First code

#include <utility>

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

    template<typename T>
    struct FunctionType<T(*)()>
    {
        using ReturnType = T;
        using ParameterType = void;
        using Signature = ReturnType();
    };
    template<typename T, typename P>
    struct FunctionType<T (*)( P )>
    {
        using ReturnType = T;
        using ParameterType = P;
        using Signature = typename Type<ReturnType( ParameterType )>::type;
    };
    template<typename T, typename P>
    struct FunctionType<T(*)( P... )>
    {
        using ReturnType = T;
        using ParameterType = P;
        using Signature = typename Type<ReturnType( ParameterType... )>::type;
    };
    template<typename T, typename ...U>
    auto apply_impl( typename FunctionType<T>::Signature && 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... ) );
    }

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

int foo ( int a ){
    return a;
}

int main()
{
    using FunctionTraits::tag;
    auto bar = []( tag , tag ) ->void { return; };
    return 0;
}