 1
arg size : 2
arg 0 type : 1
is mutable : 0

test

paiza.io

gist

how to get lambda return type