Code 2

#include <utility>
#include <locale>

namespace FunctionTraits
{
    template<typename T>
    struct Type{
        using type = T;
    };

    template<typename T>
    struct FunctionType;

    template<typename T>
    struct FunctionType<T(*)()>
    {
        using ReturnType = typename Type<T>::type;
        using ParameterType = void;
        using Signature = ReturnType();
    };
    template<typename T, typename P>
    struct FunctionType<T (*)( P )>
    {
        using ReturnType = typename Type<T>::type;
        using ParameterType = typename Type<P>::type;
        using Signature = ReturnType(*)( ParameterType );
    };
    template<typename T, typename ...P>
    struct FunctionType<T(*)( P... )>
    {
        using ReturnType = T;
        //~ using ParameterType = typename Type<P...>::type; ???
        using Signature = ReturnType(*)( P... );
    };

    template<class T, class ...U>
    using Signature = typename FunctionType<T(*)(U...)>::Signature;

    template<typename T, typename ...U>
    auto apply_impl( Signature<T, U...> && function, U && ...args ) ->
                                    decltype( function ( std::forward<U...>( args...)) ){
        return function( std::forward<U...>( args... ));
    }
    template<typename T, typename ...U>
    auto apply( T && a, U &&...args ) -> decltype ( apply_impl( std::forward<T>( a ), std::forward<U...>( args... ) ) ){
        using tag = typename FunctionType<T>::Signature;
        return apply_impl( std::forward<T>( a ), std::forward<U...>( args... ) );
    }
}

int foo ( int a, int ){
    return a;
}

int main()
{
    auto ch = FunctionTraits::apply( ::tolower, 'A' ); //expects 'a'
    auto ch_2 = FunctionTraits::apply( foo, 1, 3 ); //expects 1
    return 0;
}