libstdc++: Implement P1739R4 changes to views::take/drop/counted

This implements P1739R4 along with the resolution for LWG 3407 which
corrects the paper's wording.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_util.h (views::_Drop): Forward declare.
	(subrange): Befriend views::_Drop.
	(subrange::_S_store_size): Declare constexpr instead of just
	const, remove obsolete comment.
	* include/std/ranges (views::__detail::__is_empty_view): Define.
	(views::__detail::__is_basic_string_view): Likewise.
	(views::__detail::__is_subrange): Likewise.
	(views::__detail::__is_iota_view): Likewise.
	(views::__detail::__can_take_view): Rename template parm _Tp to _Dp.
	(views::_Take): Rename template parm _Tp to _Dp, make it non-deducible
	and fix it to range_difference_t<_Range>.  Implement P1739R4 and
	LWG 3407 changes.
	(views::__detail::__can_drop_view): Rename template parm _Tp to _Dp.
	(views::_Drop): As with views::_Take.
	(views::_Counted): Implement P1739R4 changes.
	* include/std/span (__detail::__is_std_span): Rename to ...
	(__detail::__is_span): ... this and turn it into a variable
	template.
	(__detail::__is_std_array): Turn it into a variable template.
	(span::span): Adjust uses of __is_std_span and __is_std_array
	accordingly.
	* testsuite/std/ranges/adaptors/p1739.cc: New test.

4 files changed, 193 insertions, 21 deletions

diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index a52b8fc..060e665 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -216,6 +216,8 @@ namespace ranges
 
   } // namespace __detail
 
+  namespace views { struct _Drop; } // defined in <ranges>
+
   enum class subrange_kind : bool { unsized, sized };
 
   /// The ranges::subrange class template
@@ -226,10 +228,11 @@ namespace ranges
     class subrange : public view_interface<subrange<_It, _Sent, _Kind>>
     {
     private:
-      // XXX: gcc complains when using constexpr here
-      static const bool _S_store_size
+      static constexpr bool _S_store_size
 	= _Kind == subrange_kind::sized && !sized_sentinel_for<_Sent, _It>;
 
+      friend struct views::_Drop; // Needs to inspect _S_store_size.
+
       _It _M_begin = _It();
       [[no_unique_address]] _Sent _M_end = _Sent();
 
diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index 30ba060..d6bb78c 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -42,6 +42,7 @@
 #include <initializer_list>
 #include <iterator>
 #include <optional>
+#include <span>
 #include <tuple>
 #include <bits/ranges_util.h>
 #include <bits/refwrap.h>
@@ -2125,19 +2126,67 @@ namespace views::__adaptor
   {
     namespace __detail
     {
-      template<typename _Range, typename _Tp>
+      template<typename _Range>
+	inline constexpr bool __is_empty_view = false;
+
+      template<typename _Tp>
+	inline constexpr bool __is_empty_view<empty_view<_Tp>> = true;
+
+      template<typename _Range>
+	inline constexpr bool __is_basic_string_view = false;
+
+      template<typename _CharT, typename _Traits>
+	inline constexpr bool __is_basic_string_view<basic_string_view<_CharT, _Traits>>
+	  = true;
+
+      template<typename _Range>
+	inline constexpr bool __is_subrange = false;
+
+      template<typename _Iter, typename _Sent, subrange_kind _Kind>
+	inline constexpr bool __is_subrange<subrange<_Iter, _Sent, _Kind>> = true;
+
+      template<typename _Range>
+	inline constexpr bool __is_iota_view = false;
+
+      template<typename _Winc, typename _Bound>
+	inline constexpr bool __is_iota_view<iota_view<_Winc, _Bound>> = true;
+
+      template<typename _Range, typename _Dp>
 	concept __can_take_view
-	  = requires { take_view(std::declval<_Range>(), std::declval<_Tp>()); };
+	  = requires { take_view(std::declval<_Range>(), std::declval<_Dp>()); };
     } // namespace __detail
 
