[libc++] Rationalize our treatment of contiguous iterators and __unwrap_iter().

- Implement C++20's changes to `reverse_iterator`, so that it won't be
    accidentally counted as a contiguous iterator in C++20 mode.
- Implement C++20's changes to `move_iterator` as well.
- `move_iterator` should not be contiguous. This fixes a bug where
    we optimized `std::copy`-of-move-iterators in an observable way.
    Add a regression test for that bugfix.
- Add libcxx tests for `__is_cpp17_contiguous_iterator` of all relevant
    standard iterator types. Particularly check that vector::iterator
    is still considered contiguous in all C++ modes, even C++03.

After this patch, there continues to be no supported way to write your
own iterator type in C++17-and-earlier such that libc++ will consider it
"contiguous"; however, we now fully support the C++20 approach (in C++20
mode only). If you want user-defined contiguous iterators in C++17-and-earlier,
libc++'s position is "please upgrade to C++20."

Differential Revision: https://reviews.llvm.org/D94807

Author: Arthur O'Dwyer

libcxx/include/__memory/pointer_traits.h

@@ -162,6 +162,70 @@ struct __rebind_pointer {
 #endif
 };
 
+// to_address
+
+template <bool _UsePointerTraits> struct __to_address_helper;
+
+template <> struct __to_address_helper<true> {
+    template <class _Pointer>
+    using __return_type = decltype(pointer_traits<_Pointer>::to_address(_VSTD::declval<const _Pointer&>()));
+
+    template <class _Pointer>
+    _LIBCPP_CONSTEXPR
+    static __return_type<_Pointer>
+    __do_it(const _Pointer &__p) _NOEXCEPT { return pointer_traits<_Pointer>::to_address(__p); }
+};
+
+template <class _Pointer, bool _Dummy = true>
+using __choose_to_address = __to_address_helper<_IsValidExpansion<__to_address_helper<_Dummy>::template __return_type, _Pointer>::value>;
+
+template <class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+_Tp*
+__to_address(_Tp* __p) _NOEXCEPT
+{
+    static_assert(!is_function<_Tp>::value, "_Tp is a function type");
+    return __p;
+}
+
+template <class _Pointer>
+inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
+typename __choose_to_address<_Pointer>::template __return_type<_Pointer>
+__to_address(const _Pointer& __p) _NOEXCEPT
+{
+    return __choose_to_address<_Pointer>::__do_it(__p);
+}
+
+template <> struct __to_address_helper<false> {
+    template <class _Pointer>
+    using __return_type = typename pointer_traits<_Pointer>::element_type*;
+
+    template <class _Pointer>
+    _LIBCPP_CONSTEXPR
+    static __return_type<_Pointer>
+    __do_it(const _Pointer &__p) _NOEXCEPT { return _VSTD::__to_address(__p.operator->()); }
+};
+
+
+#if _LIBCPP_STD_VER > 17
+template <class _Tp>
+inline _LIBCPP_INLINE_VISIBILITY constexpr
+_Tp*
+to_address(_Tp* __p) _NOEXCEPT
+{
+    static_assert(!is_function_v<_Tp>, "_Tp is a function type");
+    return __p;
+}
+
+template <class _Pointer>
+inline _LIBCPP_INLINE_VISIBILITY constexpr
+auto
+to_address(const _Pointer& __p) _NOEXCEPT
+{
+    return _VSTD::__to_address(__p);
+}
+#endif
+
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS

libcxx/include/algorithm

@@ -1639,68 +1639,42 @@ search_n(_ForwardIterator __first, _ForwardIterator __last, _Size __count, const
                            __value_, __equal_to<__v, _Tp>());
 }
 
