[WIP][libc++] Use checked iterators with std::span when the debug mode is enabled

[ldionne]

Summary

Previously, we'd use raw pointers instead of __wrap_iter, which means
we wouldn't get out-of-range checking with std::span's iterators, which
is really useful. Unfortunately, the current implementation of the debug
mode implies that a bunch of non-trivial calls need to be inserted in
span's constructors and destructors, which is not ideal. In particular,
it means that we cannot keep std::span trivially copyable and destructible
when the debug mode is enabled.

As a fly-by change, this commit removes the _LIBCPP_ABI_SPAN_POINTER_ITERATORS
knob -- not using raw pointers in place of iterators is extremely useful
for users not to start depending on properties of iterators that are not
guaranteed, which allows them to e.g. enable the debug mode without fixing
their code.

As a follow-up change, I suggest reworking the underlying implementation
of the debug mode such that containers don't need to be registered in
order to get bounds-checking iterators.

[ldionne]

Upon making more progress on the successor to this patch, I think it might make more sense to skip directly to the "real" thing. Putting on the ice for now.

libcxx/include/__config

 #  define _LIBCPP_ABI_NO_ITERATOR_BASES
 // Use the smallest possible integer type to represent the index of the variant.
 // Previously libc++ used "unsigned int" exclusively.
 #  define _LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION
 // Unstable attempt to provide a more optimized std::function
 #  define _LIBCPP_ABI_OPTIMIZED_FUNCTION
 // All the regex constants must be distinct and nonzero.
 #  define _LIBCPP_ABI_REGEX_CONSTANTS_NONZERO
-// Use raw pointers, not wrapped ones, for std::span's iterator type.
-#  define _LIBCPP_ABI_SPAN_POINTER_ITERATORS
 // Re-worked external template instantiations for std::string with a focus on
 // performance and fast-path inlining.
 #  define _LIBCPP_ABI_STRING_OPTIMIZED_EXTERNAL_INSTANTIATION
 // Enable clang::trivial_abi on std::unique_ptr.
 #  define _LIBCPP_ABI_ENABLE_UNIQUE_PTR_TRIVIAL_ABI
 // Enable clang::trivial_abi on std::shared_ptr and std::weak_ptr
 #  define _LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
 // std::random_device holds some state when it uses an implementation that gets

libcxx/include/span

concept __span_compatible_range =
   ranges::sized_range<_Range> &&
   (ranges::borrowed_range<_Range> || is_const_v<_ElementType>) &&
   !__is_std_span<remove_cvref_t<_Range>>::value  &&
   !__is_std_array<remove_cvref_t<_Range>>::value &&
   !is_array_v<remove_cvref_t<_Range>> &&
   is_convertible_v<remove_reference_t<ranges::range_reference_t<_Range>>(*)[], _ElementType(*)[]>;
 #endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 
-#if defined(_LIBCPP_ENABLE_DEBUG_MODE) || defined(_LIBCPP_ABI_SPAN_POINTER_ITERATORS)
-#   define _LIBCPP_SPAN_USE_POINTER_ITERATOR
-#endif
-
 template <typename _Tp, size_t _Extent>
 class _LIBCPP_TEMPLATE_VIS span {
 public:
 //  constants and types
     using element_type           = _Tp;
     using value_type             = remove_cv_t<_Tp>;
     using size_type              = size_t;
     using difference_type        = ptrdiff_t;
     using pointer                = _Tp *;
     using const_pointer          = const _Tp *;
     using reference              = _Tp &;
     using const_reference        = const _Tp &;
-#ifdef _LIBCPP_SPAN_USE_POINTER_ITERATOR
-    using iterator               = pointer;
-#else
     using iterator               = __wrap_iter<pointer>;
-#endif
     using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
 
     static constexpr size_type extent = _Extent;
 
 // [span.cons], span constructors, copy, assignment, and destructor
     template <size_t _Sz = _Extent, enable_if_t<_Sz == 0, nullptr_t> = nullptr>
-    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr} {}
+    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr} {
+        std::__debug_db_insert_c(this);
+    }
 
-    constexpr span           (const span&) noexcept = default;
+#ifndef _LIBCPP_ENABLE_DEBUG_MODE
+    constexpr span(const span&) noexcept = default;
+#else
+    constexpr span(const span& __other) noexcept
+        : __data(__other.__data)
+    {
+        std::__debug_db_insert_c(this);
+    }
+#endif
     constexpr span& operator=(const span&) noexcept = default;
 
     template <class _It,
               enable_if_t<contiguous_iterator<_It> &&
                               is_convertible_v<remove_reference_t<iter_reference_t<_It>>(*)[], element_type (*)[]>,
                           nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
     constexpr explicit span(_It __first, size_type __count)
         : __data{_VSTD::to_address(__first)} {
+      std::__debug_db_insert_c(this);
       (void)__count;
       _LIBCPP_ASSERT(_Extent == __count, "size mismatch in span's constructor (iterator, len)");
     }
 
     template <
         class _It, class _End,
         enable_if_t<is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]> &&
                         contiguous_iterator<_It> && sized_sentinel_for<_End, _It> && !is_convertible_v<_End, size_t>,
                     nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
     constexpr explicit span(_It __first, _End __last) : __data{_VSTD::to_address(__first)} {
+      std::__debug_db_insert_c(this);
       (void)__last;
       _LIBCPP_ASSERT((__last - __first >= 0), "invalid range in span's constructor (iterator, sentinel)");
       _LIBCPP_ASSERT(__last - __first == _Extent,
                      "invalid range in span's constructor (iterator, sentinel): last - first != extent");
     }
 
