Re: [ PATCH ] C++20 <span>

[Jonathan Wakely <jwakely@redhat.com>]

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On 05/09/19 12:27 +0100, Jonathan Wakely wrote:
>On 04/09/19 18:47 -0400, JeanHeyd Meneide wrote:
>>    Thank you for the thorough review!
>>
>>On Tue, Sep 3, 2019 at 9:31 AM Jonathan Wakely <jwakely@redhat.com> wrote:
>>>It looks like__adl_begin and __adl_end should be guarded by #ifdef
>>>_GLIBCXX_P1394 because they're not needed otherwise.
>>
>>    I'm absolutely going to need it for the bit data structures (and
>>in tuning up other parts of the library). It will also be important
>>for things that need to behave nicely when working with things needing
>>[range.access] internally in libstdc++,
>>while we wait for the cmcstl2 implementation. I did gate
>>__adl_to_address behind p1394, though, because that is definitely
>>specific to that paper (and its friend, p1474).
>
>OK, then it's fine as is, thanks.
>
>>+/** @file span
>>+ *  This is a Standard C++ Library header.
>>+ */
>>+
>>+//
>>+// P0122 span library
>>+// Contributed by ThePhD
>>+//
>>+
>>+#ifndef _GLIBCXX_SPAN
>>+#define _GLIBCXX_SPAN 1
>>+
>>+#pragma GCC system_header
>>+
>>+#if __cplusplus > 201703L
>>+
>>+#include <cstddef>
>
>This header isn't needed. <bits/c++config.h> defines std::size_t and
>std::ptrdiff_t, and every libstdc++ header should include
><bits/c++config.h> already. Not including <cstddef> is a minor
>optimisation for compile time, because we don't need to open <cstddef>
>and <stddef.h>.
>
>>+    template<typename _Tp, size_t _Num>
>>+      inline constexpr bool
>>+      __is_std_array_v<_GLIBCXX_STD_C::array<_Tp, _Num>> = true;
>>+
>>+#ifdef _GLIBCXX_DEBUG
>>+    template<typename _Tp, size_t _Num>
>>+      inline constexpr bool
>>+      __is_std_array_v<std::__debug::array<_Tp, _Num>> = true;
>>+#endif // debug/array
>>+
>>+    template<size_t _Extent>
>>+      class __extent_storage
>>+      {
>>+      public:
>>+
>>+        constexpr
>>+        __extent_storage() noexcept = default;
>
>Something else I forgot to mention (which I'll add to the coding
>conventions documentation) is that we use TABs for indentation at the
>start of lines. Tabstop should be set to 8, so anything indented by 8
>chars or more should use TABs. Anything indented by fewer than 8 chars
>just uses spaces.
>
>>+
>>+        constexpr
>>+        __extent_storage(size_t) noexcept
>>+        {}
>>+
>>+        static constexpr size_t
>>+        _M_extent() noexcept
>>+        { return _Extent; }
>>+      };
>>+
>>+    template<>
>>+      class __extent_storage<static_cast<size_t>(-1)>
>>+      {
>>+      public:
>>+
>>+        constexpr
>>+        __extent_storage() noexcept : _M_extent_value(0){};
>>+
>>+        constexpr
>>+        __extent_storage(size_t __extent) noexcept
>>+        : _M_extent_value(__extent)
>>+        {}
>>+
>>+        constexpr size_t
>>+        _M_extent() const noexcept
>>+        { return this->_M_extent_value; }
>>+
>>+      private:
>>+        size_t _M_extent_value;
>>+      };
>>+
>>+  } // namespace __detail
>>+
>>+  template<typename _Type, size_t _Extent = dynamic_extent>
>>+    class span : private __detail::__extent_storage<_Extent>
>>+    {
>>+    public:
>>+      // member types
>>+      using value_type             = remove_cv_t<_Type>;
>>+      using element_type           = _Type;
>>+      using index_type             = size_t;
>>+      using reference              = element_type&;
>>+      using const_reference        = const element_type&;
>>+      using pointer                = _Type*;
>>+      using const_pointer          = const _Type*;
>>+      using iterator               = __gnu_cxx::__normal_iterator<pointer, span>;
>>+      using const_iterator         = __gnu_cxx::__normal_iterator<const_pointer, span>;
>>+      using reverse_iterator       = ::std::reverse_iterator<iterator>;
>>+      using const_reverse_iterator = ::std::reverse_iterator<const_iterator>;
>>+      using difference_type        = ptrdiff_t;
>>+      // Official wording has no size_type -- why??
>>+      // using size_type = size_t;
>>+
>>+      // member constants
>>+      static inline constexpr size_t extent = _Extent;
>>+
>>+    private:
>>+      using __base_t = __detail::__extent_storage<extent>;
>>+
>>+    public:
>>+      // constructors
>>+      template <typename _Dummy = _Type, enable_if_t<is_same_v<_Dummy, _Type> && (_Extent == dynamic_extent || _Extent == 0)>* = nullptr>
>
>This line is too long, it needs to be split before 80 columns.
>
>>+        constexpr span() noexcept : __base_t(0), _M_ptr(nullptr)
>>+        {}
>>+
>>+      constexpr
>>+      span(const span&) noexcept = default;
>>+
>>+      template<size_t _ArrayExtent,
>>+        enable_if_t<
>>+          (_Extent == dynamic_extent || _ArrayExtent == _Extent) &&
>
>N.B. line breaks should come before logical operators like && e.g.
>
>     foo
>     && bar
>
>Not:
>
>     foo &&
>     bar
>
>The rationale for this is to make it easy to see the operator. You can
>look at the start of  the line (at a predictable, easily findable
>position) instead of having to look at the right hand side to see if
>it's a && or || operator.
>
>
>>+      template<typename _Container,
>>+        enable_if_t<
>>+          (_Extent == dynamic_extent) &&
>>+          !is_same_v<remove_cvref_t<_Container>, span> &&
>>+          !__detail::__is_std_array_v<remove_cvref_t<_Container>> &&
>>+          !is_array_v<remove_cvref_t<_Container>> &&
>
>Here's a good example where the && operators are all at different
>columns, but if you put line breaks before the operator it looks like:
>
>         (_Extent == dynamic_extent)
>         && !is_same_v<remove_cvref_t<_Container>, span>
>         && !__detail::__is_std_array_v<remove_cvref_t<_Container>>
>         && !is_array_v<remove_cvref_t<_Container>>
>         && ...
>
>This is easier to see that it's a series of AND conditions with a
>quick glance.
>
>As discussed on IRC the new XFAIL tests need some adjustment. The
>attached patch changes the whitespace as discussed above, and fixes
>the tests.
>
>Our convention for XFAIL tests is to call them foo_neg.cc (not
>fail.foo.cc like libc++ uses).
>
>The attached patch is what I've tested and committed to trunk - thanks
>very much!
>
>I'll make some additional fixes (e.g. LWG 3103) and update the docs
>later today.