-// copy
-template <class _Iter>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-_Iter
-__unwrap_iter(_Iter __i)
-{
-    return __i;
-}
-
-template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-typename enable_if
-<
-    is_trivially_copy_assignable<_Tp>::value,
-    _Tp*
->::type
-__unwrap_iter(move_iterator<_Tp*> __i)
-{
-    return __i.base();
-}
+// __unwrap_iter
+
+// The job of __unwrap_iter is to lower iterators-that-are-tantamount-to-pointers
+// (such as vector<T>::iterator) into pointers, to reduce the number of template
+// instantiations and to enable pointer-based optimizations e.g. in std::copy.
+// In debug mode, we don't do this.
+
+template <class _Iter, bool = __is_cpp17_contiguous_iterator<_Iter>::value>
+struct __unwrap_iter_impl {
+    static _LIBCPP_CONSTEXPR _Iter
+    __apply(_Iter __i) _NOEXCEPT {
+        return __i;
+    }
+};
 
 #if _LIBCPP_DEBUG_LEVEL < 2
 
-template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-typename enable_if
-<
-    is_trivially_copy_assignable<_Tp>::value,
-    _Tp*
->::type
-__unwrap_iter(__wrap_iter<_Tp*> __i)
-{
-    return __i.base();
-}
-
-template <class _Tp>
-inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-typename enable_if
-<
-    is_trivially_copy_assignable<_Tp>::value,
-    const _Tp*
->::type
-__unwrap_iter(__wrap_iter<const _Tp*> __i)
-{
-    return __i.base();
-}
+template <class _Iter>
+struct __unwrap_iter_impl<_Iter, true> {
+    static _LIBCPP_CONSTEXPR decltype(_VSTD::__to_address(declval<_Iter>()))
+    __apply(_Iter __i) _NOEXCEPT {
+        return _VSTD::__to_address(__i);
+    }
+};
 
-#else
+#endif  // _LIBCPP_DEBUG_LEVEL < 2
 
-template <class _Tp>
+template<class _Iter, class _Impl = __unwrap_iter_impl<_Iter> >
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR
-typename enable_if
-<
-    is_trivially_copy_assignable<_Tp>::value,
-    __wrap_iter<_Tp*>
->::type
-__unwrap_iter(__wrap_iter<_Tp*> __i)
+decltype(_Impl::__apply(_VSTD::declval<_Iter>()))
+__unwrap_iter(_Iter __i) _NOEXCEPT
 {
-    return __i;
+    return _Impl::__apply(__i);
 }
 
-#endif  // _LIBCPP_DEBUG_LEVEL < 2
+// copy
 
 template <class _InputIterator, class _OutputIterator>
 inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX17
@@ -1894,7 +1868,7 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 typename enable_if
 <
     is_same<typename remove_const<_Tp>::type, _Up>::value &&
-    is_trivially_copy_assignable<_Up>::value,
+    is_trivially_move_assignable<_Up>::value,
     _Up*
 >::type
 __move(_Tp* __first, _Tp* __last, _Up* __result)
@@ -1942,7 +1916,7 @@ inline _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR_AFTER_CXX14
 typename enable_if
 <
     is_same<typename remove_const<_Tp>::type, _Up>::value &&
-    is_trivially_copy_assignable<_Up>::value,
+    is_trivially_move_assignable<_Up>::value,
     _Up*
 >::type
 __move_backward(_Tp* __first, _Tp* __last, _Up* __result)

libcxx/include/filesystem

@@ -1495,7 +1495,7 @@ _LIBCPP_INLINE_VISIBILITY _LIBCPP_DEPRECATED_WITH_CHAR8_T
       "'char' or 'char8_t'");
 #if defined(_LIBCPP_WIN32API)
   using _Traits = __is_pathable<_Source>;
-  return u8path(__unwrap_iter(_Traits::__range_begin(__s)), __unwrap_iter(_Traits::__range_end(__s)));
+  return u8path(_VSTD::__unwrap_iter(_Traits::__range_begin(__s)), _VSTD::__unwrap_iter(_Traits::__range_end(__s)));
 #else
   return path(__s);
 #endif