Index: libcxx/include/iterator
===================================================================
--- libcxx/include/iterator
+++ libcxx/include/iterator
@@ -399,6 +399,11 @@
 // 24.8, container access:
 template <class C> constexpr auto size(const C& c) -> decltype(c.size());         // C++17
 template <class T, size_t N> constexpr size_t size(const T (&array)[N]) noexcept; // C++17
+
+template <class C> constexpr auto ssize(const C& c)
+    -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;				       // C++20
+template <class T, ptrdiff_t> constexpr ptrdiff_t ssize(const T (&array)[N]) noexcept; // C++20
+
 template <class C> constexpr auto empty(const C& c) -> decltype(c.empty());       // C++17
 template <class T, size_t N> constexpr bool empty(const T (&array)[N]) noexcept;  // C++17
 template <class E> constexpr bool empty(initializer_list<E> il) noexcept;         // C++17
@@ -1858,6 +1863,18 @@
 inline _LIBCPP_INLINE_VISIBILITY
 constexpr size_t size(const _Tp (&)[_Sz]) noexcept { return _Sz; }
 
+#if _LIBCPP_STD_VER > 17
+template <class _Cont>
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr auto ssize(const _Cont& __c)
+->                   common_type_t<ptrdiff_t, make_signed_t<decltype(__c.size())>>
+{ return static_cast<common_type_t<ptrdiff_t, make_signed_t<decltype(__c.size())>>>(__c.size()); }
+
+template <class _Tp, ptrdiff_t _Sz>
+inline _LIBCPP_INLINE_VISIBILITY
+constexpr ptrdiff_t ssize(const _Tp (&)[_Sz]) noexcept { return _Sz; }
+#endif
+
 template <class _Cont>
 _LIBCPP_NODISCARD_AFTER_CXX17 inline _LIBCPP_INLINE_VISIBILITY
 constexpr auto empty(const _Cont& __c)
Index: libcxx/test/std/iterators/iterator.container/ssize.pass.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/iterators/iterator.container/ssize.pass.cpp
@@ -0,0 +1,118 @@
+//===----------------------------------------------------------------------===//
+//
+// 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++98, c++03, c++11, c++14, c++17
+
+// <iterator>
+// template <class C> constexpr auto ssize(const C& c)
+//     -> common_type_t<ptrdiff_t, make_signed_t<decltype(c.size())>>;                    // C++20
+// template <class T, ptrdiff_t> constexpr ptrdiff_t ssize(const T (&array)[N]) noexcept; // C++20
+
+#include <iterator>
+#include <cassert>
+#include <vector>
+#include <array>
+#include <list>
+#include <initializer_list>
+#include <string_view>
+
+#include "test_macros.h"
+
+
+struct short_container {
+    uint16_t size() const { return 60000; }
+    };
+
+template<typename C>
+void test_const_container(const C& c)
+{
+//  Can't say noexcept here because the container might not be
+    static_assert( std::is_signed_v<decltype(std::ssize(c))>, "");
+    assert ( std::ssize(c)   == static_cast<decltype(std::ssize(c))>(c.size()));
+}
+
+template<typename T>
+void test_const_container(const std::initializer_list<T>& c)
+{
+//  ASSERT_NOEXCEPT(std::size(c));
+    static_assert( std::is_signed_v<decltype(std::ssize(c))>, "");
+//  For some reason, there isn't a std::size() for initializer lists, so it uses the generic one
+    assert ( std::ssize(c)   == static_cast<decltype(std::ssize(c))>(c.size()));
+}
+
+template<typename C>
+void test_container(C& c)
+{
+//  Can't say noexcept here because the container might not be
+    static_assert( std::is_signed_v<decltype(std::ssize(c))>, "");
+    assert ( std::ssize(c)   == static_cast<decltype(std::ssize(c))>(c.size()));
+}
+
+template<typename T>
+void test_container(std::initializer_list<T>& c)
+{
+//  ASSERT_NOEXCEPT(std::size(c));
+//  For some reason, there isn't a std::size() for initializer lists, so it uses the generic one
+    static_assert( std::is_signed_v<decltype(std::ssize(c))>, "");
+    assert ( std::ssize(c)   == static_cast<decltype(std::ssize(c))>(c.size()));
+}
+
+template<typename T, size_t Sz>
+void test_const_array(const T (&array)[Sz])
+{
+    ASSERT_NOEXCEPT(std::ssize(array));
+    assert ( std::ssize(array) == Sz );
+}
+
+int main(int, char**)
+{
+    std::vector<int> v; v.push_back(1);
+    std::list<int>   l; l.push_back(2);
+    std::array<int, 1> a; a[0] = 3;
+    std::initializer_list<int> il = { 4 };
+    test_container ( v );
+    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(v)));
+    test_container ( l );
+    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(l)));
+    test_container ( a );
+    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(a)));
+    test_container ( il );
+    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(il)));
+
+    test_const_container ( v );
+    test_const_container ( l );
+    test_const_container ( a );
+    test_const_container ( il );
+
+    std::string_view sv{"ABC"};
+    test_container ( sv );
+    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(sv)));
+    test_const_container ( sv );
+
+    static constexpr int arrA [] { 1, 2, 3 };
+    ASSERT_SAME_TYPE(ptrdiff_t, decltype(std::ssize(arrA)));
+    static_assert( std::is_signed_v<decltype(std::ssize(arrA))>, "");
+    test_const_array ( arrA );
+
+//  From P1227R2:
+//     Note that the code does not just return the std::make_signed variant of
+//     the container's size() method, because it's conceivable that a container
+//     might choose to represent its size as a uint16_t, supporting up to
+//     65,535 elements, and it would be a disaster for std::ssize() to turn a
+//     size of 60,000 into a size of -5,536.
+
+    short_container sc;
+//  is the return type signed? Is it bit enough to hold 60K?
+//  is the "signed version" of sc.size() too small?
+    static_assert( std::is_signed_v<                      decltype(std::ssize(sc))>, "");
+    static_assert( std::numeric_limits<                   decltype(std::ssize(sc))>::max()  > 60000, "");
+    static_assert( std::numeric_limits<std::make_signed_t<decltype(std:: size(sc))>>::max() < 60000, "");
+    assert (std::ssize(sc) == 60000);
+    
+  return 0;
+}