Here's the follow-up patch.

Tested x86_64-linux, committed to trunk.



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commit 8b9507a5e5a4f98cace57780d638f24dccef5756
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 5 13:26:14 2019 +0100

    Improve precondition checks for std::span
    
            * doc/xml/manual/status_cxx2020.xml: Update status for P0122R7 and
            P1024R3. Remove entry for P0920R2.
            * include/std/span  (__cpp_lib_span): Change value.
            (__extent_storage, __extent_storage<dynamic_extent>): Remove default
            constructor.
            (span): Replace __extent_storage base class with data member.
            (span::_S_subspan_extent): New function.
            (span::empty()): Add nodiscard attribute.
            (span::front, span::back, span::operator[]): Check preconditions.
            (span::first, span::last, span::subspan): Add noexcept. Improve
            precondition checks (LWG 3103).
            (get): Remove redundant condition from static_assert.
            (tuple_element<I, span<T, E>>): Fix static_assert message and simplify.
            (as_writable_bytes): Add inline specifier.
            * include/std/version (__cpp_lib_span): Change value.
            * testsuite/23_containers/span/back_neg.cc: Remove stray semi-colon.
            * testsuite/23_containers/span/front_neg.cc: Likewise.
            * testsuite/23_containers/span/index_op_neg.cc: Likewise.
            * testsuite/23_containers/span/last_neg.cc: Improve test.
            * testsuite/23_containers/span/subspan_neg.cc: Likewise.
            * testsuite/23_containers/span/1.cc: New test.
            * testsuite/23_containers/span/2.cc: New test.
            * testsuite/23_containers/span/back_assert_neg.cc: New test.
            * testsuite/23_containers/span/first_2_assert_neg.cc: New test.
            * testsuite/23_containers/span/first_assert_neg.cc: New test.
            * testsuite/23_containers/span/first_neg.cc: New test.
            * testsuite/23_containers/span/front_assert_neg.cc: New test.
            * testsuite/23_containers/span/index_op_assert_neg.cc: New test.
            * testsuite/23_containers/span/last_2_assert_neg.cc: New test.
            * testsuite/23_containers/span/last_assert_neg.cc: New test.
            * testsuite/23_containers/span/subspan_2_assert_neg.cc: New test.
            * testsuite/23_containers/span/subspan_3_assert_neg.cc: New test.
            * testsuite/23_containers/span/subspan_4_assert_neg.cc: New test.
            * testsuite/23_containers/span/subspan_5_assert_neg.cc: New test.
            * testsuite/23_containers/span/subspan_6_assert_neg.cc: New test.
            * testsuite/23_containers/span/subspan_assert_neg.cc: New test.

