Index: libcxx/include/iterator
===================================================================
--- libcxx/include/iterator
+++ libcxx/include/iterator
@@ -1442,7 +1442,7 @@
     template <class _Up> friend class __wrap_iter;
     template <class _CharT, class _Traits, class _Alloc> friend class basic_string;
     template <class _Tp, class _Alloc> friend class _LIBCPP_TEMPLATE_VIS vector;
-    template <class _Tp, ptrdiff_t> friend class _LIBCPP_TEMPLATE_VIS span;
+    template <class _Tp, size_t> friend class _LIBCPP_TEMPLATE_VIS span;
 
     template <class _Iter1, class _Iter2>
     _LIBCPP_CONSTEXPR_IF_NODEBUG friend
Index: libcxx/include/span
===================================================================
--- libcxx/include/span
+++ libcxx/include/span
@@ -16,36 +16,36 @@
 namespace std {
 
 // constants
-inline constexpr ptrdiff_t dynamic_extent = -1;
+inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
 
 // [views.span], class template span
-template <class ElementType, ptrdiff_t Extent = dynamic_extent>
+template <class ElementType, size_t Extent = dynamic_extent>
     class span;
 
 // [span.objectrep], views of object representation
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, size_t Extent>
     span<const byte, ((Extent == dynamic_extent) ? dynamic_extent :
         (static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent))> as_bytes(span<ElementType, Extent> s) noexcept;
 
-template <class ElementType, ptrdiff_t Extent>
+template <class ElementType, size_t Extent>
     span<      byte, ((Extent == dynamic_extent) ? dynamic_extent :
         (static_cast<ptrdiff_t>(sizeof(ElementType)) * Extent))> as_writable_bytes(span<ElementType, Extent> s) noexcept;
 
 
 namespace std {
-template <class ElementType, ptrdiff_t Extent = dynamic_extent>
+template <class ElementType, size_t Extent = dynamic_extent>
 class span {
 public:
     // constants and types
     using element_type = ElementType;
     using value_type = remove_cv_t<ElementType>;
-    using index_type = ptrdiff_t;
+    using index_type = size_t;
     using difference_type = ptrdiff_t;
     using pointer = element_type*;
     using const_pointer = const element_type*;
     using reference = element_type&;
-    using iterator = implementation-defined;
     using const_reference = const element_type&;
+    using iterator = implementation-defined;
     using const_iterator = implementation-defined;
     using reverse_iterator = std::reverse_iterator<iterator>;
     using const_reverse_iterator = std::reverse_iterator<const_iterator>;
@@ -66,17 +66,17 @@
     template <class Container>
         constexpr span(const Container& cont);
     constexpr span(const span& other) noexcept = default;
-    template <class OtherElementType, ptrdiff_t OtherExtent>
+    template <class OtherElementType, size_t OtherExtent>
         constexpr span(const span<OtherElementType, OtherExtent>& s) noexcept;
     ~span() noexcept = default;
     constexpr span& operator=(const span& other) noexcept = default;
 
     // [span.sub], span subviews
-    template <ptrdiff_t Count>
+    template <size_t Count>
         constexpr span<element_type, Count> first() const;
-    template <ptrdiff_t Count>
+    template <size_t Count>
         constexpr span<element_type, Count> last() const;
-    template <ptrdiff_t Offset, ptrdiff_t Count = dynamic_extent>
+    template <size_t Offset, size_t Count = dynamic_extent>
         constexpr span<element_type, see below> subspan() const;
 
     constexpr span<element_type, dynamic_extent> first(index_type count) const;
@@ -143,14 +143,14 @@
 
 #if _LIBCPP_STD_VER > 17
 
-inline constexpr ptrdiff_t dynamic_extent = -1;
-template <typename _Tp, ptrdiff_t _Extent = dynamic_extent> class span;
+inline constexpr size_t dynamic_extent = numeric_limits<size_t>::max();
+template <typename _Tp, size_t _Extent = dynamic_extent> class span;
 
 
 template <class _Tp>
 struct __is_span_impl : public false_type {};
 
