Remove the enable_view heuristic (#1455)

This is to more closely harmonize range-v3 with C++20's std::ranges. Refers to cplusplus/nbballot#278

.vscode/settings.json

@@ -90,7 +90,8 @@
         "__string": "cpp",
         "algorithm": "cpp",
         "codecvt": "cpp",
-        "valarray": "cpp"
+        "valarray": "cpp",
+        "version": "cpp"
     },
     "C_Cpp.configurationWarnings": "Disabled"
 }

include/concepts/concepts.hpp

@@ -1167,38 +1167,42 @@ namespace concepts
             convertible_to<T, T>;
 
         template<typename T>
-        CPP_concept_bool copy_constructible =
-            move_constructible<T> &&
+        CPP_concept_fragment(copy_constructible_,
+            requires()(0) &&
             constructible_from<T, T &> &&
             constructible_from<T, T const &> &&
             constructible_from<T, T const> &&
             convertible_to<T &, T> &&
             convertible_to<T const &, T> &&
-            convertible_to<T const, T>;
+            convertible_to<T const, T>
+        );
+        template<typename T>
+        CPP_concept_bool copy_constructible =
+            move_constructible<T> &&
+            CPP_fragment(concepts::copy_constructible_, T);
 
+        template<typename T>
+        CPP_concept_fragment(move_assignable_,
+            requires()(0) &&
+            assignable_from<T &, T>
+        );
         template<typename T>
         CPP_concept_bool movable =
             std::is_object<T>::value &&
             move_constructible<T> &&
-            assignable_from<T &, T> &&
+            CPP_fragment(concepts::move_assignable_, T) &&
             swappable<T>;
 
+        template<typename T>
+        CPP_concept_fragment(copy_assignable_,
+            requires()(0) &&
+            assignable_from<T &, T const &>
+        );
         template<typename T>
         CPP_concept_bool copyable =
             copy_constructible<T> &&
             movable<T> &&
-            assignable_from<T &, T const &>;
-
-#if !CPP_CXX_CONCEPTS
-        template<>
-        CPP_concept_bool copy_constructible<void> = false;
-
-        template<>
-        CPP_concept_bool movable<void> = false;
-
-        template<>
-        CPP_concept_bool copyable<void> = false;
-#endif
+            CPP_fragment(concepts::copy_assignable_, T);
 
         template<typename T>
         CPP_concept_bool semiregular =

include/range/v3/iterator/concepts.hpp

@@ -226,22 +226,23 @@ namespace ranges
         weakly_incrementable<I> &&
         CPP_fragment(ranges::incrementable_, I);
 
+    template<typename I>
+    CPP_concept_fragment(input_or_output_iterator_,
+        requires()(0) &&
+        detail::dereferenceable_<I&>
+    );
+
     template<typename I>
     CPP_concept_bool input_or_output_iterator =
         weakly_incrementable<I> &&
-        detail::dereferenceable_<I&>;
+        CPP_fragment(ranges::input_or_output_iterator_, I);
 
     template<typename S, typename I>
     CPP_concept_bool sentinel_for =
         semiregular<S> &&
         input_or_output_iterator<I> &&
         detail::weakly_equality_comparable_with_<S, I>;
 
-#if !CPP_CXX_CONCEPTS
-    template<>
-    CPP_concept_bool input_or_output_iterator<void> = false;
-#endif
-
     namespace defer
     {
         template<typename S, typename I>

include/range/v3/range/access.hpp

@@ -18,11 +18,6 @@
 #include <initializer_list>
 #include <iterator>
 #include <limits>
-#ifdef __has_include
-#if __has_include(<string_view>)
-#include <string_view>
-#endif
-#endif
 #include <utility>
 
 #include <range/v3/range_fwd.hpp>
@@ -32,6 +27,28 @@
 #include <range/v3/iterator/traits.hpp>
 #include <range/v3/utility/static_const.hpp>
 
+#ifndef RANGES_NO_STD_FORWARD_DECLARATIONS
+// Non-portable forward declarations of standard library components
+RANGES_BEGIN_NAMESPACE_STD
+    RANGES_BEGIN_NAMESPACE_VERSION
+        template<typename ElementType, size_t Extent>
+        class span;
+
+        template<typename CharT, typename Traits>
+        class basic_string_view;
+    RANGES_END_NAMESPACE_VERSION
+RANGES_END_NAMESPACE_STD
+#else
+#ifdef __has_include
+#if __has_include(<span>)
+#include <span>
+#endif
+#if __has_include(<string_view>)
+#include <string_view>
+#endif
+#endif
+#endif
+
 #include <range/v3/detail/disable_warnings.hpp>
 
 namespace ranges
@@ -42,6 +59,11 @@ namespace ranges
         enable_safe_range<std::basic_string_view<CharT, Traits>> = true;
 #endif
 
