_NODISCARD for function objects

[celonymire]

Very likely partially addresses #206.

Note: _NODISCARD is already present for ranges function objects.

[ghost]

All CLA requirements met.

[celonymire]

There seems to be a missing test case for compare_three_way (noticed while I was fixing the test), should it be added?

[StephanTLavavej]

That file is indeed missing compare_three_way coverage, but we have extensive tests in a different file:

STL/tests/std/tests/P0896R4_P1614R2_comparisons/test.cpp

Lines 222 to 296 in 19c683d

	// Validate static properties of compare_three_way, compare_three_way_result, and compare_three_way_result_t
	template <class T>
	concept is_trait = requires {
	typename T::type;
	};
	template <class T, class U>
	concept can_three_way = requires(T const& t, U const& u) {
	t <=> u;
	};
	template <class T, class U, class Cat>
	constexpr bool test_compare_three_way() {
	STATIC_ASSERT(same_as<T, std::remove_cvref_t<T>>);
	STATIC_ASSERT(same_as<U, std::remove_cvref_t<U>>);
	STATIC_ASSERT(can_three_way<T, U> == !std::is_void_v<Cat>);
	STATIC_ASSERT(can_three_way<U, T> == !std::is_void_v<Cat>);
	if constexpr (can_three_way<T, U>) {
	STATIC_ASSERT(same_as<decltype(std::declval<T const&>() <=> std::declval<U const&>()), Cat>);
	STATIC_ASSERT(same_as<decltype(std::declval<U const&>() <=> std::declval<T const&>()), Cat>);
	STATIC_ASSERT(same_as<compare_three_way_result_t<T, U>, Cat>);
	STATIC_ASSERT(same_as<compare_three_way_result_t<U, T>, Cat>);
	STATIC_ASSERT(same_as<typename compare_three_way_result<T, U>::type, Cat>);
	STATIC_ASSERT(same_as<typename compare_three_way_result<U, T>::type, Cat>);
	STATIC_ASSERT(same_as<decltype(compare_three_way{}(std::declval<T const&>(), std::declval<U const&>())), Cat>);
	STATIC_ASSERT(same_as<decltype(compare_three_way{}(std::declval<U const&>(), std::declval<T const&>())), Cat>);
	} else {
	STATIC_ASSERT(!is_trait<compare_three_way_result<T, U>>);
	STATIC_ASSERT(!is_trait<compare_three_way_result<U, T>>);
	}
	return true;
	}
	enum class some_enum { value };
	STATIC_ASSERT(test_compare_three_way<int, int, strong_ordering>());
	STATIC_ASSERT(test_compare_three_way<int, long, strong_ordering>());
	STATIC_ASSERT(test_compare_three_way<float, float, partial_ordering>());
	STATIC_ASSERT(test_compare_three_way<float, double, partial_ordering>());
	STATIC_ASSERT(test_compare_three_way<long, double, partial_ordering>());
	STATIC_ASSERT(test_compare_three_way<bool, int, void>());
	STATIC_ASSERT(test_compare_three_way<some_enum, some_enum, strong_ordering>());
	STATIC_ASSERT(test_compare_three_way<some_enum, int, void>());
	STATIC_ASSERT(test_compare_three_way<int*, int*, strong_ordering>());
	STATIC_ASSERT(test_compare_three_way<int*, void*, strong_ordering>());
	STATIC_ASSERT(test_compare_three_way<int (*)(), int (*)(), void>());
	STATIC_ASSERT(test_compare_three_way<int (*)(), void (*)(), void>());
	template <class Cat>
	struct compares_as {};
	template <class Cat1, class Cat2>
	bool operator==(compares_as<Cat1> const&, compares_as<Cat2> const&);
	template <class Cat1, class Cat2>
	common_comparison_category_t<Cat1, Cat2> operator<=>(compares_as<Cat1> const&, compares_as<Cat2> const&);
	template <class Cat1, class Cat2>
	struct std::common_type<compares_as<Cat1>, compares_as<Cat2>> {
	using type = common_comparable;
	};
	STATIC_ASSERT(test_compare_three_way<compares_as<partial_ordering>, compares_as<partial_ordering>, partial_ordering>());
	STATIC_ASSERT(test_compare_three_way<compares_as<partial_ordering>, compares_as<weak_ordering>, partial_ordering>());
	STATIC_ASSERT(test_compare_three_way<compares_as<partial_ordering>, compares_as<strong_ordering>, partial_ordering>());
	STATIC_ASSERT(test_compare_three_way<compares_as<weak_ordering>, compares_as<weak_ordering>, weak_ordering>());
	STATIC_ASSERT(test_compare_three_way<compares_as<weak_ordering>, compares_as<strong_ordering>, weak_ordering>());
	STATIC_ASSERT(test_compare_three_way<compares_as<strong_ordering>, compares_as<strong_ordering>, strong_ordering>());
	// Validate dynamic properties of compare_three_way, ranges::equal_to, ranges::not_equal_to, ranges::less,
	// ranges::less_equal, ranges::greater, ranges::greater_equal
	#define assert_three_way(t, u, result) assert(compare_three_way{}((t), (u)) == (result))

So you don't need to add anything. (You can if you really want to.)

[celonymire]

Ah okay. I'll just leave it as it is.

[StephanTLavavej]

Everything looks great here. 😸

[StephanTLavavej]

Status update: We simultaneously merge changes on GitHub and our MS-internal repo, so after Thanksgiving we should be able to batch up your PR with everything else that’s ready to merge and get this checked in for VS 2019 16.9 Preview 3.

[StephanTLavavej]

Thanks for improving the user experience with function objects, and congratulations on your first microsoft/STL commit! 🎉 😺 🚀

This will ship in VS 2019 16.9 Preview 3.