Simplify sol::wrap

[Smertig]

Now sol::wrap is implemented as follows:

sol2/include/sol/function_types_templated.hpp

Lines 132 to 139 in a7da2a8

	template <typename F, F f>
	struct wrap {
	typedef F type;
	static int call(lua_State* L) noexcept(noexcept(c_call<type, f>(L))) {
	return c_call<type, f>(L);
	}
	};

This is not so user-friendly API, because one should use sol::wrap<decltype(<expr>), <expr>> (or write macro to avoid copy-pasting).

I suggest replacing it with:

template <auto F>
struct wrap {
    using type = decltype(F);

    ...
};

• sol2 requires C++17, so there should be no concern about using auto in non-type template parameters
• this is a breaking change, however upcoming v4.0.0 release is the right time to do it
• if you don't want to break anything, you can simply add template <auto F> struct foobar with different name, but sol::wrap is a really good name for such a wrapper

---

P.S. you can also refactor sol::function_detail with auto F feature without breaking sol2 API:

sol2/include/sol/function_types_templated.hpp

Lines 30 to 115 in a7da2a8

	namespace function_detail {
	template <typename F, F fx>
	inline int call_wrapper_variable(std::false_type, lua_State* L) {
	typedef meta::bind_traits<meta::unqualified_t<F>> traits_type;
	typedef typename traits_type::args_list args_list;
	typedef meta::tuple_types<typename traits_type::return_type> return_type;
	return stack::call_into_lua(return_type(), args_list(), L, 1, fx);
	}
	template <typename R, typename V, V, typename T>
	inline int call_set_assignable(std::false_type, T&&, lua_State* L) {
	return luaL_error(L, "cannot write to this type: copy assignment/constructor not available");
	}
	template <typename R, typename V, V variable, typename T>
	inline int call_set_assignable(std::true_type, lua_State* L, T&& mem) {
	(mem.*variable) = stack::get<R>(L, 2);
	return 0;
	}
	template <typename R, typename V, V, typename T>
	inline int call_set_variable(std::false_type, lua_State* L, T&&) {
	return luaL_error(L, "cannot write to a const variable");
	}
	template <typename R, typename V, V variable, typename T>
	inline int call_set_variable(std::true_type, lua_State* L, T&& mem) {
	return call_set_assignable<R, V, variable>(std::is_assignable<std::add_lvalue_reference_t<R>, R>(), L, std::forward<T>(mem));
	}
	template <typename V, V variable>
	inline int call_wrapper_variable(std::true_type, lua_State* L) {
	typedef meta::bind_traits<meta::unqualified_t<V>> traits_type;
	typedef typename traits_type::object_type T;
	typedef typename traits_type::return_type R;
	auto& mem = stack::get<T>(L, 1);
	switch (lua_gettop(L)) {
	case 1: {
	decltype(auto) r = (mem.*variable);
	stack::push_reference(L, std::forward<decltype(r)>(r));
	return 1;
	}
	case 2:
	return call_set_variable<R, V, variable>(meta::neg<std::is_const<R>>(), L, mem);
	default:
	return luaL_error(L, "incorrect number of arguments to member variable function call");
	}
	}
	template <typename F, F fx>
	inline int call_wrapper_function(std::false_type, lua_State* L) {
	return call_wrapper_variable<F, fx>(std::is_member_object_pointer<F>(), L);
	}
	template <typename F, F fx>
	inline int call_wrapper_function(std::true_type, lua_State* L) {
	return call_detail::call_wrapped<void, false, false>(L, fx);
	}
	template <typename F, F fx>
	int call_wrapper_entry(lua_State* L) noexcept(meta::bind_traits<F>::is_noexcept) {
	return call_wrapper_function<F, fx>(std::is_member_function_pointer<meta::unqualified_t<F>>(), L);
	}
	template <typename... Fxs>
	struct c_call_matcher {
	template <typename Fx, std::size_t I, typename R, typename... Args>
	int operator()(types<Fx>, meta::index_value<I>, types<R>, types<Args...>, lua_State* L, int, int) const {
	typedef meta::at_in_pack_t<I, Fxs...> target;
	return target::call(L);
	}
	};
	template <typename F, F fx>
	inline int c_call_raw(std::true_type, lua_State* L) {
	return fx(L);
	}
	template <typename F, F fx>
	inline int c_call_raw(std::false_type, lua_State* L) {
	#ifdef __clang__
	return detail::trampoline(L, function_detail::call_wrapper_entry<F, fx>);
	#else
	return detail::typed_static_trampoline<decltype(&function_detail::call_wrapper_entry<F, fx>), (&function_detail::call_wrapper_entry<F, fx>)>(L);
	#endif // fuck you clang :c
	}