[STLForwardCompat] Implement llvm::type_identity (#146390)

A basic implementation until we get it in `std` in C++20.

Author: kparzysz

clang/include/clang/Tooling/Transformer/Transformer.h

@@ -42,11 +42,6 @@ class TransformerImpl {
   virtual void
   onMatchImpl(const ast_matchers::MatchFinder::MatchResult &Result) = 0;
 };
-
-// FIXME: Use std::type_identity or backport when available.
-template <class T> struct type_identity {
-  using type = T;
-};
 } // namespace detail
 
 template <typename T> struct TransformerResult {
@@ -95,8 +90,8 @@ class Transformer : public ast_matchers::MatchFinder::MatchCallback {
   template <typename MetadataT>
   explicit Transformer(
       transformer::RewriteRuleWith<MetadataT> Rule,
-      std::function<void(llvm::Expected<TransformerResult<
-                             typename detail::type_identity<MetadataT>::type>>)>
+      std::function<void(
+          llvm::Expected<TransformerResult<llvm::type_identity_t<MetadataT>>>)>
           Consumer);
 
   /// N.B. Passes `this` pointer to `MatchFinder`.  So, this object should not
@@ -200,8 +195,8 @@ template <typename T> class WithMetadataImpl final : public TransformerImpl {
 template <typename MetadataT>
 Transformer::Transformer(
     transformer::RewriteRuleWith<MetadataT> Rule,
-    std::function<void(llvm::Expected<TransformerResult<
-                           typename detail::type_identity<MetadataT>::type>>)>
+    std::function<void(
+        llvm::Expected<TransformerResult<llvm::type_identity_t<MetadataT>>>)>
         Consumer)
     : Impl(std::make_unique<detail::WithMetadataImpl<MetadataT>>(
           std::move(Rule), std::move(Consumer))) {}

llvm/include/llvm/ADT/STLForwardCompat.h

@@ -36,6 +36,19 @@ template <typename T>
 using remove_cvref_t // NOLINT(readability-identifier-naming)
     = typename llvm::remove_cvref<T>::type;
 
+// TODO: Remove this in favor of std::type_identity<T> once we switch to C++23.
+template <typename T>
+struct type_identity // NOLINT(readability-identifier-naming)
+{
+  using type = T;
+};
+
+// TODO: Remove this in favor of std::type_identity_t<T> once we switch to
+// C++23.
+template <typename T>
+using type_identity_t // NOLINT(readability-identifier-naming)
+    = typename llvm::type_identity<T>::type;
+
 //===----------------------------------------------------------------------===//
 //     Features from C++23
 //===----------------------------------------------------------------------===//

llvm/unittests/ADT/STLForwardCompatTest.cpp

@@ -45,6 +45,25 @@ TYPED_TEST(STLForwardCompatRemoveCVRefTest, RemoveCVRefT) {
                             llvm::remove_cvref_t<From>>::value));
 }
 
+template <typename T> class TypeIdentityTest : public ::testing::Test {
+public:
+  using TypeIdentity = llvm::type_identity<T>;
+};
+
+struct A {
+  struct B {};
+};
+using TypeIdentityTestTypes =
+    ::testing::Types<int, volatile int, A, const A::B>;
+
+TYPED_TEST_SUITE(TypeIdentityTest, TypeIdentityTestTypes, /*NameGenerator*/);
+
+TYPED_TEST(TypeIdentityTest, Identity) {
+  // TestFixture is the instantiated TypeIdentityTest.
+  EXPECT_TRUE(
+      (std::is_same_v<TypeParam, typename TestFixture::TypeIdentity::type>));
+}
+
 TEST(TransformTest, TransformStd) {
   std::optional<int> A;