diff --git a/libstdc++-v3/doc/xml/manual/status_cxx2020.xml b/libstdc++-v3/doc/xml/manual/status_cxx2020.xml
index 3a5de38ad22..ffd73aefc96 100644
--- a/libstdc++-v3/doc/xml/manual/status_cxx2020.xml
+++ b/libstdc++-v3/doc/xml/manual/status_cxx2020.xml
@@ -237,15 +237,14 @@ Feature-testing recommendations for C++</link>.
     </row>
 
     <row>
-      <?dbhtml bgcolor="#C8B0B0" ?>
       <entry>  <code>&lt;span&gt;</code> </entry>
       <entry>
         <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0122r7.pdf">
 	P0122R7
 	</link>
       </entry>
-      <entry align="center"> </entry>
-      <entry />
+      <entry align="center"> 10.1 </entry>
+      <entry> <code>__cpp_lib_span &gt;= 201803L</code> </entry>
     </row>
 
     <row>
@@ -1111,7 +1110,6 @@ Feature-testing recommendations for C++</link>.
     </row>
 
     <row>
-      <?dbhtml bgcolor="#C8B0B0" ?>
       <entry>
         Usability Enhancements for <classname>std::span</classname>
       </entry>
@@ -1120,20 +1118,8 @@ Feature-testing recommendations for C++</link>.
         P1024R3
 	</link>
       </entry>
-      <entry align="center"> </entry>
-      <entry />
-    </row>
-
-    <row>
-      <?dbhtml bgcolor="#C8B0B0" ?>
-      <entry> Precalculated hash values in lookup </entry>
-      <entry>
-        <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0920r2.html">
-        P0920R2
-	</link>
-      </entry>
-      <entry align="center"> </entry>
-      <entry />
+      <entry align="center"> 10.1 </entry>
+      <entry> <code>__cpp_lib_span &gt;= 201902L</code> </entry>
     </row>
 
     <row>
diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index 4ce2f31a131..95d778b104b 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -49,9 +49,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
-// FIXME: they forgot this feature test macro
-// get on someone's back about it in Belfast!!!
-#define __cpp_lib_span 201911
+#define __cpp_lib_span 201902L
 
   inline constexpr size_t dynamic_extent = static_cast<size_t>(-1);
 
@@ -78,9 +76,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       class __extent_storage
       {
       public:
-	constexpr
-	__extent_storage() noexcept = default;
-
 	constexpr
 	__extent_storage(size_t) noexcept
 	{ }
@@ -91,13 +86,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       };
 
     template<>
-      class __extent_storage<static_cast<size_t>(-1)>
+      class __extent_storage<dynamic_extent>
       {
       public:
-	constexpr
-	__extent_storage() noexcept : _M_extent_value(0)
-	{ };
-
 	constexpr
 	__extent_storage(size_t __extent) noexcept
 	: _M_extent_value(__extent)
@@ -114,8 +105,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   } // namespace __detail
 
   template<typename _Type, size_t _Extent = dynamic_extent>
-    class span : private __detail::__extent_storage<_Extent>
+    class span
     {
+      template<size_t _Offset, size_t _Count>
+	static constexpr size_t
+	_S_subspan_extent()
+	{
+	  if constexpr (_Count != dynamic_extent)
+	    return _Count;
+	  else if constexpr (extent != dynamic_extent)
+	    return _Extent - _Offset;
+	  else
+	    return dynamic_extent;
+	}
+
     public:
       // member types
       using value_type             = remove_cv_t<_Type>;
@@ -138,17 +141,13 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       // member constants
       static inline constexpr size_t extent = _Extent;
 