-    _LIBCPP_INLINE_VISIBILITY constexpr span(type_identity_t<element_type> (&__arr)[_Extent]) noexcept : __data{__arr} {}
+    _LIBCPP_INLINE_VISIBILITY constexpr span(type_identity_t<element_type> (&__arr)[_Extent]) noexcept : __data{__arr} {
+        std::__debug_db_insert_c(this);
+    }
 
     template <class _OtherElementType,
               enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
+    constexpr span(array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {
+        std::__debug_db_insert_c(this);
+    }
 
     template <class _OtherElementType,
               enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {}
+    constexpr span(const array<_OtherElementType, _Extent>& __arr) noexcept : __data{__arr.data()} {
+        std::__debug_db_insert_c(this);
+    }
 
 #if defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
     template <class _Container>
         requires __span_compatible_range<_Container, element_type>
     _LIBCPP_INLINE_VISIBILITY
     constexpr explicit span(_Container& __c) : __data{std::data(__c)} {
       _LIBCPP_ASSERT(std::size(__c) == _Extent, "size mismatch in span's constructor (range)");
+      std::__debug_db_insert_c(this);
     }
     template <class _Container>
         requires __span_compatible_range<const _Container, element_type>
     _LIBCPP_INLINE_VISIBILITY
     constexpr explicit span(const _Container& __c) : __data{std::data(__c)} {
       _LIBCPP_ASSERT(std::size(__c) == _Extent, "size mismatch in span's constructor (range)");
+      std::__debug_db_insert_c(this);
     }
 #else
     template <__span_compatible_range<element_type> _Range>
     _LIBCPP_INLINE_VISIBILITY
     constexpr explicit span(_Range&& __r) : __data{ranges::data(__r)} {
       _LIBCPP_ASSERT(ranges::size(__r) == _Extent, "size mismatch in span's constructor (range)");
+      std::__debug_db_insert_c(this);
     }
 #endif
 
     template <class _OtherElementType>
     _LIBCPP_INLINE_VISIBILITY
         constexpr span(const span<_OtherElementType, _Extent>& __other,
                        enable_if_t<
                           is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                           nullptr_t> = nullptr)
-        : __data{__other.data()} {}
+        : __data{__other.data()} {
+        std::__debug_db_insert_c(this);
+    }
 
     template <class _OtherElementType>
     _LIBCPP_INLINE_VISIBILITY
         constexpr explicit span(const span<_OtherElementType, dynamic_extent>& __other,
                        enable_if_t<
                           is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                           nullptr_t> = nullptr) noexcept
-        : __data{__other.data()} { _LIBCPP_ASSERT(_Extent == __other.size(), "size mismatch in span's constructor (other span)"); }
-
+        : __data{__other.data()} {
+        std::__debug_db_insert_c(this);
+        _LIBCPP_ASSERT(_Extent == __other.size(), "size mismatch in span's constructor (other span)");
+    }
 
-//  ~span() noexcept = default;
+#ifdef _LIBCPP_ENABLE_DEBUG_MODE
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 ~span() noexcept {
+        std::__debug_db_erase_c(this);
+    }
+#endif
 
     template <size_t _Count>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, _Count> first() const noexcept
     {
-        static_assert(_Count <= _Extent, "Count out of range in span::first()");
+        static_assert(_Count <= _Extent, "span<T, N>::first<Count>(): Count out of range");
         return span<element_type, _Count>{data(), _Count};
     }
 
     template <size_t _Count>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, _Count> last() const noexcept
     {
-        static_assert(_Count <= _Extent, "Count out of range in span::last()");
+        static_assert(_Count <= _Extent, "span<T, N>::last<Count>(): Count out of range");
         return span<element_type, _Count>{data() + size() - _Count, _Count};
     }
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> first(size_type __count) const noexcept
     {
-        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::first(count)");
+        _LIBCPP_ASSERT(__count <= size(), "span<T, N>::first(count): count out of range");
         return {data(), __count};
     }
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> last(size_type __count) const noexcept
     {
-        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
+        _LIBCPP_ASSERT(__count <= size(), "span<T, N>::last(count): count out of range");
         return {data() + size() - __count, __count};
     }
 
     template <size_t _Offset, size_t _Count = dynamic_extent>
     _LIBCPP_INLINE_VISIBILITY
     constexpr auto subspan() const noexcept
         -> span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>
     {
-        static_assert(_Offset <= _Extent, "Offset out of range in span::subspan()");
-        static_assert(_Count == dynamic_extent || _Count <= _Extent - _Offset, "Offset + count out of range in span::subspan()");
+        static_assert(_Offset <= _Extent, "span<T, N>::subspan<Offset, Count>(): Offset out of range");
+        static_assert(_Count == dynamic_extent || _Count <= _Extent - _Offset, "span<T, N>::subspan<Offset, Count>(): Offset + Count out of range");
 
         using _ReturnType = span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>;
         return _ReturnType{data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
     }
 
