libstdc++: Implement ranges::iota from P2440R1

libstdc++-v3/ChangeLog:

	* include/bits/ranges_algo.h (out_value_result): Define.
	(iota_result): Define.
	(__iota_fn, iota): Define.
	* testsuite/25_algorithms/iota/1.cc: New test.

libstdc++-v3/include/bits/ranges_algo.h

@@ -3517,6 +3517,54 @@ namespace ranges
   };
 
   inline constexpr __contains_subrange_fn contains_subrange{};
+
+  template<typename _Out, typename _Tp>
+    struct out_value_result
+    {
+      [[no_unique_address]] _Out out;
+      [[no_unique_address]] _Tp value;
+
+      template<typename _Out2, typename _Tp2>
+	requires convertible_to<const _Out&, _Out2>
+	  && convertible_to<const _Tp&, _Tp2>
+	constexpr
+	operator out_value_result<_Out2, _Tp2>() const &
+	{ return {out, value}; }
+
+      template<typename _Out2, typename _Tp2>
+	requires convertible_to<_Out, _Out2>
+	  && convertible_to<_Tp, _Tp2>
+	constexpr
+	operator out_value_result<_Out2, _Tp2>() &&
+	{ return {std::move(out), std::move(value)}; }
+    };
+
+  template<typename _Out, typename _Tp>
+    using iota_result = out_value_result<_Out, _Tp>;
+
+  struct __iota_fn
+  {
+    template<input_or_output_iterator _Out, sentinel_for<_Out> _Sent, weakly_incrementable _Tp>
+      requires indirectly_writable<_Out, const _Tp&>
+      constexpr iota_result<_Out, _Tp>
+      operator()(_Out __first, _Sent __last, _Tp __value) const
+      {
+	while (__first != __last)
+	  {
+	    *__first = static_cast<const _Tp&>(__value);
+	    ++__first;
+	    ++__value;
+	  }
+	return {std::move(__first), std::move(__value)};
+      }
+
+    template<weakly_incrementable _Tp, output_range<const _Tp&> _Range>
+      constexpr iota_result<borrowed_iterator_t<_Range>, _Tp>
+      operator()(_Range&& __r, _Tp __value) const
+      { return (*this)(ranges::begin(__r), ranges::end(__r), std::move(__value)); }
+  };
+
+  inline constexpr __iota_fn iota{};
 #endif // C++23
 } // namespace ranges
 

libstdc++-v3/testsuite/25_algorithms/iota/1.cc

@@ -0,0 +1,29 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do run { target c++23 } }
+
+#include <algorithm>
+#include <testsuite_hooks.h>
+#include <testsuite_iterators.h>
+
+namespace ranges = std::ranges;
+
+void
+test01()
+{
+  int x[3] = {};
+  __gnu_test::test_output_range<int> rx(x);
+  auto r0 = ranges::iota(rx, 0);
+  VERIFY( r0.out.ptr == x+3 );
+  VERIFY( r0.value == 3 );
+  VERIFY( ranges::equal(x, (int[]){0,1,2}) );
+  auto r1 = ranges::iota(x, x+2, 5);
+  VERIFY( r1.out == x+2 );
+  VERIFY( r1.value == 7 );
+  VERIFY( ranges::equal(x, (int[]){5,6,2}) );
+}
+
+int
+main()
+{
+  test01();
+}