-    private:
-      using __base_t = __detail::__extent_storage<extent>;
-
-    public:
       // constructors
 
       template <typename _Dummy = _Type,
 		enable_if_t<is_same_v<_Dummy, _Type>
 		  && (_Extent == dynamic_extent || _Extent == 0)>* = nullptr>
 	constexpr
-	span() noexcept : __base_t(0), _M_ptr(nullptr)
+	span() noexcept : _M_extent(0), _M_ptr(nullptr)
 	{ }
 
       constexpr
@@ -234,7 +233,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	constexpr
 	span(_ContiguousIterator __first, index_type __count)
 	noexcept(noexcept(::std::__adl_to_address(__first)))
-	: __base_t(__count), _M_ptr(::std::__adl_to_address(__first))
+	: _M_extent(__count), _M_ptr(::std::__adl_to_address(__first))
 	{ __glibcxx_assert(_Extent == dynamic_extent || __count == _Extent); }
 
 #else
@@ -276,7 +275,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       constexpr
       span(pointer __first, index_type __count) noexcept
-      : __base_t(__count), _M_ptr(static_cast<pointer>(__first))
+      : _M_extent(__count), _M_ptr(static_cast<pointer>(__first))
       { __glibcxx_assert(_Extent == dynamic_extent || __count == _Extent); }
 
       constexpr
@@ -290,41 +289,52 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr span&
       operator=(const span&) noexcept = default;
 
-      // observers: element access
+      // observers
+
+      constexpr index_type
+      size() const noexcept
+      { return this->_M_extent._M_extent(); }
+
+      constexpr index_type
+      size_bytes() const noexcept
+      { return this->_M_extent._M_extent() * sizeof(element_type); }
+
+      [[nodiscard]] constexpr bool
+      empty() const noexcept
+      { return size() == 0; }
+
+      // element access
 
       constexpr reference
       front() const noexcept
-      { return *this->begin(); }
+      {
+	static_assert(extent != 0);
+	__glibcxx_assert(!empty());
+	return *this->_M_ptr;
+      }
 
       constexpr reference
       back() const noexcept
       {
-	iterator __it = this->end();
-	--__it;
-	return *__it;
+	static_assert(extent != 0);
+	__glibcxx_assert(!empty());
+	return *(this->_M_ptr + (size() - 1));
       }
 
       constexpr reference
       operator[](index_type __idx) const noexcept
-      { return *(this->_M_ptr + __idx); }
+      {
+	static_assert(extent != 0);
+	__glibcxx_assert(__idx < size());
+	return *(this->_M_ptr + __idx);
+      }
 
       constexpr pointer
       data() const noexcept
       { return this->_M_ptr; }
 
-      constexpr index_type
-      size() const noexcept
-      { return this->__base_t::_M_extent(); }
+      // iterator support
 
-      constexpr index_type
-      size_bytes() const noexcept
-      { return this->__base_t::_M_extent() * sizeof(element_type); }
-
-      constexpr bool
-      empty() const noexcept
-      { return size() == 0; }
-
-      // observers: iterator support
       constexpr iterator
       begin() const noexcept
       { return iterator(this->_M_ptr); }
@@ -357,70 +367,85 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       crend() const noexcept
       { return const_reverse_iterator(this->cbegin()); }
 
-      // observers: subranges
+      // subviews
+
       template<size_t _Count>
 	constexpr span<element_type, _Count>
-	first() const
+	first() const noexcept
 	{
-	  __glibcxx_assert(_Count < size());
+	  if constexpr (_Extent == dynamic_extent)
+	    __glibcxx_assert(_Count <= size());
+	  else
+	    static_assert(_Count <= extent);
 	  return { this->data(), _Count };
 	}
 
       constexpr span<element_type, dynamic_extent>
-      first(index_type __count) const
+      first(index_type __count) const noexcept
       {
-	__glibcxx_assert(__count < size());
+	__glibcxx_assert(__count <= size());
 	return { this->data(), __count };
       }
 
       template<size_t _Count>
 	constexpr span<element_type, _Count>
-	last() const
+	last() const noexcept
 	{
-	  static_assert(_Count == dynamic_extent ||
-			  _Extent == dynamic_extent || _Count <= _Extent,
-	    "Count or Extent are dynamic, "
-	    "or the Count is less than the static extent");
-	  __glibcxx_assert(_Count <= size());
+	  if constexpr (_Extent == dynamic_extent)
+	    __glibcxx_assert(_Count <= size());
+	  else
+	    static_assert(_Count <= extent);
 	  return { this->data() + (this->size() - _Count), _Count };
 	}
 