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent>
        subspan(size_type __offset, size_type __count = dynamic_extent) const noexcept
     {
-        _LIBCPP_ASSERT(__offset <= size(), "Offset out of range in span::subspan(offset, count)");
-        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "Count out of range in span::subspan(offset, count)");
+        _LIBCPP_ASSERT(__offset <= size(), "span<T, N>::subspan(offset, count): offset out of range");
+        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "span<T, N>::subspan(offset, count): count out of range");
         if (__count == dynamic_extent)
             return {data() + __offset, size() - __offset};
-        _LIBCPP_ASSERT(__count <= size() - __offset, "Offset + count out of range in span::subspan(offset, count)");
+        _LIBCPP_ASSERT(__count <= size() - __offset, "span<T, N>::subspan(offset, count): offset + count out of range");
         return {data() + __offset, __count};
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size()           const noexcept { return _Extent; }
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes()     const noexcept { return _Extent * sizeof(element_type); }
     [[nodiscard]] _LIBCPP_INLINE_VISIBILITY constexpr bool empty() const noexcept { return _Extent == 0; }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
     {
-        _LIBCPP_ASSERT(__idx < size(), "span<T,N>[] index out of bounds");
+        _LIBCPP_ASSERT(__idx < size(), "span<T, N>::operator[](index): index out of range");
         return __data[__idx];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
     {
         _LIBCPP_ASSERT(!empty(), "span<T, N>::front() on empty span");
         return __data[0];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
     {
         _LIBCPP_ASSERT(!empty(), "span<T, N>::back() on empty span");
         return __data[size()-1];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
 // [span.iter], span iterator support
     _LIBCPP_INLINE_VISIBILITY constexpr iterator begin() const noexcept {
-#ifdef _LIBCPP_SPAN_USE_POINTER_ITERATOR
-        return iterator(data());
-#else
         return iterator(this, data());
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY constexpr iterator end() const noexcept {
-#ifdef _LIBCPP_SPAN_USE_POINTER_ITERATOR
-        return iterator(data() + size());
-#else
         return iterator(this, data() + size());
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
 
     _LIBCPP_INLINE_VISIBILITY span<const byte, _Extent * sizeof(element_type)> __as_bytes() const noexcept
     { return span<const byte, _Extent * sizeof(element_type)>{reinterpret_cast<const byte *>(data()), size_bytes()}; }
 
     _LIBCPP_INLINE_VISIBILITY span<byte, _Extent * sizeof(element_type)> __as_writable_bytes() const noexcept
     { return span<byte, _Extent * sizeof(element_type)>{reinterpret_cast<byte *>(data()), size_bytes()}; }
 
+#ifdef _LIBCPP_ENABLE_DEBUG_MODE
+
+    using __debug_iterator = iterator;
+
+    bool __dereferenceable(const iterator* __i) const {
+        return __data <= __i->base() && __i->base() < __data + extent;
+    }
+
+    bool __decrementable(const iterator* __i) const {
+        return __data < __i->base() && __i->base() <= __data + extent;
+    }
+
+    bool __addable(const iterator* __i, ptrdiff_t __n) const {
+        const_pointer __p = __i->base() + __n;
+        return __data <= __p && __p <= __data + extent;
+    }
+
+    bool __subscriptable(const iterator* __i, ptrdiff_t __n) const {
+        const_pointer __p = __i->base() + __n;
+        return __data <= __p && __p < __data + extent;
+    }
+
+#endif // _LIBCPP_ENABLE_DEBUG_MODE
+
 private:
     pointer    __data;
 
 };
 
 
 template <typename _Tp>
 class _LIBCPP_TEMPLATE_VIS span<_Tp, dynamic_extent> {
 private:
 
 public:
 //  constants and types
     using element_type           = _Tp;
     using value_type             = remove_cv_t<_Tp>;
     using size_type              = size_t;
     using difference_type        = ptrdiff_t;
     using pointer                = _Tp *;
     using const_pointer          = const _Tp *;
     using reference              = _Tp &;
     using const_reference        = const _Tp &;
-#ifdef _LIBCPP_SPAN_USE_POINTER_ITERATOR
-    using iterator               = pointer;
-#else
     using iterator               = __wrap_iter<pointer>;
-#endif
     using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
 
     static constexpr size_type extent = dynamic_extent;
 
