Is it possible to figure out the parameter type and return type of a lambda?

Question

Given a lambda, is it possible to figure out it\'s parameter type and return type? If yes, how?

Basically, I want lambda_traits which can be used in fol

Answer 1

The specialization method shown in @KennyTMs answer can be extended to cover all cases, including variadic and mutable lambdas:

template 
struct closure_traits : closure_traits {};

#define REM_CTOR(...) __VA_ARGS__
#define SPEC(cv, var, is_var)                                              \
template                         \
struct closure_traits                  \
{                                                                          \
    using arity = std::integral_constant;   \
    using is_variadic = std::integral_constant;              \
    using is_const    = std::is_const;                             \
                                                                           \
    using result_type = R;                                                 \
                                                                           \
    template                                                \
    using arg = typename std::tuple_element>::type; \
};

SPEC(const, (,...), 1)
SPEC(const, (), 0)
SPEC(, (,...), 1)
SPEC(, (), 0)

Demo.

Note that the arity is not adjusted for variadic operator()s. Instead one can also consider is_variadic.