     struct _Take : __adaptor::_RangeAdaptor<_Take>
     {
-      template<viewable_range _Range, typename _Tp>
-	requires __detail::__can_take_view<_Range, _Tp>
+      template<viewable_range _Range, typename _Dp = range_difference_t<_Range>>
+	requires __detail::__can_take_view<_Range, _Dp>
 	constexpr auto
-	operator() [[nodiscard]] (_Range&& __r, _Tp&& __n) const
+	operator() [[nodiscard]] (_Range&& __r, type_identity_t<_Dp> __n) const
 	{
-	  return take_view(std::forward<_Range>(__r), std::forward<_Tp>(__n));
+	  using _Tp = remove_cvref_t<_Range>;
+	  if constexpr (__detail::__is_empty_view<_Tp>)
+	    return _Tp();
+	  else if constexpr (random_access_range<_Tp>
+			     && sized_range<_Tp>
+			     && (std::__detail::__is_span<_Tp>
+				 || __detail::__is_basic_string_view<_Tp>
+				 || __detail::__is_subrange<_Tp>
+				 || __detail::__is_iota_view<_Tp>))
+	    {
+	      __n = std::min<_Dp>(ranges::distance(__r), __n);
+	      auto __begin = ranges::begin(__r);
+	      auto __end = __begin + __n;
+	      if constexpr (std::__detail::__is_span<_Tp>)
+		return span<typename _Tp::element_type>(__begin, __end);
+	      else if constexpr (__detail::__is_basic_string_view<_Tp>)
+		return _Tp(__begin, __end);
+	      else if constexpr (__detail::__is_subrange<_Tp>)
+		return subrange<iterator_t<_Tp>>(__begin, __end);
+	      else
+		return iota_view(*__begin, *__end);
+	    }
+	  else
+	    return take_view(std::forward<_Range>(__r), __n);
 	}
 
       using _RangeAdaptor<_Take>::operator();
@@ -2372,19 +2421,49 @@ namespace views::__adaptor
   {
     namespace __detail
     {
-      template<typename _Range, typename _Tp>
+      template<typename _Range, typename _Dp>
 	concept __can_drop_view
-	  = requires { drop_view(std::declval<_Range>(), std::declval<_Tp>()); };
+	  = requires { drop_view(std::declval<_Range>(), std::declval<_Dp>()); };
     } // namespace __detail
 
     struct _Drop : __adaptor::_RangeAdaptor<_Drop>
     {
-      template<viewable_range _Range, typename _Tp>
-	requires __detail::__can_drop_view<_Range, _Tp>
+      template<viewable_range _Range, typename _Dp = range_difference_t<_Range>>
+	requires __detail::__can_drop_view<_Range, _Dp>
 	constexpr auto
-	operator() [[nodiscard]] (_Range&& __r, _Tp&& __n) const
+	operator() [[nodiscard]] (_Range&& __r, type_identity_t<_Dp> __n) const
 	{
-	  return drop_view(std::forward<_Range>(__r), std::forward<_Tp>(__n));
+	  using _Tp = remove_cvref_t<_Range>;
+	  if constexpr (__detail::__is_empty_view<_Tp>)
+	    return _Tp();
+	  else if constexpr (random_access_range<_Tp>
+			     && sized_range<_Tp>
+			     && (std::__detail::__is_span<_Tp>
+				 || __detail::__is_basic_string_view<_Tp>
+				 || __detail::__is_iota_view<_Tp>
+				 || __detail::__is_subrange<_Tp>))
+	    {
+	      __n = std::min<_Dp>(ranges::distance(__r), __n);
+	      auto __begin = ranges::begin(__r) + __n;
+	      auto __end = ranges::end(__r);
+	      if constexpr (std::__detail::__is_span<_Tp>)
+		return span<typename _Tp::element_type>(__begin, __end);
+	      else if constexpr (__detail::__is_subrange<_Tp>)
+		{
+		  if constexpr (_Tp::_S_store_size)
+		    {
+		      using ranges::__detail::__to_unsigned_like;
+		      auto __m = ranges::distance(__r) - __n;
+		      return _Tp(__begin, __end, __to_unsigned_like(__m));
+		    }
+		  else
+		    return _Tp(__begin, __end);
+		}
+	      else
+		return _Tp(__begin, __end);
+	    }
+	  else
+	    return drop_view(std::forward<_Range>(__r), __n);
 	}
 
       using _RangeAdaptor<_Drop>::operator();
@@ -3519,7 +3598,9 @@ namespace views::__adaptor
       constexpr auto
       operator() [[nodiscard]] (_Iter __i, iter_difference_t<_Iter> __n) const
       {
-	if constexpr (random_access_iterator<_Iter>)
+	if constexpr (contiguous_iterator<_Iter>)
+	  return span(std::__to_address(__i), __n);
+	else if constexpr (random_access_iterator<_Iter>)
 	  return subrange(__i, __i + __n);
 	else
 	  return subrange(counted_iterator(std::move(__i), __n),
diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
index af0d24b..61824de 100644
--- a/libstdc++-v3/include/std/span
+++ b/libstdc++-v3/include/std/span
@@ -58,16 +58,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   namespace __detail
   {
     template<typename _Tp>
-      struct __is_std_span : false_type { };
+      inline constexpr bool __is_span = false;
 
