c++ - how to correctly find out the return type of a lambda

Question

basically how to make following code compile?

I know it failed because compiler was trying to evaluate something like ([](int &i){})(0) but how to solve the problem?

template <class TElement>
struct foo {
    TElement _e;
    foo(TElement e) : _e(e){}
    template <class Lambda>
    void bar(Lambda f) {
        using TResult = decltype(std::declval<Lambda>()(std::declval<TElement>()));
    }
};

int main() {

    foo<int>(0).bar([](int i){}); // compile
    foo<int>(0).bar([](int &&i){}); // compile
    foo<int>(0).bar([](int const &i){}); // compile
    foo<int>(0).bar([](int &i){}); // failed

}

Answer 1

You may use following traits:

template <typename T>
struct return_type : return_type<decltype(&T::operator())>
{};
// For generic types, directly use the result of the signature of its 'operator()'

template <typename ClassType, typename ReturnType, typename... Args>
struct return_type<ReturnType(ClassType::*)(Args...) const>
{
    using type = ReturnType;
};