Restore string_view detection check

PiperOrigin-RevId: 623195368
Change-Id: Iadb9bdedee4d9b5ced4fff9e6316ee63f9a89ea5

absl/meta/type_traits.h

@@ -630,6 +630,11 @@ template <typename T>
 struct IsView : std::integral_constant<bool, std::is_pointer<T>::value ||
                                                  IsViewImpl<T>::value> {};
 
+#ifdef ABSL_HAVE_STD_STRING_VIEW
+template <typename Char, typename Traits>
+struct IsView<std::basic_string_view<Char, Traits>> : std::true_type {};
+#endif
+
 #ifdef __cpp_lib_span
 template <typename T>
 struct IsView<std::span<T>> : std::true_type {};

absl/meta/type_traits_test.cc

@@ -45,6 +45,12 @@ static_assert(IsOwnerAndNotView<std::string>::value,
               "string is an owner, not a view");
 static_assert(IsOwnerAndNotView<std::wstring>::value,
               "wstring is an owner, not a view");
+#ifdef ABSL_HAVE_STD_STRING_VIEW
+static_assert(!IsOwnerAndNotView<std::string_view>::value,
+              "string_view is a view, not an owner");
+static_assert(!IsOwnerAndNotView<std::wstring_view>::value,
+              "wstring_view is a view, not an owner");
+#endif
 
 template <class T, class U>
 struct simple_pair {

absl/strings/string_view.h

@@ -173,6 +173,7 @@ class string_view {
   using reverse_iterator = const_reverse_iterator;
   using size_type = size_t;
   using difference_type = std::ptrdiff_t;
+  using absl_internal_is_view = std::true_type;
 
   static constexpr size_type npos = static_cast<size_type>(-1);
 

absl/strings/string_view_test.cc

@@ -47,6 +47,14 @@
 
 namespace {
 
+static_assert(!absl::type_traits_internal::IsOwner<absl::string_view>::value &&
+                  absl::type_traits_internal::IsView<absl::string_view>::value,
+              "string_view is a view, not an owner");
+
+static_assert(absl::type_traits_internal::IsLifetimeBoundAssignment<
+                  absl::string_view, std::string>::value,
+              "lifetimebound assignment not detected");
+
 // A minimal allocator that uses malloc().
 template <typename T>
 struct Mallocator {