     template<typename _Tp, size_t _Num>
-      struct __is_std_span<span<_Tp, _Num>> : true_type { };
+      inline constexpr bool __is_span<span<_Tp, _Num>> = true;
 
     template<typename _Tp>
-      struct __is_std_array : false_type { };
+      inline constexpr bool __is_std_array = false;
 
     template<typename _Tp, size_t _Num>
-      struct __is_std_array<std::array<_Tp, _Num>> : true_type { };
+      inline constexpr bool __is_std_array<std::array<_Tp, _Num>> = true;
 
     template<size_t _Extent>
       class __extent_storage
@@ -203,8 +203,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Range>
 	requires ranges::contiguous_range<_Range> && ranges::sized_range<_Range>
 	  && (ranges::borrowed_range<_Range> || is_const_v<element_type>)
-	  && (!__detail::__is_std_span<remove_cvref_t<_Range>>::value)
-	  && (!__detail::__is_std_array<remove_cvref_t<_Range>>::value)
+	  && (!__detail::__is_span<remove_cvref_t<_Range>>)
+	  && (!__detail::__is_std_array<remove_cvref_t<_Range>>)
 	  && (!is_array_v<remove_cvref_t<_Range>>)
 	  && __is_compatible_ref<ranges::range_reference_t<_Range>>::value
 	constexpr explicit(extent != dynamic_extent)
diff --git a/libstdc++-v3/testsuite/std/ranges/adaptors/p1739.cc b/libstdc++-v3/testsuite/std/ranges/adaptors/p1739.cc
new file mode 100644
index 0000000..3e51aa0
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/ranges/adaptors/p1739.cc
@@ -0,0 +1,88 @@
+// { dg-options "-std=gnu++20" }
+// { dg-do run { target c++20 } }
+
+#include <algorithm>
+#include <ranges>
+#include <testsuite_hooks.h>
+
+// Verify P1739R4 (and LWG 3407) changes to views::take/drop/counted.
+
+namespace ranges = std::ranges;
+namespace views = ranges::views;
+
+void
+test01()
+{
+  ranges::empty_view<int> v;
+  using ty = decltype(v);
+  using ty = decltype(views::take(v, 42));
+  using ty = decltype(views::drop(v, 42));
+}
+
+void
+test02()
+{
+  int x[] = {1,2,3,4,5};
+  std::span r(x, x+5);
+  using ty = decltype(r);
+
+  std::same_as<ty> auto v1 = views::take(r, 2);
+  VERIFY( ranges::equal(v1, (int[]){1,2}) );
+
+  std::same_as<ty> auto v2 = views::drop(r, 3);
+  VERIFY( ranges::equal(v2, (int[]){4,5}) );
+
+  std::same_as<ty> auto v3 = views::counted(r.begin(), 4);
+  VERIFY( ranges::equal(v3, (int[]){1,2,3,4}) );
+}
+
+void
+test03()
+{
+  using namespace std::literals;
+  std::string_view r = "hello world"sv;
+  using ty = decltype(r);
+
+  std::same_as<ty> auto v1 = views::take(r, 5);
+  VERIFY( ranges::equal(v1, "hello"sv) );
+
+  std::same_as<ty> auto v2 = views::drop(r, 6);
+  VERIFY( ranges::equal(v2, "world"sv) );
+}
+
+void
+test04()
+{
+  int x[] = {1,2,3,4,5};
+  ranges::subrange r(x, x+5);
+  using ty = decltype(r);
+
+  std::same_as<ty> auto v1 = views::take(r, 2);
+  VERIFY( ranges::equal(v1, (int[]){1,2}) );
+
+  std::same_as<ty> auto v2 = views::drop(r, 1);
+  VERIFY( ranges::equal(v2, (int[]){2,3,4,5}) );
+}
+
+void
+test05()
+{
+  auto r1 = views::iota(1, 6);
+  using ty1 = decltype(r1);
+
+  std::same_as<ty1> auto v1 = views::take(r1, 2);
+  VERIFY( ranges::equal(v1, (int[]){1,2}) );
+
+  std::same_as<ty1> auto v2 = views::drop(r1, 3);
+  VERIFY( ranges::equal(v2, (int[]){4,5}) );
+}
+
+int
+main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
+  test05();
+}