 // [span.cons], span constructors, copy, assignment, and destructor
-    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}, __size{0} {}
+    _LIBCPP_INLINE_VISIBILITY constexpr span() noexcept : __data{nullptr}, __size{0} {
+        std::__debug_db_insert_c(this);
+    }
 
-    constexpr span           (const span&) noexcept = default;
+#ifndef _LIBCPP_ENABLE_DEBUG_MODE
+    constexpr span(const span&) noexcept = default;
+#else
+    constexpr span(const span& __other) noexcept : __data(__other.__data), __size(__other.__size) {
+        std::__debug_db_insert_c(this);
+    }
+
+#endif
     constexpr span& operator=(const span&) noexcept = default;
 
     template <class _It,
               enable_if_t<contiguous_iterator<_It> &&
                               is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]>,
                           nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(_It __first, size_type __count)
-        : __data{_VSTD::to_address(__first)}, __size{__count} {}
+        : __data{_VSTD::to_address(__first)}, __size{__count} {
+        std::__debug_db_insert_c(this);
+    }
 
     template <
         class _It, class _End,
         enable_if_t<is_convertible_v<remove_reference_t<iter_reference_t<_It> > (*)[], element_type (*)[]> &&
                         contiguous_iterator<_It> && sized_sentinel_for<_End, _It> && !is_convertible_v<_End, size_t>,
                     nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span(_It __first, _End __last)
-        : __data(_VSTD::to_address(__first)), __size(__last - __first) {}
+        : __data(_VSTD::to_address(__first)), __size(__last - __first) {
+        std::__debug_db_insert_c(this);
+    }
 
     template <size_t _Sz>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(type_identity_t<element_type> (&__arr)[_Sz]) noexcept : __data{__arr}, __size{_Sz} {}
+    constexpr span(type_identity_t<element_type> (&__arr)[_Sz]) noexcept : __data{__arr}, __size{_Sz} {
+        std::__debug_db_insert_c(this);
+    }
 
     template <class _OtherElementType, size_t _Sz,
               enable_if_t<is_convertible_v<_OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
+    constexpr span(array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {
+        std::__debug_db_insert_c(this);
+    }
 
     template <class _OtherElementType, size_t _Sz,
               enable_if_t<is_convertible_v<const _OtherElementType(*)[], element_type (*)[]>, nullptr_t> = nullptr>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {}
+    constexpr span(const array<_OtherElementType, _Sz>& __arr) noexcept : __data{__arr.data()}, __size{_Sz} {
+        std::__debug_db_insert_c(this);
+    }
 
 #if defined(_LIBCPP_HAS_NO_INCOMPLETE_RANGES)
     template <class _Container>
         requires __span_compatible_range<_Container, element_type>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(_Container& __c) : __data(std::data(__c)), __size{std::size(__c)} {}
+    constexpr span(_Container& __c) : __data(std::data(__c)), __size{std::size(__c)} {
+        std::__debug_db_insert_c(this);
+    }
     template <class _Container>
         requires __span_compatible_range<const _Container, element_type>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(const _Container& __c) : __data(std::data(__c)), __size{std::size(__c)} {}
+    constexpr span(const _Container& __c) : __data(std::data(__c)), __size{std::size(__c)} {
+        std::__debug_db_insert_c(this);
+    }
 #else
     template <__span_compatible_range<element_type> _Range>
     _LIBCPP_INLINE_VISIBILITY
-    constexpr span(_Range&& __r) : __data(ranges::data(__r)), __size{ranges::size(__r)} {}
+    constexpr span(_Range&& __r) : __data(ranges::data(__r)), __size{ranges::size(__r)} {
+        std::__debug_db_insert_c(this);
+    }
 #endif
 
     template <class _OtherElementType, size_t _OtherExtent>
     _LIBCPP_INLINE_VISIBILITY
         constexpr span(const span<_OtherElementType, _OtherExtent>& __other,
                        enable_if_t<
                           is_convertible_v<_OtherElementType(*)[], element_type (*)[]>,
                           nullptr_t> = nullptr) noexcept
-        : __data{__other.data()}, __size{__other.size()} {}
+        : __data{__other.data()}, __size{__other.size()} {
+        std::__debug_db_insert_c(this);
+    }
 
-//    ~span() noexcept = default;
+#ifdef _LIBCPP_ENABLE_DEBUG_MODE
+    _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_AFTER_CXX17 ~span() noexcept {
+        std::__debug_db_erase_c(this);
+    }
+#endif
 
     template <size_t _Count>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, _Count> first() const noexcept
     {
-        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
+        _LIBCPP_ASSERT(_Count <= size(), "span<T>::first<Count>(): Count out of range");
         return span<element_type, _Count>{data(), _Count};
     }
 
     template <size_t _Count>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, _Count> last() const noexcept
     {
-        _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
+        _LIBCPP_ASSERT(_Count <= size(), "span<T>::last<Count>(): Count out of range");
         return span<element_type, _Count>{data() + size() - _Count, _Count};
     }
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> first(size_type __count) const noexcept
     {
-        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::first(count)");
+        _LIBCPP_ASSERT(__count <= size(), "span<T>::first(count): count out of range");
         return {data(), __count};
     }
 