-template <class _Tp, ptrdiff_t _Extent>
+template <class _Tp, size_t _Extent>
 struct __is_span_impl<span<_Tp, _Extent>> : public true_type {};
 
 template <class _Tp>
@@ -189,18 +189,18 @@
     : public true_type {};
 
 
-template <typename _Tp, ptrdiff_t _Extent>
+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 index_type             = ptrdiff_t;
+    using index_type             = size_t;
     using difference_type        = ptrdiff_t;
     using pointer                = _Tp *;
-    using const_pointer          = const _Tp *;
+    using const_pointer          = const _Tp *; // not in standard
     using reference              = _Tp &;
-    using const_reference        = const _Tp &;
+    using const_reference        = const _Tp &; // not in standard
     using iterator               =  __wrap_iter<pointer>;
     using const_iterator         =  __wrap_iter<const_pointer>;
     using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
@@ -244,20 +244,18 @@
 
 //  ~span() noexcept = default;
 
-    template <ptrdiff_t _Count>
+    template <size_t _Count>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> first() const noexcept
     {
-        static_assert(_Count >= 0, "Count must be >= 0 in span::first()");
         static_assert(_Count <= _Extent, "Count out of range in span::first()");
         return {data(), _Count};
     }
 
-    template <ptrdiff_t _Count>
+    template <size_t _Count>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> last() const noexcept
     {
-        static_assert(_Count >= 0, "Count must be >= 0 in span::last()");
         static_assert(_Count <= _Extent, "Count out of range in span::last()");
         return {data() + size() - _Count, _Count};
     }
@@ -276,7 +274,7 @@
         return {data() + size() - __count, __count};
     }
 
-    template <ptrdiff_t _Offset, ptrdiff_t _Count = dynamic_extent>
+    template <size_t _Offset, size_t _Count = dynamic_extent>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr auto subspan() const noexcept
         -> span<element_type, _Count != dynamic_extent ? _Count : _Extent - _Offset>
@@ -359,12 +357,12 @@
 //  constants and types
     using element_type           = _Tp;
     using value_type             = remove_cv_t<_Tp>;
-    using index_type             = ptrdiff_t;
+    using index_type             = size_t;
     using difference_type        = ptrdiff_t;
     using pointer                = _Tp *;
-    using const_pointer          = const _Tp *;
+    using const_pointer          = const _Tp *; // not in standard
     using reference              = _Tp &;
-    using const_reference        = const _Tp &;
+    using const_reference        = const _Tp &; // not in standard
     using iterator               =  __wrap_iter<pointer>;
     using const_iterator         =  __wrap_iter<const_pointer>;
     using reverse_iterator       = _VSTD::reverse_iterator<iterator>;
@@ -379,7 +377,7 @@
     constexpr span& operator=(const span&) noexcept = default;
 
     _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __ptr, index_type __count) : __data{__ptr}, __size{__count} {}
-    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __f, pointer __l) : __data{__f}, __size{distance(__f, __l)} {}
+    _LIBCPP_INLINE_VISIBILITY constexpr span(pointer __f, pointer __l) : __data{__f}, __size{static_cast<size_t>(distance(__f, __l))} {}
 
     template <size_t _Sz>
     inline _LIBCPP_INLINE_VISIBILITY
@@ -406,7 +404,7 @@
         : __data{_VSTD::data(__c)}, __size{(index_type) _VSTD::size(__c)} {}
 
 
-    template <class _OtherElementType, ptrdiff_t _OtherExtent>
+    template <class _OtherElementType, size_t _OtherExtent>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span(const span<_OtherElementType, _OtherExtent>& __other,
                        enable_if_t<
@@ -416,20 +414,18 @@
 
 //    ~span() noexcept = default;
 
-    template <ptrdiff_t _Count>
+    template <size_t _Count>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> first() const noexcept
     {
-        static_assert(_Count >= 0, "Count must be >= 0 in span::first()");
         _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::first()");
         return {data(), _Count};
     }
 
