[libcxx][NFC] removes `swap`'s dependency on `swap_ranges`

[cjdb]

Summary

Under the as-if rule, we can directly implement the array overload for
std::swap. By removing this circular dependency where swap is
implemented in terms of swap_ranges and swap_ranges is defined in
terms of swap, we can split them into their own headers. This will:

(a) limit the surface area in which Hyrum's law can bite us;
(b) force users to include the correct headers;
(c) make finding the definitions trivial (swap is a utility,

`swap_ranges` is an algorithm).

[• Quuxplusone]

I initially expected the call to swap, from within a template named swap, to be problematic... but https://godbolt.org/z/hxYaob3eT demonstrates that name lookup seems to be doing the right thing. So, this LGTM, mod two nits!

libcxx/include/type_traits

4192–4193

If what you're doing below works (I think it does), then you should remove lines 4192-4196 because the forward declaration of swap_ranges is no longer needed.

libcxx/test/std/utilities/utility/utility.swap/swap_array.pass.cpp

18 ↗

(On Diff #353840)

Style in libcxx/test/ is generally to put the header-under-test first in the list of includes, even though that is out of alphabetical order. Please leave this file untouched. (Leaving the tests untouched is also a good way to prove that your change is really "NFC".)

[cjdb]

• removes forward declaration of swap_ranges
• fixes header mishap

[mizvekov]

libcxx/include/type_traits

4191

Minor minor nit: might as well remove this comment here.

libcxx/include/type_traits

 _LIBCPP_INLINE_VAR constexpr bool is_nothrow_invocable_r_v
     = is_nothrow_invocable_r<_Ret, _Fn, _Args...>::value;
 
 #endif // _LIBCPP_STD_VER > 14
 
 template <class _Tp> struct __is_swappable;
 template <class _Tp> struct __is_nothrow_swappable;
 
 // swap, swap_ranges
 
-template <class _ForwardIterator1, class _ForwardIterator2>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
-_ForwardIterator2
-swap_ranges(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2);
-
 #ifndef _LIBCPP_CXX03_LANG
 template <class _Tp>
 using __swap_result_t = typename enable_if<is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value>::type;
 #else
 template <class>
 using __swap_result_t = void;
 #endif
 
 template <class _Tp>

 
 template<class _Tp, size_t _Np>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 typename enable_if<
     __is_swappable<_Tp>::value
 >::type
 swap(_Tp (&__a)[_Np], _Tp (&__b)[_Np]) _NOEXCEPT_(__is_nothrow_swappable<_Tp>::value)
 {
-    _VSTD::swap_ranges(__a, __a + _Np, __b);
+    for (size_t __i = 0; __i != _Np; ++__i) {
+        swap(__a[__i], __b[__i]);
+    }
 }
 
 template <class _ForwardIterator1, class _ForwardIterator2>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
 _ForwardIterator2
 swap_ranges(_ForwardIterator1 __first1, _ForwardIterator1 __last1, _ForwardIterator2 __first2)
 {
     for(; __first1 != __last1; ++__first1, (void) ++__first2)