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, dynamic_extent> last (size_type __count) const noexcept
     {
-        _LIBCPP_ASSERT(__count <= size(), "Count out of range in span::last(count)");
+        _LIBCPP_ASSERT(__count <= size(), "span<T>::last(count): count out of range");
         return {data() + size() - __count, __count};
     }
 
     template <size_t _Offset, size_t _Count = dynamic_extent>
     _LIBCPP_INLINE_VISIBILITY
     constexpr span<element_type, _Count> subspan() const noexcept
     {
-        _LIBCPP_ASSERT(_Offset <= size(), "Offset out of range in span::subspan()");
-        _LIBCPP_ASSERT(_Count == dynamic_extent || _Count <= size() - _Offset, "Offset + count out of range in span::subspan()");
+        _LIBCPP_ASSERT(_Offset <= size(), "span<T>::subspan<Offset, Count>(): Offset out of range");
+        _LIBCPP_ASSERT(_Count == dynamic_extent || _Count <= size() - _Offset, "span<T>::subspan<Offset, Count>(): Offset + Count out of range");
         return span<element_type, _Count>{data() + _Offset, _Count == dynamic_extent ? size() - _Offset : _Count};
     }
 
     constexpr span<element_type, dynamic_extent>
     _LIBCPP_INLINE_VISIBILITY
     subspan(size_type __offset, size_type __count = dynamic_extent) const noexcept
     {
-        _LIBCPP_ASSERT(__offset <= size(), "Offset out of range in span::subspan(offset, count)");
-        _LIBCPP_ASSERT(__count  <= size() || __count == dynamic_extent, "count out of range in span::subspan(offset, count)");
+        _LIBCPP_ASSERT(__offset <= size(), "span<T>::subspan(offset, count): offset out of range");
+        _LIBCPP_ASSERT(__count <= size() || __count == dynamic_extent, "span<T>::subspan(offset, count): count out of range");
         if (__count == dynamic_extent)
             return {data() + __offset, size() - __offset};
-        _LIBCPP_ASSERT(__count <= size() - __offset, "Offset + count out of range in span::subspan(offset, count)");
+        _LIBCPP_ASSERT(__count <= size() - __offset, "span<T>::subspan(offset, count): offset + count out of range");
         return {data() + __offset, __count};
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size()           const noexcept { return __size; }
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes()     const noexcept { return __size * sizeof(element_type); }
     [[nodiscard]] _LIBCPP_INLINE_VISIBILITY constexpr bool empty() const noexcept { return __size == 0; }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
     {
-        _LIBCPP_ASSERT(__idx < size(), "span<T>[] index out of bounds");
+        _LIBCPP_ASSERT(__idx < size(), "span<T>::operator[](index): index out of range");
         return __data[__idx];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
     {
-        _LIBCPP_ASSERT(!empty(), "span<T>[].front() on empty span");
+        _LIBCPP_ASSERT(!empty(), "span<T>::front() on empty span");
         return __data[0];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference back() const noexcept
     {
-        _LIBCPP_ASSERT(!empty(), "span<T>[].back() on empty span");
+        _LIBCPP_ASSERT(!empty(), "span<T>::back() on empty span");
         return __data[size()-1];
     }
 
 
     _LIBCPP_INLINE_VISIBILITY constexpr pointer data()                         const noexcept { return __data; }
 
 // [span.iter], span iterator support
     _LIBCPP_INLINE_VISIBILITY constexpr iterator begin() const noexcept {
-#ifdef _LIBCPP_SPAN_USE_POINTER_ITERATOR
-        return iterator(data());
-#else
         return iterator(this, data());
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY constexpr iterator end() const noexcept {
-#ifdef _LIBCPP_SPAN_USE_POINTER_ITERATOR
-        return iterator(data() + size());
-#else
         return iterator(this, data() + size());
-#endif
     }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator        rbegin() const noexcept { return reverse_iterator(end()); }
     _LIBCPP_INLINE_VISIBILITY constexpr reverse_iterator          rend() const noexcept { return reverse_iterator(begin()); }
 
     _LIBCPP_INLINE_VISIBILITY span<const byte, dynamic_extent> __as_bytes() const noexcept
     { return {reinterpret_cast<const byte *>(data()), size_bytes()}; }
 
     _LIBCPP_INLINE_VISIBILITY span<byte, dynamic_extent> __as_writable_bytes() const noexcept
     { return {reinterpret_cast<byte *>(data()), size_bytes()}; }
 