-    template <ptrdiff_t _Count>
+    template <size_t _Count>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<element_type, _Count> last() const noexcept
     {
-        static_assert(_Count >= 0, "Count must be >= 0 in span::last()");
         _LIBCPP_ASSERT(_Count <= size(), "Count out of range in span::last()");
         return {data() + size() - _Count, _Count};
     }
@@ -448,7 +444,7 @@
         return {data() + size() - __count, __count};
     }
 
-    template <ptrdiff_t _Offset, ptrdiff_t _Count = dynamic_extent>
+    template <size_t _Offset, size_t _Count = dynamic_extent>
     inline _LIBCPP_INLINE_VISIBILITY
         constexpr span<_Tp, dynamic_extent> subspan() const noexcept
     {
@@ -527,17 +523,17 @@
 };
 
 //  as_bytes & as_writeable_bytes
-template <class _Tp, ptrdiff_t _Extent>
+template <class _Tp, size_t _Extent>
     auto as_bytes(span<_Tp, _Extent> __s) noexcept
     -> decltype(__s.__as_bytes())
     { return __s.__as_bytes(); }
 
-template <class _Tp, ptrdiff_t _Extent>
+template <class _Tp, size_t _Extent>
     auto as_writeable_bytes(span<_Tp, _Extent> __s) noexcept
     -> typename enable_if<!is_const_v<_Tp>, decltype(__s.__as_writeable_bytes())>::type
     { return __s.__as_writeable_bytes(); }
 
-template <class _Tp, ptrdiff_t _Extent>
+template <class _Tp, size_t _Extent>
     constexpr void swap(span<_Tp, _Extent> &__lhs, span<_Tp, _Extent> &__rhs) noexcept
     { __lhs.swap(__rhs); }
 
Index: libcxx/test/std/containers/views/span.cons/default.fail.cpp
===================================================================
--- libcxx/test/std/containers/views/span.cons/default.fail.cpp
+++ libcxx/test/std/containers/views/span.cons/default.fail.cpp
@@ -24,7 +24,7 @@
 
 int main(int, char**)
 {
-    std::span<int, 2> s; // expected-error-re@span:* {{static_assert failed{{( due to requirement '2[LL]{0,2} == 0')?}} "Can't default construct a statically sized span with size > 0"}}
+    std::span<int, 2> s; // expected-error-re@span:* {{static_assert failed{{( due to requirement '.*')?}} "Can't default construct a statically sized span with size > 0"}}
 
 //  TODO: This is what I want:
 // eXpected-error {{no matching constructor for initialization of 'std::span<int, 2>'}}
Index: libcxx/test/std/containers/views/span.iterators/end.pass.cpp
===================================================================
--- libcxx/test/std/containers/views/span.iterators/end.pass.cpp
+++ libcxx/test/std/containers/views/span.iterators/end.pass.cpp
@@ -38,8 +38,8 @@
         ret = ret &&  (&*(ce-1) == last);
     }
 
-    ret = ret &&  (( e -  s.begin()) == s.size());
-    ret = ret &&  ((ce - s.cbegin()) == s.size());
+    ret = ret &&  (static_cast<size_t>( e -  s.begin()) == s.size());
+    ret = ret &&  (static_cast<size_t>(ce - s.cbegin()) == s.size());
 
     ret = ret &&  (e == ce);
     return ret;
@@ -64,8 +64,8 @@
         assert( &*(ce-1) == last);
     }
 
-    assert(( e -  s.begin()) == s.size());
-    assert((ce - s.cbegin()) == s.size());
+    assert(static_cast<size_t>( e -  s.begin()) == s.size());
+    assert(static_cast<size_t>(ce - s.cbegin()) == s.size());
 
     assert(e == ce);
 }
Index: libcxx/test/std/containers/views/span.iterators/rend.pass.cpp
===================================================================
--- libcxx/test/std/containers/views/span.iterators/rend.pass.cpp
+++ libcxx/test/std/containers/views/span.iterators/rend.pass.cpp
@@ -35,8 +35,8 @@
         ret = ret &&  (ce != s.crbegin());
     }
 