       constexpr span<element_type, dynamic_extent>
-      last(index_type __count) const
+      last(index_type __count) const noexcept
       {
-	__glibcxx_assert(__count < size());
-	index_type __offset = (this->size() - __count);
-	return { this->data() + __offset, __count };
+	__glibcxx_assert(__count <= size());
+	return { this->data() + (this->size() - __count), __count };
       }
 
       template<size_t _Offset, size_t _Count = dynamic_extent>
 	constexpr auto
-	subspan() const
+	subspan() const noexcept
+	-> span<element_type, _S_subspan_extent<_Offset, _Count>()>
 	{
-	  static_assert(_Count == dynamic_extent ||
-			  _Extent == dynamic_extent ||
-			  (_Offset + _Count) <= _Extent,
-	    "Count or Extent are dynamic, "
-	    "or the Count + Offset is less than the static extent");
-	  constexpr size_t __span_extent =
-	    (_Count != dynamic_extent
-		? _Count
-		: (_Extent != dynamic_extent ? _Extent - _Offset
-						    : dynamic_extent));
-	  using __span_t = span<element_type, __span_extent>;
-	  if constexpr(_Count != dynamic_extent)
+	  if constexpr (_Extent == dynamic_extent)
+	    __glibcxx_assert(_Offset <= size());
+	  else
+	    static_assert(_Offset <= extent);
+
+	  if constexpr (_Count == dynamic_extent)
+	    return { this->data() + _Offset, this->size() - _Offset };
+	  else
 	    {
-	      __glibcxx_assert((_Offset + _Count) < size());
+	      if constexpr (_Extent == dynamic_extent)
+		{
+		  __glibcxx_assert(_Count <= size());
+		  __glibcxx_assert(_Count <= (size() - _Offset));
+		}
+	      else
+		{
+		  static_assert(_Count <= extent);
+		  static_assert(_Count <= (extent - _Offset));
+		}
+	      return { this->data() + _Offset, _Count };
 	    }
-	  return __span_t(this->data() + _Offset,
-	    (_Count == dynamic_extent ? this->size() - _Offset : _Count));
 	}
 
       constexpr span<element_type, dynamic_extent>
       subspan(index_type __offset, index_type __count = dynamic_extent) const
+      noexcept
       {
-	return {this->data() + __offset,
-	  __count == dynamic_extent ? this->size() - __offset : __count};
+	__glibcxx_assert(__offset <= size());
+	if (__count == dynamic_extent)
+	  __count = this->size() - __offset;
+	else
+	  {
+	    __glibcxx_assert(__count <= size());
+	    __glibcxx_assert(__offset + __count <= size());
+	  }
+	return {this->data() + __offset, __count};
       }
 
       // observers: range helpers
@@ -434,51 +459,54 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       { return __sp.end(); }
 
     private:
+      [[no_unique_address]] __detail::__extent_storage<extent> _M_extent;
       pointer _M_ptr;
     };
 
   // deduction guides
   template<typename _Type, size_t _ArrayExtent>
-    span(_Type(&)[_ArrayExtent])->span<_Type, _ArrayExtent>;
+    span(_Type(&)[_ArrayExtent]) -> span<_Type, _ArrayExtent>;
 
   template<typename _Type, size_t _ArrayExtent>
-    span(array<_Type, _ArrayExtent>&)->span<_Type, _ArrayExtent>;
+    span(array<_Type, _ArrayExtent>&) -> span<_Type, _ArrayExtent>;
 
   template<typename _Type, size_t _ArrayExtent>
     span(const array<_Type, _ArrayExtent>&)
-      ->span<const _Type, _ArrayExtent>;
+      -> span<const _Type, _ArrayExtent>;
 
 #if defined(_GLIBCXX_P1394) && _GLIBCXX_P1394
 
   template<typename _ContiguousIterator, typename _Sentinel>
     span(_ContiguousIterator, _Sentinel)
-      ->span<remove_reference_t<
+      -> span<remove_reference_t<
 	typename iterator_traits<_ContiguousIterator>::reference>>;
 
   template<typename _Range>
     span(_Range &&)
-      ->span<remove_reference_t<typename iterator_traits<decltype(
-	::std::__adl_begin(::std::declval<_Range&>()))>::reference>>;
+      -> span<remove_reference_t<typename iterator_traits<
+	  decltype(std::__adl_begin(::std::declval<_Range&>()))>::reference>>;
 