+#ifdef _LIBCPP_ENABLE_DEBUG_MODE
+
+    using __debug_iterator = iterator;
+
+    bool __dereferenceable(const iterator* __i) const {
+        return __data <= __i->base() && __i->base() < __data + __size;
+    }
+
+    bool __decrementable(const iterator* __i) const {
+        return __data < __i->base() && __i->base() <= __data + __size;
+    }
+
+    bool __addable(const iterator* __i, ptrdiff_t __n) const {
+        const_pointer __p = __i->base() + __n;
+        return __data <= __p && __p <= __data + __size;
+    }
+
+    bool __subscriptable(const iterator* __i, ptrdiff_t __n) const {
+        const_pointer __p = __i->base() + __n;
+        return __data <= __p && __p < __data + __size;
+    }
+
+#endif // _LIBCPP_ENABLE_DEBUG_MODE
+
 private:
     pointer   __data;
     size_type __size;
 };
 
 template <class _Tp, size_t _Extent>
 inline constexpr bool ranges::enable_borrowed_range<span<_Tp, _Extent> > = true;
 

libcxx/test/libcxx/containers/views/views.span/debug.iterator-indexing.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// Make sure that std::span's iterators check for OOB accesses when the debug mode is enabled.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: !libcpp-has-debug-mode, c++03
+
+#include <span>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    auto check = [](auto const& span) {
+        {
+            auto it = span.begin();
+            TEST_LIBCPP_ASSERT_FAILURE(it + 4, "Attempted to add/subtract an iterator outside its valid range");
+        }
+        {
+            auto it = span.begin();
+            TEST_LIBCPP_ASSERT_FAILURE(it - 4, "Attempted to add/subtract an iterator outside its valid range");
+        }
+        {
+            auto it = span.begin();
+            TEST_LIBCPP_ASSERT_FAILURE(--it, "Attempted to decrement a non-decrementable iterator");
+        }
+        {
+            auto it = span.begin();
+            TEST_LIBCPP_ASSERT_FAILURE(it--, "Attempted to decrement a non-decrementable iterator");
+        }
+        {
+            auto it = span.end();
+            TEST_LIBCPP_ASSERT_FAILURE(++it, "Attempted to increment a non-incrementable iterator");
+        }
+        {
+            auto it = span.end();
+            TEST_LIBCPP_ASSERT_FAILURE(it++, "Attempted to increment a non-incrementable iterator");
+        }
+        {
+            auto it = span.end();
+            TEST_LIBCPP_ASSERT_FAILURE(*it, "Attempted to dereference a non-dereferenceable iterator");
+        }
+    };
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const span(array, 3);
+        check(span);
+    }
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 3> const span(array, 3);
+        check(span);
+    }
+
+    return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.elem/assert.back.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <span>
+//
+// constexpr reference back() const noexcept;
+
+// Make sure that accessing a span out-of-bounds triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <span>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const s(&array[0], 0);
+        TEST_LIBCPP_ASSERT_FAILURE(s.back(), "span<T>::back() on empty span");
+    }
+
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 0> const s(&array[0], 0);
+        TEST_LIBCPP_ASSERT_FAILURE(s.back(), "span<T, N>::back() on empty span");
+    }
+
+    return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.elem/assert.front.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <span>
+//
+// constexpr reference front() const noexcept;
+
+// Make sure that accessing a span out-of-bounds triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <span>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const s(&array[0], 0);
+        TEST_LIBCPP_ASSERT_FAILURE(s.front(), "span<T>::front() on empty span");
+    }
+
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 0> const s(&array[0], 0);
+        TEST_LIBCPP_ASSERT_FAILURE(s.front(), "span<T, N>::front() on empty span");
+    }
+
+    return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.elem/assert.op_idx.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <span>
+//
+// constexpr reference operator[](size_type idx) const;
+
+// Make sure that accessing a span out-of-bounds triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <span>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const s(array, 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s[3], "span<T>::operator[](index): index out of range");
+    }
+
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 3> const s(array, 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s[3], "span<T, N>::operator[](index): index out of range");
+    }
+
+    return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.sub/assert.first.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <span>
+//
+// constexpr span<element_type, dynamic_extent> first(size_type count) const;
+
+// Make sure that creating a sub-span with an incorrect number of elements triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <span>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const s(&array[0], 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s.first(4), "span<T>::first(count): count out of range");
+        TEST_LIBCPP_ASSERT_FAILURE(s.first<4>(), "span<T>::first<Count>(): Count out of range");
+    }
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 3> const s(&array[0], 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s.first(4), "span<T, N>::first(count): count out of range");
+        // s.first<4>() caught at compile-time (tested elsewhere)
+    }
+
+    return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.sub/assert.last.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <span>
+//
+// constexpr span<element_type, dynamic_extent> last(size_type count) const;
+
+// Make sure that creating a sub-span with an incorrect number of elements triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <span>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const s(&array[0], 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s.last(4), "span<T>::last(count): count out of range");
+        TEST_LIBCPP_ASSERT_FAILURE(s.last<4>(), "span<T>::last<Count>(): Count out of range");
+    }
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 3> const s(&array[0], 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s.last(4), "span<T, N>::last(count): count out of range");
+        // s.last<4>() caught at compile-time (tested elsewhere)
+    }
+
+    return 0;
+}

