[libc++] Implement LWG3549: view_interface need not inherit from view_base

Implement LWG3549 by making `view_interface` not inherit from `view_base`. Types
are still views if they have a public and unambiguous derivation from
`view_interface`, so adjust the `enable_view` machinery as such to account for
that.

Differential Revision: https://reviews.llvm.org/D117714

4 files changed, 22 insertions, 4 deletions

diff --git a/libcxx/docs/Status/Cxx2bIssues.csv b/libcxx/docs/Status/Cxx2bIssues.csv
index 726668f3bdb2..a91e3b52a170 100644
--- a/libcxx/docs/Status/Cxx2bIssues.csv
+++ b/libcxx/docs/Status/Cxx2bIssues.csv
@@ -91,7 +91,7 @@
 `3544 <https://wg21.link/LWG3544>`__,"``format-arg-store::args`` is unintentionally not exposition-only","June 2021","","","|format|"
 `3546 <https://wg21.link/LWG3546>`__,"``common_iterator``'s postfix-proxy is not quite right","June 2021","","","|ranges|"
 `3548 <https://wg21.link/LWG3548>`__,"``shared_ptr`` construction from ``unique_ptr`` should move (not copy) the deleter","June 2021","",""
-`3549 <https://wg21.link/LWG3549>`__,"``view_interface`` is overspecified to derive from ``view_base``","June 2021","","","|ranges|"
+`3549 <https://wg21.link/LWG3549>`__,"``view_interface`` is overspecified to derive from ``view_base``","June 2021","|Complete|","14.0","|ranges|"
 `3551 <https://wg21.link/LWG3551>`__,"``borrowed_{iterator,subrange}_t`` are overspecified","June 2021","","","|ranges|"
 `3552 <https://wg21.link/LWG3552>`__,"Parallel specialized memory algorithms should require forward iterators","June 2021","",""
 `3553 <https://wg21.link/LWG3553>`__,"Useless constraint in ``split_view::outer-iterator::value_type::begin()``","June 2021","","","|ranges|"
diff --git a/libcxx/include/__ranges/enable_view.h b/libcxx/include/__ranges/enable_view.h
index a09de11da81e..e1daec046fc0 100644
--- a/libcxx/include/__ranges/enable_view.h
+++ b/libcxx/include/__ranges/enable_view.h
@@ -12,6 +12,7 @@
 
 #include <__config>
 #include <concepts>
+#include <type_traits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -25,8 +26,17 @@ namespace ranges {
 
 struct view_base { };
 
+template<class _Derived>
+  requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
+class view_interface;
+
+template<class _Op, class _Yp>
+  requires is_convertible_v<_Op*, view_interface<_Yp>*>
+void __is_derived_from_view_interface(const _Op*, const view_interface<_Yp>*);
+
 template <class _Tp>
-inline constexpr bool enable_view = derived_from<_Tp, view_base>;
+inline constexpr bool enable_view = derived_from<_Tp, view_base> ||
+  requires { ranges::__is_derived_from_view_interface((_Tp*)nullptr, (_Tp*)nullptr); };
 
 } // end namespace ranges
 
diff --git a/libcxx/include/__ranges/view_interface.h b/libcxx/include/__ranges/view_interface.h
index 8a1f5d8c9251..c5215cbcb8e3 100644
--- a/libcxx/include/__ranges/view_interface.h
+++ b/libcxx/include/__ranges/view_interface.h
@@ -18,7 +18,6 @@
 #include <__ranges/access.h>
 #include <__ranges/concepts.h>
 #include <__ranges/empty.h>
-#include <__ranges/enable_view.h>
 #include <concepts>
 #include <type_traits>
 
@@ -40,7 +39,7 @@ void __implicitly_convert_to(type_identity_t<_Tp>) noexcept;
 
 template<class _Derived>
   requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
-class view_interface : public view_base {
+class view_interface {
   _LIBCPP_HIDE_FROM_ABI
   constexpr _Derived& __derived() noexcept {
     static_assert(sizeof(_Derived) && derived_from<_Derived, view_interface> && view<_Derived>);
diff --git a/libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp b/libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp
index ebc443ebe08d..b3f9b972a0cb 100644
--- a/libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp
+++ b/libcxx/test/std/ranges/range.utility/view.interface/view.interface.pass.cpp
@@ -79,6 +79,11 @@ struct MoveOnlyForwardRange : std::ranges::view_interface<MoveOnlyForwardRange>
 };
 static_assert(std::ranges::view<MoveOnlyForwardRange>);
 
+struct MI : std::ranges::view_interface<InputRange>,
+            std::ranges::view_interface<MoveOnlyForwardRange> {
+};
+static_assert(!std::ranges::view<MI>);
+
 struct EmptyIsTrue : std::ranges::view_interface<EmptyIsTrue> {
   int buff[8] = {0, 1, 2, 3, 4, 5, 6, 7};
   constexpr ForwardIter begin() const { return ForwardIter(const_cast<int*>(buff)); }
@@ -300,6 +305,10 @@ constexpr bool testFrontBack() {
   return true;
 }
 
+struct V1 : std::ranges::view_interface<V1> { };
+struct V2 : std::ranges::view_interface<V2> { V1 base_; };
+static_assert(sizeof(V2) == sizeof(V1));
+
 int main(int, char**) {
   testEmpty();
   static_assert(testEmpty());