 #else
 
   template<typename _Container>
-    span(_Container&)->span<typename _Container::value_type>;
+    span(_Container&) -> span<typename _Container::value_type>;
 
   template<typename _Container>
-    span(const _Container&)->span<const typename _Container::value_type>;
+    span(const _Container&) -> span<const typename _Container::value_type>;
 
 #endif // P1394
 
   template<typename _Type, size_t _Extent>
+    inline
     span<const byte, _Extent == dynamic_extent
 	? dynamic_extent : _Extent * sizeof(_Type)>
-    inline as_bytes(span<_Type, _Extent> __sp) noexcept
+    as_bytes(span<_Type, _Extent> __sp) noexcept
     {
       return {reinterpret_cast<const byte*>(__sp.data()), __sp.size_bytes()};
     }
 
   template<typename _Type, size_t _Extent>
+    inline
     span<byte, _Extent == dynamic_extent
        ? dynamic_extent : _Extent * sizeof(_Type)>
     as_writable_bytes(span<_Type, _Extent> __sp) noexcept
@@ -491,9 +519,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr _Type&
     get(span<_Type, _Extent> __sp) noexcept
     {
-      static_assert(_Extent != dynamic_extent
-	&& (_Extent > 0) && (_Index < _Extent),
-	"std::get can only be used with a span of non-dynamic (fixed) extent");
+      static_assert(_Extent != dynamic_extent && _Index < _Extent,
+	"get<I> can only be used with a span of non-dynamic (fixed) extent");
       return __sp[_Index];
     }
 
@@ -508,10 +535,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   template<size_t _Index, typename _Type, size_t _Extent>
     struct tuple_element<_Index, span<_Type, _Extent>>
     {
-      static_assert(_Extent != dynamic_extent, "tuple_size can only "
+      static_assert(_Extent != dynamic_extent, "tuple_element can only "
 	"be used with a span of non-dynamic (fixed) extent");
       static_assert(_Index < _Extent, "Index is less than Extent");
-      using type = typename span<_Type, _Extent>::element_type;
+      using type = _Type;
     };
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index 4341c9f0c1b..7625d397a0d 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -166,10 +166,8 @@
 #endif
 #define __cpp_lib_list_remove_return_type 201806L
 #define __cpp_lib_math_constants 201907L
+#define __cpp_lib_span 201902L
 #define __cpp_lib_to_array 201907L
-// FIXME: they forgot this feature test macro
-// get on someone's back about it in Belfast!!!
-#define __cpp_lib_span 201911
 #endif // C++2a
 #endif // C++17
 #endif // C++14
diff --git a/libstdc++-v3/testsuite/23_containers/span/1.cc b/libstdc++-v3/testsuite/23_containers/span/1.cc
new file mode 100644
index 00000000000..c31d8d400ce
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/1.cc
@@ -0,0 +1,27 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <span>
+
+#ifndef __cpp_lib_span
+# error "Feature-test macro for span missing in <span>"
+#elif __cpp_lib_span != 201902L
+# error "Feature-test macro for span has wrong value in <span>"
+#endif
diff --git a/libstdc++-v3/testsuite/23_containers/span/2.cc b/libstdc++-v3/testsuite/23_containers/span/2.cc
new file mode 100644
index 00000000000..1870ba16973
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/2.cc
@@ -0,0 +1,27 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <version>
+
+#ifndef __cpp_lib_span
+# error "Feature-test macro for span missing in <version>"
+#elif __cpp_lib_span != 201902L
+# error "Feature-test macro for span has wrong value in <version>"
+#endif
diff --git a/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc
new file mode 100644
index 00000000000..0b8fc672a20
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/back_assert_neg.cc
@@ -0,0 +1,28 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  std::span<int, std::dynamic_extent> s;
+  s.back();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/back_neg.cc b/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
new file mode 100644
index 00000000000..b1bb10f40d1
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/back_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <span>
+
+void
+test01()
+{
+  std::span<int, 0> s;
+  s.back(); // { dg-error "here" }
+}
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc
new file mode 100644
index 00000000000..fd9c9204d1e
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/first_2_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.first(5);
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc
new file mode 100644
index 00000000000..94f1f03a64a
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/first_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.first<5>();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/first_neg.cc b/libstdc++-v3/testsuite/23_containers/span/first_neg.cc
new file mode 100644
index 00000000000..42729347d33
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/first_neg.cc
@@ -0,0 +1,30 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <span>
+
+void
+test01()
+{
+  int a[4];
+  std::span<int, 4> s(a);
+  s.first<5>(); // { dg-error "here" }
+}
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc
new file mode 100644
index 00000000000..516bf2a2dcb
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/front_assert_neg.cc
@@ -0,0 +1,28 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  std::span<int, std::dynamic_extent> s;
+  s.front();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/front_neg.cc b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
new file mode 100644
index 00000000000..4c2b6e39c70
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/front_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <span>
+
+void
+test01()
+{
+  std::span<int, 0> s;
+  s.front(); // { dg-error "here" }
+}
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc
new file mode 100644
index 00000000000..4109c4cdc7a
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/index_op_assert_neg.cc
@@ -0,0 +1,28 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  std::span<int, std::dynamic_extent> s;
+  s[99];
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
new file mode 100644
index 00000000000..5bd1e11dda0
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/index_op_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+#include <span>
+
+void
+test01()
+{
+  std::span<int, 0> s;
+  s[99]; // { dg-error "here" }
+}
+// { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc
new file mode 100644
index 00000000000..1cc060f4735
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/last_2_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.last(5);
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc
new file mode 100644
index 00000000000..31b6cc0816f
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/last_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.last<5>();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/last_neg.cc b/libstdc++-v3/testsuite/23_containers/span/last_neg.cc
index 8f0145f7012..ac77dbe4438 100644
--- a/libstdc++-v3/testsuite/23_containers/span/last_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/last_neg.cc
@@ -15,15 +15,16 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-options "-std=c++2a" }
+// { dg-options "-std=gnu++2a" }
 // { dg-do compile { target c++2a } }
 