libcxx/test/libcxx/containers/views/views.span/span.sub/assert.subspan.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <span>
+//
+// constexpr span<element_type, dynamic_extent> subspan(
+//   size_type offset, size_type count = dynamic_extent) const;
+//
+//  Requires: (0 <= Offset && Offset <= size())
+//      && (Count == dynamic_extent || Count >= 0 && Offset + Count <= size())
+
+// Make sure that creating a sub-span with an incorrect number of elements triggers an assertion.
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03
+// XFAIL: use_system_cxx_lib && target={{.+}}-apple-macosx{{10.9|10.10|10.11|10.12|10.13|10.14|10.15|11.0|12.0}}
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+#include <span>
+#include <cstddef>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+    {
+        int array[] = {0, 1, 2};
+        std::span<int> const s(&array[0], 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan(4), "span<T>::subspan(offset, count): offset out of range");
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan<4>(), "span<T>::subspan<Offset, Count>(): Offset out of range");
+
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan(0, 4), "span<T>::subspan(offset, count): count out of range");
+        TEST_LIBCPP_ASSERT_FAILURE((s.subspan<0, 4>()), "span<T>::subspan<Offset, Count>(): Offset + Count out of range");
+
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan(1, 3), "span<T>::subspan(offset, count): offset + count out of range");
+        TEST_LIBCPP_ASSERT_FAILURE((s.subspan<1, 3>()), "span<T>::subspan<Offset, Count>(): Offset + Count out of range");
+    }
+    {
+        int array[] = {0, 1, 2};
+        std::span<int, 3> const s(&array[0], 3);
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan(4), "span<T, N>::subspan(offset, count): offset out of range");
+        // s.subspan<4>() caught at compile-time (tested elsewhere)
+
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan(0, 4), "span<T, N>::subspan(offset, count): count out of range");
+        // s.subspan<0, 4>() caught at compile-time (tested elsewhere)
+
+        TEST_LIBCPP_ASSERT_FAILURE(s.subspan(1, 3), "span<T, N>::subspan(offset, count): offset + count out of range");
+        // s.subspan<1, 3>() caught at compile-time (tested elsewhere)
+    }
+
+    return 0;
+}

libcxx/test/std/containers/views/views.span/span.cons/span.dtor.compile.pass.cpp

 // ~span() = default;
 
 #include <span>
 #include <type_traits>
 
 template <class T>
 constexpr void testDestructor() {
   static_assert(std::is_nothrow_destructible_v<T>);
+#ifdef _LIBCPP_ENABLE_DEBUG_MODE
+  static_assert(!std::is_trivially_destructible_v<T>);
+#else
   static_assert(std::is_trivially_destructible_v<T>);
+#endif
 }
 
 void test() {
   testDestructor<std::span<int, 1>>();
   testDestructor<std::span<int>>();
 }

libcxx/test/std/containers/views/views.span/span.sub/first.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
 
 // template<size_t Count>
 //  constexpr span<element_type, Count> first() const;
 //
-// constexpr span<element_type, dynamic_extent> first(size_type count) const;
-//
 //  Requires: Count <= size().
 
 #include <span>
-
 #include <cstddef>
 
-#include "test_macros.h"
-
-constexpr int carr[] = {1, 2, 3, 4};
-
-int main(int, char**) {
-  std::span<const int, 4> sp(carr);
+void f() {
+  int array[] = {1, 2, 3, 4};
+  std::span<const int, 4> sp(array);
 
   //  Count too large
-  {
-    [[maybe_unused]] auto s1 = sp.first<5>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Count out of range in span::first()"}}
-  }
+  [[maybe_unused]] auto s1 = sp.first<5>(); // expected-error@span:* {{span<T, N>::first<Count>(): Count out of range}}
 
   //  Count numeric_limits
-  {
-    [[maybe_unused]] auto s1 = sp.first<std::size_t(-1)>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Count out of range in span::first()"}}
-  }
-
-  return 0;
+  [[maybe_unused]] auto s2 = sp.first<std::size_t(-1)>(); // expected-error@span:* {{span<T, N>::first<Count>(): Count out of range}}
 }

libcxx/test/std/containers/views/views.span/span.sub/last.verify.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // <span>
 
 // template<size_t Count>
 //  constexpr span<element_type, Count> last() const;
 //
-// constexpr span<element_type, dynamic_extent> last(size_type count) const;
-//
 //  Requires: Count <= size().
 
