From 8918d4c9d68792ecc648bca06e19b170b2c9e29e Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Thu, 9 Sep 2021 17:47:56 -0700
Subject: [PATCH] Implement LWG-3549

Which (1) makes `enable_view<T>` be `true` for a type `T` which is publicly-and-unambigously derived from a single specialization of `view_interface`, and (2) removes `view_interface`'s inheritance of `view_base`.

Fixes #2178
---
 stl/inc/xutility                              | 17 +++++++++--
 .../P0896R4_ranges_range_machinery/test.cpp   | 29 ++++++++++++++++++-
 2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/stl/inc/xutility b/stl/inc/xutility
index f8e5f13df53..9a299f419dd 100644
--- a/stl/inc/xutility
+++ b/stl/inc/xutility
@@ -2681,8 +2681,21 @@ namespace ranges {
 
     struct view_base {};
 
+    template <template <class...> class _Template, class... _Args>
+    void _Derived_from_specialization_impl(const _Template<_Args...>&);
+
+    template <class _Ty, template <class...> class _Template>
+    concept _Derived_from_specialization_of = requires(const _Ty& _Obj) {
+        _Derived_from_specialization_impl<_Template>(_Obj);
+    };
+
+    template <class _Derived>
+        requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
+    class view_interface;
+
     template <class _Ty>
-    inline constexpr bool enable_view = derived_from<_Ty, view_base>;
+    inline constexpr bool enable_view =
+        derived_from<_Ty, view_base> || _Derived_from_specialization_of<_Ty, view_interface>;
 
 #ifdef __cpp_lib_ranges // TRANSITION, GH-1814
     template <class _Ty>
@@ -3016,7 +3029,7 @@ namespace ranges {
     // clang-format off
     template <class _Derived>
         requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
-    class view_interface : public view_base {
+    class view_interface {
     private:
         _NODISCARD constexpr _Derived& _Cast() noexcept {
             static_assert(derived_from<_Derived, view_interface>,
diff --git a/tests/std/tests/P0896R4_ranges_range_machinery/test.cpp b/tests/std/tests/P0896R4_ranges_range_machinery/test.cpp
index 12c68e5d24d..fd24b9934ba 100644
--- a/tests/std/tests/P0896R4_ranges_range_machinery/test.cpp
+++ b/tests/std/tests/P0896R4_ranges_range_machinery/test.cpp
@@ -1578,6 +1578,7 @@ using immutable_badsized_range    = badsized_range<immutable_sized_range>;
 template <class T>
 constexpr bool ranges::disable_sized_range<badsized_range<T>> = true;
 
+// "strange" in that const-ness affects the iterator type
 struct strange_view {
     strange_view()               = default;
     strange_view(strange_view&&) = default;
@@ -1591,7 +1592,15 @@ struct strange_view {
 };
 
 struct strange_view2 : strange_view, ranges::view_base {};
-struct strange_view3 : strange_view2 {};
+struct strange_view3 : strange_view2 {}; // not truly a view since enable_view<strange_view3> is false (see below)
+struct strange_view4 : strange_view, ranges::view_interface<strange_view4> {};
+struct strange_view5 : strange_view4, ranges::view_interface<strange_view5> {
+    // not truly a view since enable_view<strange_view5> is false due to multiple inheritance from view_interface
+};
+
+// Verify that specializations of view_interface do not inherit from view_base
+STATIC_ASSERT(!std::is_base_of_v<std::ranges::view_base, ranges::view_interface<strange_view4>>);
+STATIC_ASSERT(!std::is_base_of_v<std::ranges::view_base, ranges::view_interface<strange_view5>>);
 
 template <>
 inline constexpr bool ranges::enable_view<strange_view> = true;
@@ -1620,6 +1629,7 @@ namespace exhaustive_size_and_view_test {
 
     using I  = int*;
     using CI = int const*;
+    using D  = std::ptrdiff_t;
     using S  = std::size_t;
     using UC = unsigned char;
 
@@ -1682,6 +1692,23 @@ namespace exhaustive_size_and_view_test {
     STATIC_ASSERT(test<strange_view3&, false, I, S>());
     STATIC_ASSERT(test<strange_view3 const, false, CI, S>());
     STATIC_ASSERT(test<strange_view3 const&, false, CI, S>());
+
+    STATIC_ASSERT(test<strange_view4, true, I, D>());
+    STATIC_ASSERT(test<strange_view4&, false, I, D>());
+    STATIC_ASSERT(test<strange_view4 const, false, CI, D>());
+    STATIC_ASSERT(test<strange_view4 const&, false, CI, D>());
+
+    template <class = void>
+    constexpr bool strict_test_case() {
+        if constexpr (!is_permissive) {
+            STATIC_ASSERT(test<strange_view5, false, I, S>());
+        }
+        return true;
+    }
+    STATIC_ASSERT(strict_test_case());
+    STATIC_ASSERT(test<strange_view5&, false, I, S>());
+    STATIC_ASSERT(test<strange_view5 const, false, CI, S>());
+    STATIC_ASSERT(test<strange_view5 const&, false, CI, S>());
 } // namespace exhaustive_size_and_view_test
 
 // Validate output_range