 #include <span>
 
-int
-main()
+void
+test01()
 {
-  std::span<int, 2> myspan(nullptr, 2);
-  myspan.last<3>(); // { dg-error "here" }
+  int a[2];
+  std::span<int, 2> s(a);
+  s.last<3>(); // { dg-error "here" }
 }
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc
new file mode 100644
index 00000000000..355a71c9d9d
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_2_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.subspan<2, 5>();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc
new file mode 100644
index 00000000000..bdf4abad2c6
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_3_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.subspan<2, 3>();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc
new file mode 100644
index 00000000000..37ebed038fa
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_4_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.subspan(5, 0);
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc
new file mode 100644
index 00000000000..615ea0ba559
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_5_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.subspan(2, 5);
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc
new file mode 100644
index 00000000000..db5f427b35f
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_6_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.subspan(2, 3);
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc
new file mode 100644
index 00000000000..d120d436c23
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_assert_neg.cc
@@ -0,0 +1,29 @@
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { xfail c++2a } }
+
+#define _GLIBCXX_ASSERTIONS
+#include <span>
+
+int main()
+{
+  int a[4];
+  std::span<int, std::dynamic_extent> s(a);
+  s.subspan<5, 0>();
+}
diff --git a/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc b/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc
index 5ecbff937cc..70fcda6e91a 100644
--- a/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc
+++ b/libstdc++-v3/testsuite/23_containers/span/subspan_neg.cc
@@ -15,15 +15,33 @@
 // with this library; see the file COPYING3.  If not see
 // <http://www.gnu.org/licenses/>.
 
-// { dg-options "-std=c++2a" }
+// { dg-options "-std=gnu++2a" }
 // { dg-do compile { target c++2a } }
 
 #include <span>
 
-int
-main()
+void
+test01()
 {
-  std::span<int, 2> myspan(nullptr, 2);
-  myspan.subspan<3, 1>(); // { dg-error "here" }
+  int a[4];
+  std::span<int, 4> s(a);
+  s.subspan<5, 0>(); // { dg-error "here" }
 }
+
+void
+test02()
+{
+  int a[4];
+  std::span<int, 4> s(a);
+  s.subspan<3, 5>(); // { dg-error "here" }
+}
+
+void
+test03()
+{
+  int a[4];
+  std::span<int, 4> s(a);
+  s.subspan<3, 2>(); // { dg-error "here" }
+}
+
 // { dg-error "static assertion failed" "" { target *-*-* } 0 }