-    ret = ret &&  (( e -  s.rbegin()) == s.size());
-    ret = ret &&  ((ce - s.crbegin()) == s.size());
+    ret = ret &&  (static_cast<size_t>( e -  s.rbegin()) == s.size());
+    ret = ret &&  (static_cast<size_t>(ce - s.crbegin()) == s.size());
 
     ret = ret &&  (e == ce);
     return ret;
@@ -58,8 +58,8 @@
         assert(ce != s.crbegin());
     }
 
-    assert(( e -  s.rbegin()) == s.size());
-    assert((ce - s.crbegin()) == s.size());
+    assert(static_cast<size_t>( e -  s.rbegin()) == s.size());
+    assert(static_cast<size_t>(ce - s.crbegin()) == s.size());
 
     assert(e == ce);
 }
Index: libcxx/test/std/containers/views/span.obs/size.pass.cpp
===================================================================
--- libcxx/test/std/containers/views/span.obs/size.pass.cpp
+++ libcxx/test/std/containers/views/span.obs/size.pass.cpp
@@ -22,7 +22,7 @@
 
 
 template <typename Span>
-constexpr bool testConstexprSpan(Span sp, ptrdiff_t sz)
+constexpr bool testConstexprSpan(Span sp, size_t sz)
 {
     ASSERT_NOEXCEPT(sp.size());
     return sp.size() == sz;
@@ -30,7 +30,7 @@
 
 
 template <typename Span>
-void testRuntimeSpan(Span sp, ptrdiff_t sz)
+void testRuntimeSpan(Span sp, size_t sz)
 {
     ASSERT_NOEXCEPT(sp.size());
     assert(sp.size() == sz);
Index: libcxx/test/std/containers/views/types.pass.cpp
===================================================================
--- libcxx/test/std/containers/views/types.pass.cpp
+++ libcxx/test/std/containers/views/types.pass.cpp
@@ -16,7 +16,7 @@
 //  // constants and types
 //  using element_type           = ElementType;
 //  using value_type             = remove_cv_t<ElementType>;
-//  using index_type             = ptrdiff_t;
+//  using index_type             = size_t;
 //  using difference_type        = ptrdiff_t;
 //  using pointer                = element_type *;
 //  using reference              = element_type &;
@@ -63,12 +63,12 @@
     ASSERT_SAME_TYPE(typename ItT::difference_type,   typename S::difference_type);
 }
 
-template <typename S, typename ElementType, std::ptrdiff_t Size>
+template <typename S, typename ElementType, size_t Size>
 void testSpan()
 {
     ASSERT_SAME_TYPE(typename S::element_type,    ElementType);
     ASSERT_SAME_TYPE(typename S::value_type,      std::remove_cv_t<ElementType>);
-    ASSERT_SAME_TYPE(typename S::index_type,      std::ptrdiff_t);
+    ASSERT_SAME_TYPE(typename S::index_type,      size_t);
     ASSERT_SAME_TYPE(typename S::difference_type, std::ptrdiff_t);
     ASSERT_SAME_TYPE(typename S::pointer,         ElementType *);
     ASSERT_SAME_TYPE(typename S::const_pointer,   const ElementType *);
@@ -87,10 +87,10 @@
 template <typename T>
 void test()
 {
-    testSpan<std::span<               T>,                T, -1>();
-    testSpan<std::span<const          T>, const          T, -1>();
-    testSpan<std::span<      volatile T>,       volatile T, -1>();
-    testSpan<std::span<const volatile T>, const volatile T, -1>();
+    testSpan<std::span<               T>,                T, std::dynamic_extent>();
+    testSpan<std::span<const          T>, const          T, std::dynamic_extent>();
+    testSpan<std::span<      volatile T>,       volatile T, std::dynamic_extent>();
+    testSpan<std::span<const volatile T>, const volatile T, std::dynamic_extent>();
 
     testSpan<std::span<               T, 5>,                T, 5>();
     testSpan<std::span<const          T, 5>, const          T, 5>();