[PATCH] libstdc++: Implement P2474R2 changes to views::take/drop

[Patrick Palka via Gcc-patches]

Tested on x86_64-pc-linux-gnu, does this look OK for trunk?

libstdc++-v3/ChangeLog:

        * include/std/ranges (views::__detail::__is_repeat_view): Define
        and later define a partial specialization.
        (views::__detail::__take_of_repeat_view): Declare and later define.
        (views::__detail::__drop_of_repeat_view): Likewise.
        (views::_Take::operator()): Return a repeat_view if the argument
        is a repeat_view as per P2474R2.
        (views::_Drop::operator()): Likewise.
        (repeat_view): Befriend __take/drop_of_repeat_view.
        * testsuite/std/ranges/repeat/1.cc (test04): New test.
---
 libstdc++-v3/include/std/ranges               | 56 +++++++++++++++++++
 libstdc++-v3/testsuite/std/ranges/repeat/1.cc | 33 +++++++++++
 2 files changed, 89 insertions(+)

diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
index b29264931cc..6de08244bd1 100644
--- a/libstdc++-v3/include/std/ranges
+++ b/libstdc++-v3/include/std/ranges
@@ -2257,6 +2257,13 @@ namespace views::__adaptor
       template<typename _Winc, typename _Bound>
        inline constexpr bool __is_iota_view<iota_view<_Winc, _Bound>> = true;
 
+      template<typename _Range>
+       inline constexpr bool __is_repeat_view = false;
+
+      template<typename _Range>
+       constexpr auto
+       __take_of_repeat_view(_Range&&, range_difference_t<_Range>); // defined 
later
+
       template<typename _Range, typename _Dp>
        concept __can_take_view
          = requires { take_view(std::declval<_Range>(), std::declval<_Dp>()); 
};
@@ -2291,6 +2298,8 @@ namespace views::__adaptor
              else
                return iota_view(*__begin, *__end);
            }
+         else if constexpr (__detail::__is_repeat_view<_Tp>)
+           return __detail::__take_of_repeat_view(std::forward<_Range>(__r), 
__n);
          else
            return take_view(std::forward<_Range>(__r), __n);
        }
@@ -2527,6 +2536,10 @@ namespace views::__adaptor
   {
     namespace __detail
     {
+      template<typename _Range>
+       constexpr auto
+       __drop_of_repeat_view(_Range&&, range_difference_t<_Range>); // defined 
later
+
       template<typename _Range, typename _Dp>
        concept __can_drop_view
          = requires { drop_view(std::declval<_Range>(), std::declval<_Dp>()); 
};
@@ -2568,6 +2581,8 @@ namespace views::__adaptor
              else
                return _Tp(__begin, __end);
            }
+         else if constexpr (__detail::__is_repeat_view<_Tp>)
+           return __detail::__drop_of_repeat_view(std::forward<_Range>(__r), 
__n);
          else
            return drop_view(std::forward<_Range>(__r), __n);
        }
@@ -7367,6 +7382,14 @@ namespace views::__adaptor
 
     struct _Iterator;
 
+    template<typename _Range>
+      friend constexpr auto
+      views::__detail::__take_of_repeat_view(_Range&&, 
range_difference_t<_Range>);
+
+    template<typename _Range>
+      friend constexpr auto
+      views::__detail::__drop_of_repeat_view(_Range&&, 
range_difference_t<_Range>);
+
   public:
     repeat_view() requires default_initializable<_Tp> = default;
 
@@ -7540,6 +7563,9 @@ namespace views::__adaptor
   {
     namespace __detail
     {
+      template<typename _Tp, typename _Bound>
+       inline constexpr bool __is_repeat_view<repeat_view<_Tp, _Bound>> = true;
+
       template<typename _Tp>
        concept __can_repeat_view
          = requires { repeat_view(std::declval<_Tp>()); };
@@ -7565,6 +7591,36 @@ namespace views::__adaptor
     };
 
     inline constexpr _Repeat repeat;
+
+    namespace __detail
+    {
+      template<typename _Range>
+       constexpr auto
+       __take_of_repeat_view(_Range&& __r, range_difference_t<_Range> __n)
+       {
+         using _Tp = remove_cvref_t<_Range>;
+         static_assert(__is_repeat_view<_Tp>);
+         if constexpr (sized_range<_Tp>)
+           return views::repeat(*__r._M_value, std::min(ranges::distance(__r), 
__n));
+         else
+           return views::repeat(*__r._M_value, __n);
+       }
+
+      template<typename _Range>
+       constexpr auto
+       __drop_of_repeat_view(_Range&& __r, range_difference_t<_Range> __n)
+       {
+         using _Tp = remove_cvref_t<_Range>;
+         static_assert(__is_repeat_view<_Tp>);
+         if constexpr (sized_range<_Tp>)
+           {
+             auto __sz = ranges::distance(__r);
+             return views::repeat(*__r._M_value, __sz - std::min(__sz, __n));
+           }
+         else
+           return __r;
+       }
+    }
   }
 
   template<input_range _Vp>
diff --git a/libstdc++-v3/testsuite/std/ranges/repeat/1.cc 
b/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
index 3698ed12c14..542c8614f88 100644
--- a/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
+++ b/libstdc++-v3/testsuite/std/ranges/repeat/1.cc
@@ -84,10 +84,43 @@ test03()
   return true;
 }
 
+constexpr bool
+test04()
+{
+  // Verify P2472R2 changes to views::take/drop.
+  auto r = views::repeat(42);
+
+  auto rt = r | views::take(10);
+  static_assert(views::__detail::__is_repeat_view<decltype(rt)>);
+  VERIFY( ranges::equal(rt, views::repeat(42, 10)) );
+
+  auto rd = r | views::drop(10);
+  static_assert(std::same_as<decltype(rd), decltype(r)>);
+
+  auto br = views::repeat(42, 37);
+
+  auto brt = br | views::take(10);
+  static_assert(std::same_as<decltype(brt), decltype(br)>);
+  VERIFY( ranges::equal(brt, views::repeat(42, 10)) );
+
+  auto brt100 = br | views::take(100);
+  VERIFY( ranges::equal(brt100, br) );
+
+  auto brd = br | views::drop(10);
+  static_assert(std::same_as<decltype(brd), decltype(br)>);
+  VERIFY( ranges::equal(brd, views::repeat(42, 27)) );
+
+  auto brd100 = br | views::drop(100);
+  VERIFY( ranges::empty(brd100) );
+
+  return true;
+}
+
 int
 main()
 {
   static_assert(test01());
   static_assert(test02());
   static_assert(test03());
+  static_assert(test04());
 }
-- 
2.38.1.119.g9c32cfb49c