 #include <span>
-
 #include <cstddef>
 
-#include "test_macros.h"
-
-constexpr int carr[] = {1, 2, 3, 4};
-
-int main(int, char**) {
-  std::span<const int, 4> sp(carr);
+void f() {
+  int array[] = {1, 2, 3, 4};
+  std::span<const int, 4> sp(array);
 
   //  Count too large
-  {
-    [[maybe_unused]] auto s1 = sp.last<5>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Count out of range in span::last()"}}
-  }
+  [[maybe_unused]] auto s1 = sp.last<5>(); // expected-error@span:* {{span<T, N>::last<Count>(): Count out of range}}
 
   //  Count numeric_limits
-  {
-    [[maybe_unused]] auto s1 = sp.last<std::size_t(-1)>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Count out of range in span::last()"}}
-  }
-
-  return 0;
+  [[maybe_unused]] auto s2 = sp.last<std::size_t(-1)>(); // expected-error@span:* {{span<T, N>::last<Count>(): Count out of range}}
 }

libcxx/test/std/containers/views/views.span/span.sub/subspan.fail.cpp

-//===----------------------------------------------------------------------===//
-//
-// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
-// See https://llvm.org/LICENSE.txt for license information.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-//
-//===----------------------------------------------------------------------===//
-// UNSUPPORTED: c++03, c++11, c++14, c++17
-
-// <span>
-
-// template<size_t Offset, size_t Count = dynamic_extent>
-//   constexpr span<element_type, see below> subspan() const;
-//
-// constexpr span<element_type, dynamic_extent> subspan(
-//   size_type offset, size_type count = dynamic_extent) const;
-//
-//  Requires: offset <= size() &&
-//            (count == dynamic_extent || count <= size() - offset)
-
-#include <span>
-
-#include <cstddef>
-
-#include "test_macros.h"
-
-constexpr int carr[] = {1, 2, 3, 4};
-
-int main(int, char**) {
-  std::span<const int, 4> sp(carr);
-
-  //  Offset too large templatized
-  {
-    [[maybe_unused]] auto s1 = sp.subspan<5>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Offset out of range in span::subspan()"}}
-  }
-
-  //  Count too large templatized
-  {
-    [[maybe_unused]] auto s1 = sp.subspan<0, 5>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Offset + count out of range in span::subspan()"}}
-  }
-
-  //  Offset + Count too large templatized
-  {
-    [[maybe_unused]] auto s1 = sp.subspan<2, 3>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Offset + count out of range in span::subspan()"}}
-  }
-
-  //  Offset + Count overflow templatized
-  {
-    [[maybe_unused]] auto s1 = sp.subspan<3, std::size_t(-2)>(); // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Offset + count out of range in span::subspan()"}}, expected-error-re@span:* {{array is too large{{(.* elements)}}}}
-  }
-
-  return 0;
-}

libcxx/test/std/containers/views/views.span/span.sub/subspan.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// This test also generates spurious warnings when instantiating std::span
+// with a very large extent (like size_t(-2)) -- silence those.
+// ADDITIONAL_COMPILE_FLAGS: -Xclang -verify-ignore-unexpected=warning
+
+// <span>
+
+// template<size_t Offset, size_t Count = dynamic_extent>
+//   constexpr span<element_type, see below> subspan() const;
+//
+//  Requires: offset <= size() &&
+//            (count == dynamic_extent || count <= size() - offset)
+
+#include <span>
+#include <cstddef>
+
+void f() {
+  int array[] = {1, 2, 3, 4};
+  std::span<const int, 4> sp(array);
+
+  //  Offset too large templatized
+  [[maybe_unused]] auto s1 = sp.subspan<5>(); // expected-error@span:* {{span<T, N>::subspan<Offset, Count>(): Offset out of range"}}
+
+  //  Count too large templatized
+  [[maybe_unused]] auto s2 = sp.subspan<0, 5>(); // expected-error@span:* {{span<T, N>::subspan<Offset, Count>(): Offset + Count out of range"}}
+
+  //  Offset + Count too large templatized
+  [[maybe_unused]] auto s3 = sp.subspan<2, 3>(); // expected-error@span:* {{span<T, N>::subspan<Offset, Count>(): Offset + Count out of range"}}
+
+  //  Offset + Count overflow templatized
+  [[maybe_unused]] auto s4 = sp.subspan<3, std::size_t(-2)>(); // expected-error@span:* {{span<T, N>::subspan<Offset, Count>(): Offset + Count out of range"}}, expected-error-re@span:* {{array is too large{{(.* elements)}}}}
+}

libcxx/test/std/containers/views/views.span/trivially_copyable.compile.pass.cpp

 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17
 
 // P2251 was voted into C++23, but is supported even in C++20 mode by all vendors.
 
 // <span>
 
 #include <span>
 #include <type_traits>
 
+#ifdef _LIBCPP_ENABLE_DEBUG_MODE
+static_assert(!std::is_trivially_copyable_v<std::span<int>>);
+static_assert(!std::is_trivially_copyable_v<std::span<int, 3>>);
+#else
 static_assert(std::is_trivially_copyable_v<std::span<int>>);
 static_assert(std::is_trivially_copyable_v<std::span<int, 3>>);
+#endif