+#if defined(__cpp_lib_span) && __cpp_lib_span > 0
+    template<class T, std::size_t N>
+    RANGES_INLINE_VAR constexpr bool enable_safe_range<std::span<T, N>> = true;
+#endif
+
     namespace detail
     {
         template<typename T>

include/range/v3/range/concepts.hpp

@@ -32,35 +32,25 @@
 #include <range/v3/range/traits.hpp>
 
 #ifndef RANGES_NO_STD_FORWARD_DECLARATIONS
-// Non-portable forward declarations of standard containers
+// Non-portable forward declarations of standard library components
 RANGES_BEGIN_NAMESPACE_STD
-    RANGES_BEGIN_NAMESPACE_CONTAINER
-        template<typename Key,
-                 typename Compare /*= less<Key>*/,
-                 typename Alloc /*= allocator<Key>*/>
-        class set;
-
-        template<typename Key,
-                 typename Compare /*= less<Key>*/,
-                 typename Alloc /*= allocator<Key>*/>
-        class multiset;
-
-        template<typename Key,
-                 typename Hash /*= hash<Key>*/,
-                 typename Pred /*= equal_to<Key>*/,
-                 typename Alloc /*= allocator<Key>*/>
-        class unordered_set;
-
-        template<typename Key,
-                 typename Hash /*= hash<Key>*/,
-                 typename Pred /*= equal_to<Key>*/,
-                 typename Alloc /*= allocator<Key>*/>
-        class unordered_multiset;
-    RANGES_END_NAMESPACE_CONTAINER
+    RANGES_BEGIN_NAMESPACE_VERSION
+        template<typename ElementType, size_t Extent>
+        class span;
+
+        template<typename CharT, typename Traits>
+        class basic_string_view;
+    RANGES_END_NAMESPACE_VERSION
 RANGES_END_NAMESPACE_STD
 #else
-#include <set>
-#include <unordered_set>
+#ifdef __has_include
+#if __has_include(<span>)
+#include <span>
+#endif
+#if __has_include(<string_view>)
+#include <string_view>
+#endif
+#endif
 #endif
 
 #include <range/v3/detail/disable_warnings.hpp>
@@ -186,90 +176,30 @@ namespace ranges
     // clang-format on
 
     /// \cond
-    namespace detail
+    namespace ext
     {
-        struct enable_view_helper_
-        {
-            bool result_;
-
-            template<typename T>
-            static constexpr auto test(T const *) -> CPP_ret(bool)( //
-                requires range<T> && range<T const>)
-            {
-                return RANGES_IS_SAME(iter_reference_t<iterator_t<T>>,
-                                      iter_reference_t<iterator_t<T const>>);
-            }
-            static constexpr auto test(void const *) -> bool
-            {
-                return true;
-            }
-            template<typename T>
-            constexpr enable_view_helper_(T const * p)
-              : result_(enable_view_helper_::test(p))
-            {}
-        };
-        constexpr bool enable_view_impl_(...)
-        {
-            return false;
-        }
-        constexpr bool enable_view_impl_(view_base const *)
-        {
-            return true;
-        }
-        constexpr bool enable_view_impl_(enable_view_helper_ ev)
-        {
-            return ev.result_;
-        }
-        template<typename T>
-        constexpr bool enable_view_impl_(std::initializer_list<T> const *)
-        {
-            return false;
-        }
-        template<typename Key, typename Compare, typename Alloc>
-        constexpr bool enable_view_impl_(std::set<Key, Compare, Alloc> const *)
-        {
-            return false;
-        }
-        template<typename Key, typename Compare, typename Alloc>
-        constexpr bool enable_view_impl_(std::multiset<Key, Compare, Alloc> const *)
-        {
-            return false;
-        }
-        template<typename Key, typename Hash, typename Pred, typename Alloc>
-        constexpr bool enable_view_impl_(
-            std::unordered_set<Key, Hash, Pred, Alloc> const *)
-        {
-            return false;
-        }
-        template<typename Key, typename Hash, typename Pred, typename Alloc>
-        constexpr bool enable_view_impl_(
-            std::unordered_multiset<Key, Hash, Pred, Alloc> const *)
-        {
-            return false;
-        }
-        // BUGBUG TODO
-        // template<typename BidiIter, typename Alloc>
-        // constexpr bool enable_view_impl_(std::match_results<BidiIter, Alloc> const *)
-        // {
-        //     return false;
-        // }
         template<typename T>
-        constexpr T const * nullptr_(int)
-        {
-            return nullptr;
-        }
-        template<typename T>
-        constexpr int nullptr_(long)
-        {
-            return 0;
-        }
+        struct enable_view
+          : std::is_base_of<view_base, T>
+        {};
     } // namespace detail
     /// \endcond
 
     // Specialize this if the default is wrong.
     template<typename T>
     RANGES_INLINE_VAR constexpr bool enable_view =
-        detail::enable_view_impl_(detail::nullptr_<T>(0));
+        ext::enable_view<T>::value;
+
+#if defined(__cpp_lib_string_view) && __cpp_lib_string_view > 0
+    template<typename Char, typename Traits>
+    RANGES_INLINE_VAR constexpr bool enable_view<std::basic_string_view<Char, Traits>> =
+        true;
+#endif
+
+#if defined(__cpp_lib_span) && __cpp_lib_span > 0
+    template<typename T, std::size_t N>
+    RANGES_INLINE_VAR constexpr bool enable_view<std::span<T, N>> = N + 1 < 2;
+#endif
 
     ///
     /// View concepts below

test/debug_view.hpp

@@ -19,7 +19,7 @@
 #include <range/v3/utility/swap.hpp>
 
 template<typename T, bool Sized = true>
-struct debug_input_view
+struct debug_input_view : ranges::view_base
 {
     static_assert(std::is_object<T>::value, "");
 

test/utility/concepts.cpp

@@ -289,7 +289,7 @@ static_assert(ranges::view_<ranges::istream_view<int>>, "");
 static_assert(!ranges::common_range<ranges::istream_view<int>>, "");
 static_assert(!ranges::sized_range<ranges::istream_view<int>>, "");
 
-struct myview {
+struct myview : ranges::view_base {
     const char *begin();
     const char *end();
 };