<array>: array<const T, 0> allows fill() and swap()

[StephanTLavavej]

Similar-but-distinct from #942; we should track it as a separate issue. The following libcxx tests are skipped:

STL/tests/libcxx/expected_results.txt

Lines 543 to 545 in 9e76d8c

	# STL bug: We allow fill() and swap() for array<const T, 0>.
	std/containers/sequences/array/array.fill/fill.fail.cpp FAIL
	std/containers/sequences/array/array.swap/swap.fail.cpp FAIL

STL/tests/libcxx/skipped_tests.txt

Lines 543 to 545 in 9e76d8c

	# STL bug: We allow fill() and swap() for array<const T, 0>.
	containers\sequences\array\array.fill\fill.fail.cpp
	containers\sequences\array\array.swap\swap.fail.cpp

Because they're implemented as no-ops:

STL/stl/inc/array

Lines 600 to 622 in 9e76d8c

	template <class _Ty>
	class array<_Ty, 0> {
	public:
	using value_type = _Ty;
	using size_type = size_t;
	using difference_type = ptrdiff_t;
	using pointer = _Ty*;
	using const_pointer = const _Ty*;
	using reference = _Ty&;
	using const_reference = const _Ty&;
	using iterator = _Array_iterator<_Ty, 0>;
	using const_iterator = _Array_const_iterator<_Ty, 0>;
	using reverse_iterator = _STD reverse_iterator<iterator>;
	using const_reverse_iterator = _STD reverse_iterator<const_iterator>;
	#if _HAS_TR1_NAMESPACE
	_DEPRECATE_TR1_NAMESPACE void assign(const _Ty&) {}
	#endif // _HAS_TR1_NAMESPACE
	_CONSTEXPR20 void fill(const _Ty&) {}
	_CONSTEXPR20 void swap(array&) noexcept {}

WG21-N4861 [array.members]/3 specifies fill() with "Effects: As if by fill_n(begin(), N, u)." and /4 specifies swap() with "Effects: Equivalent to swap_ranges(begin(), end(), y.begin())."

Notably, these "equivalent to" calls are ill-formed for const T (even if there happen to be zero of them), so libcxx is correct to expect these to fail to compile (this is what meow.fail.cpp means).

We should be able to fix this without breaking ABI; static_assert should be sufficient.

[AdamBucior]

It's weird to me that the member swap does not allow non-swappable types when size is 0 but the non-member swap allows them (even though it just calls the member swap).

STL/stl/inc/array

Lines 771 to 774 in 5f3e912

	template <class _Ty, size_t _Size, enable_if_t<_Size == 0 || _Is_swappable<_Ty>::value, int> = 0>
	_CONSTEXPR20 void swap(array<_Ty, _Size>& _Left, array<_Ty, _Size>& _Right) noexcept(noexcept(_Left.swap(_Right))) {
	return _Left.swap(_Right);
	}

Is it a bug in the standard?

[CaseyCarter]

Is it a bug in the standard?

It certainly seems so. I was trying to ignore this, but now you've prompted me to poke LWG about it.

[eisenwave]

This will be addressed by paper: https://isocpp.org/files/papers/P3737R0.html

[frederick-vs-ja]

TL, DR: WG21-P3737R0 currently requires fill() and swap() to be well-formed, no-op, and noexcept for every array<T, 0>, and thus resolves this issue as invalid This issue is incorrect or by design , although we still need to add noexcept to fill.