[absl] Rename `absl::internal::identity` to `absl::internal::type_identity`

* Also does this for `absl::internal::identity_t` which is now `absl::internal::type_identity_t`
* This is clearer naming as this is a backfill of `std::type_identity` (the identity type), and not `std::identity` (the identity function)

PiperOrigin-RevId: 594316002
Change-Id: I5fb8cf7e3d07c1bc736cbecd202e7d556b6ea33e

3 files changed, 24 insertions, 21 deletions

diff --git a/absl/base/casts.h b/absl/base/casts.h
index d1958885..e0b11bbe 100644
--- a/absl/base/casts.h
+++ b/absl/base/casts.h
@@ -90,7 +90,7 @@ ABSL_NAMESPACE_BEGIN
 //
 // Such implicit cast chaining may be useful within template logic.
 template <typename To>
-constexpr To implicit_cast(typename absl::internal::identity_t<To> to) {
+constexpr To implicit_cast(typename absl::internal::type_identity_t<To> to) {
   return to;
 }
 
diff --git a/absl/base/internal/identity.h b/absl/base/internal/identity.h
index a3154ed7..365207b7 100644
--- a/absl/base/internal/identity.h
+++ b/absl/base/internal/identity.h
@@ -22,13 +22,15 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace internal {
 
+// This is a back-fill of C++20's `std::type_identity`.
 template <typename T>
-struct identity {
+struct type_identity {
   typedef T type;
 };
 
+// This is a back-fill of C++20's `std::type_identity_t`.
 template <typename T>
-using identity_t = typename identity<T>::type;
+using type_identity_t = typename type_identity<T>::type;
 
 }  // namespace internal
 ABSL_NAMESPACE_END
diff --git a/absl/base/internal/inline_variable.h b/absl/base/internal/inline_variable.h
index df933faf..09daf0f5 100644
--- a/absl/base/internal/inline_variable.h
+++ b/absl/base/internal/inline_variable.h
@@ -63,12 +63,12 @@
 // Bug: https://bugs.llvm.org/show_bug.cgi?id=35862
 //
 // Note:
-//   identity_t is used here so that the const and name are in the
+//   type_identity_t is used here so that the const and name are in the
 //   appropriate place for pointer types, reference types, function pointer
 //   types, etc..
 #if defined(__clang__)
 #define ABSL_INTERNAL_EXTERN_DECL(type, name) \
-  extern const ::absl::internal::identity_t<type> name;
+  extern const ::absl::internal::type_identity_t<type> name;
 #else  // Otherwise, just define the macro to do nothing.
 #define ABSL_INTERNAL_EXTERN_DECL(type, name)
 #endif  // defined(__clang__)
@@ -76,30 +76,31 @@
 // See above comment at top of file for details.
 #define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \
   ABSL_INTERNAL_EXTERN_DECL(type, name)                  \
-  inline constexpr ::absl::internal::identity_t<type> name = init
+  inline constexpr ::absl::internal::type_identity_t<type> name = init
 
 #else
 
 // See above comment at top of file for details.
 //
 // Note:
-//   identity_t is used here so that the const and name are in the
+//   type_identity_t is used here so that the const and name are in the
 //   appropriate place for pointer types, reference types, function pointer
 //   types, etc..
-#define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init)                  \
-  template <class /*AbslInternalDummy*/ = void>                               \
-  struct AbslInternalInlineVariableHolder##name {                             \
-    static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \
-  };                                                                          \
-                                                                              \
-  template <class AbslInternalDummy>                                          \
-  constexpr ::absl::internal::identity_t<var_type>                            \
-      AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance;   \
-                                                                              \
-  static constexpr const ::absl::internal::identity_t<var_type>&              \
-      name = /* NOLINT */                                                     \
-      AbslInternalInlineVariableHolder##name<>::kInstance;                    \
-  static_assert(sizeof(void (*)(decltype(name))) != 0,                        \
+#define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init)                 \
+  template <class /*AbslInternalDummy*/ = void>                              \
+  struct AbslInternalInlineVariableHolder##name {                            \
+    static constexpr ::absl::internal::type_identity_t<var_type> kInstance = \
+        init;                                                                \
+  };                                                                         \
+                                                                             \
+  template <class AbslInternalDummy>                                         \
+  constexpr ::absl::internal::type_identity_t<var_type>                      \
+      AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance;  \
+                                                                             \
+  static constexpr const ::absl::internal::type_identity_t<var_type>&        \
+      name = /* NOLINT */                                                    \
+      AbslInternalInlineVariableHolder##name<>::kInstance;                   \
+  static_assert(sizeof(void (*)(decltype(name))) != 0,                       \
                 "Silence unused variable warnings.")
 
 #endif  // __cpp_inline_variables