From 9bf42a9f7a686e1d31ff519a6ab875d11a5b5354 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Mon, 25 Mar 2024 00:24:07 +0800
Subject: [PATCH] Implement LWG-4012

---
 stl/inc/ranges                                |  8 +++--
 tests/std/tests/P0896R4_views_common/test.cpp | 31 +++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/stl/inc/ranges b/stl/inc/ranges
index 30d22d9c4f5..2bf24a05128 100644
--- a/stl/inc/ranges
+++ b/stl/inc/ranges
@@ -5285,7 +5285,9 @@ namespace ranges {
         }
 
         _NODISCARD constexpr auto begin() noexcept(
-            noexcept(_RANGES begin(_Base)) && is_nothrow_move_constructible_v<iterator_t<_Vw>>) /* strengthened */ {
+            noexcept(_RANGES begin(_Base)) && is_nothrow_move_constructible_v<iterator_t<_Vw>>) /* strengthened */
+            requires (!_Simple_view<_Vw>)
+        {
             if constexpr (random_access_range<_Vw> && sized_range<_Vw>) {
                 return _RANGES begin(_Base);
             } else {
@@ -5304,7 +5306,9 @@ namespace ranges {
             }
         }
 
-        _NODISCARD constexpr auto end() {
+        _NODISCARD constexpr auto end()
+            requires (!_Simple_view<_Vw>)
+        {
             if constexpr (random_access_range<_Vw> && sized_range<_Vw>) {
                 return _RANGES begin(_Base) + _RANGES distance(_Base);
             } else {
diff --git a/tests/std/tests/P0896R4_views_common/test.cpp b/tests/std/tests/P0896R4_views_common/test.cpp
index 04daf0fd82e..ed32cfa116a 100644
--- a/tests/std/tests/P0896R4_views_common/test.cpp
+++ b/tests/std/tests/P0896R4_views_common/test.cpp
@@ -541,6 +541,34 @@ constexpr bool test_lwg3717() {
     return true;
 }
 
+constexpr bool test_lwg_4012() {
+    struct simple_view_with_difference_on_const : ranges::view_interface<simple_view_with_difference_on_const> {
+        constexpr difference_type_only_iterator<int> begin() noexcept {
+            return {nullptr};
+        }
+        constexpr difference_type_only_sentinel<int> end() noexcept {
+            return {nullptr};
+        }
+        constexpr difference_type_only_iterator<int> begin() const noexcept {
+            return {p_};
+        }
+        constexpr difference_type_only_sentinel<int> end() const noexcept {
+            return {p_};
+        }
+
+        int* p_;
+    };
+
+    int n{};
+    auto cv = views::common(simple_view_with_difference_on_const{{}, &n});
+
+    assert(cv.begin() == as_const(cv).begin());
+    assert(cv.begin() == as_const(cv).end());
+    assert(as_const(cv).begin() == cv.end());
+
+    return true;
+}
+
 int main() {
     // Get full instantiation coverage
     static_assert((test_in<instantiator, const int>(), true));
@@ -551,4 +579,7 @@ int main() {
 
     assert(test_lwg3717<int>());
     assert(test_lwg3717<const int>());
+
+    static_assert(test_lwg_4012());
+    test_lwg_4012();
 }