From 9040f15c1a5589613b5bf15ceba64028800f60c5 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Sun, 31 Mar 2024 00:07:18 +0800
Subject: [PATCH] Implement LWG-3984

---
 stl/inc/ranges                                      |  2 +-
 tests/std/tests/P1206R7_ranges_to_sequence/test.cpp | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/stl/inc/ranges b/stl/inc/ranges
index 37e9db9282..83fe6046bc 100644
--- a/stl/inc/ranges
+++ b/stl/inc/ranges
@@ -10372,7 +10372,7 @@ namespace ranges {
             const auto _Xform = [](auto&& _Elem) _STATIC_CALL_OPERATOR {
                 return _RANGES to<range_value_t<_Container>>(_STD forward<decltype(_Elem)>(_Elem));
             };
-            return _RANGES to<_Container>(views::transform(_Range, _Xform), _STD forward<_Types>(_Args)...);
+            return _RANGES to<_Container>(views::transform(ref_view{_Range}, _Xform), _STD forward<_Types>(_Args)...);
         } else {
             static_assert(_Always_false<_Container>, "the program is ill-formed per N4950 [range.utility.conv.to]/2.3");
         }
diff --git a/tests/std/tests/P1206R7_ranges_to_sequence/test.cpp b/tests/std/tests/P1206R7_ranges_to_sequence/test.cpp
index e3032e3fe1..9ee7894d9f 100644
--- a/tests/std/tests/P1206R7_ranges_to_sequence/test.cpp
+++ b/tests/std/tests/P1206R7_ranges_to_sequence/test.cpp
@@ -57,6 +57,11 @@ struct sequence_instantiator {
     template <template <class...> class C, class T, class... Args>
     using deduce_container = deduce_container_impl<C>::template apply<T, Args...>;
 
+    template <class>
+    static constexpr bool is_basic_string = false;
+    template <class CharT, class Traits, class Alloc>
+    static constexpr bool is_basic_string<std::basic_string<CharT, Traits, Alloc>> = true;
+
     static constexpr auto meow = "meow"sv;
 
     template <template <class...> class C>
@@ -96,6 +101,12 @@ struct sequence_instantiator {
             std::same_as<C<char>> auto c7 = c | ranges::to<C>();
             assert(c7 == c);
         }
+        if constexpr (!is_basic_string<C<char>>) { // LWG-3984 "ranges::to's recursion branch may be ill-formed"
+            auto owning                     = std::views::all(C<C<char>>{c});
+            std::same_as<C<C<int>>> auto c8 = owning | ranges::to<C<C<int>>>();
+            assert(
+                ranges::equal(c8, C<C<char>>{c}, [](const auto& r1, const auto& r2) { return ranges::equal(r1, r2); }));
+        }
 
         return true;
     }