Add std::pair specializations for IsOwner and IsView

Also fix a typo in the comment.

Fixes: #1930
PiperOrigin-RevId: 804996432
Change-Id: Iec326a4e066a885295a09691dd6773847b9a68a3

Author: Abseil Team

absl/meta/type_traits.h

@@ -40,6 +40,7 @@
 #include <string>
 #include <string_view>
 #include <type_traits>
+#include <utility>
 #include <vector>
 
 #include "absl/base/attributes.h"
@@ -451,7 +452,7 @@ namespace type_traits_internal {
 
 // Detects if a class's definition has declared itself to be an owner by
 // declaring
-//   using absl_internal_is_view = std::true_type;
+//   using absl_internal_is_view = std::false_type;
 // as a member.
 // Types that don't want either must either omit this declaration entirely, or
 // (if e.g. inheriting from a base class) define the member to something that
@@ -479,6 +480,17 @@ struct IsOwnerImpl<
 template <typename T>
 struct IsOwner : IsOwnerImpl<T> {};
 
+// This allows incomplete types to be used for associative containers, and also
+// expands the set of types we can handle to include std::pair.
+template <typename T1, typename T2>
+struct IsOwner<std::pair<T1, T2>>
+    : std::integral_constant<
+          bool, std::conditional_t<std::is_reference_v<T1>, std::false_type,
+                                   IsOwner<std::remove_cv_t<T1>>>::value &&
+                    std::conditional_t<std::is_reference_v<T2>, std::false_type,
+                                       IsOwner<std::remove_cv_t<T2>>>::value> {
+};
+
 template <typename T, typename Traits, typename Alloc>
 struct IsOwner<std::basic_string<T, Traits, Alloc>> : std::true_type {};
 
@@ -513,6 +525,13 @@ template <typename T>
 struct IsView : std::integral_constant<bool, std::is_pointer<T>::value ||
                                                  IsViewImpl<T>::value> {};
 
+// This allows incomplete types to be used for associative containers, and also
+// expands the set of types we can handle to include std::pair.
+template <typename T1, typename T2>
+struct IsView<std::pair<T1, T2>>
+    : std::integral_constant<bool, IsView<std::remove_cv_t<T1>>::value &&
+                                       IsView<std::remove_cv_t<T2>>::value> {};
+
 template <typename Char, typename Traits>
 struct IsView<std::basic_string_view<Char, Traits>> : std::true_type {};
 

absl/meta/type_traits_test.cc

@@ -36,6 +36,9 @@ using IsOwnerAndNotView =
     absl::conjunction<absl::type_traits_internal::IsOwner<T>,
                       absl::negation<absl::type_traits_internal::IsView<T>>>;
 
+static_assert(
+    IsOwnerAndNotView<std::pair<std::vector<int>, std::string>>::value,
+    "pair of owners is an owner, not a view");
 static_assert(IsOwnerAndNotView<std::vector<int>>::value,
               "vector is an owner, not a view");
 static_assert(IsOwnerAndNotView<std::string>::value,