Add absl::NoDestructor<T>

absl::NoDestructor<T> wraps an existing type and is used for defining a
static type that does not need to be destructed upon program exit. Instead,
such an object survives during program exit, and can be safely accessed at
any time after construction.
PiperOrigin-RevId: 580566259
Change-Id: I2cbb4cd9f6b50b98a793acd2ef0a4befd541f17f

5 files changed, 653 insertions, 0 deletions

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index daa10af6..60d05a83 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -70,6 +70,14 @@ cc_library(
 )
 
 cc_library(
+    name = "no_destructor",
+    hdrs = ["no_destructor.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [":config"],
+)
+
+cc_library(
     name = "nullability",
     srcs = ["internal/nullability_impl.h"],
     hdrs = ["nullability.h"],
@@ -584,6 +592,35 @@ cc_test(
 )
 
 cc_test(
+    name = "no_destructor_test",
+    srcs = ["no_destructor_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":config",
+        ":no_destructor",
+        ":raw_logging_internal",
+        "@com_google_googletest//:gtest",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "no_destructor_benchmark",
+    testonly = 1,
+    srcs = ["no_destructor_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":no_destructor",
+        ":raw_logging_internal",
+        "@com_github_google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_test(
     name = "nullability_test",
     srcs = ["nullability_test.cc"],
     deps = [
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 3f4a1c42..9ca5cf8b 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -57,6 +57,17 @@ absl_cc_library(
 
 absl_cc_library(
   NAME
+    no_destructor
+  HDRS
+    "no_destructor.h"
+  DEPS
+    absl::config
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+absl_cc_library(
+  NAME
     nullability
   HDRS
     "nullability.h"
@@ -510,6 +521,20 @@ absl_cc_test(
 
 absl_cc_test(
   NAME
+    no_destructor_test
+  SRCS
+    "no_destructor_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::no_destructor
+    absl::config
+    absl::raw_logging_internal
+    GTest::gtest_main
+)
+
+absl_cc_test(
+  NAME
     raw_logging_test
   SRCS
     "raw_logging_test.cc"
diff --git a/absl/base/no_destructor.h b/absl/base/no_destructor.h
new file mode 100644
index 00000000..d4b16a6e
--- /dev/null
+++ b/absl/base/no_destructor.h
@@ -0,0 +1,217 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: no_destructor.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the absl::NoDestructor<T> wrapper for defining a
+// static type that does not need to be destructed upon program exit. Instead,
+// such an object survives during program exit (and can be safely accessed at
+// any time).
+//
+// Objects of such type, if constructed safely and under the right conditions,
+// provide two main benefits over other alternatives:
+//
+//   * Global objects not normally allowed due to concerns of destruction order
+//     (i.e. no "complex globals") can be safely allowed, provided that such
+//     objects can be constant initialized.
+//   * Function scope static objects can be optimized to avoid heap allocation,
+//     pointer chasing, and allow lazy construction.
+//
+// See below for complete details.
+
+
+#ifndef ABSL_BASE_NO_DESTRUCTOR_H_
+#define ABSL_BASE_NO_DESTRUCTOR_H_
+
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::NoDestructor<T>
+//
+// NoDestructor<T> is a wrapper around an object of type T that behaves as an
+// object of type T but never calls T's destructor. NoDestructor<T> makes it
+// safer and/or more efficient to use such objects in static storage contexts:
+// as global or function scope static variables.
+//
+// An instance of absl::NoDestructor<T> has similar type semantics to an
+// instance of T:
+//
+// * Constructs in the same manner as an object of type T through perfect
+//   forwarding.
+// * Provides pointer/reference semantic access to the object of type T via
+//   `->`, `*`, and `get()`.
+//   (Note that `const NoDestructor<T>` works like a pointer to const `T`.)
+//
+// An object of type NoDestructor<T> should be defined in static storage:
+// as either a global static object, or as a function scope static variable.
+//
+// Additionally, NoDestructor<T> provides the following benefits:
+//
+// * Never calls T's destructor for the object
+// * If the object is a function-local static variable, the type can be
+//   lazily constructed.
+//
+// An object of type NoDestructor<T> is "trivially destructible" in the notion
+// that its destructor is never run. Provided that an object of this type can be
+// safely initialized and does not need to be cleaned up on program shutdown,
+// NoDestructor<T> allows you to define global static variables, since Google's
+// C++ style guide ban on such objects doesn't apply to objects that are
+// trivially destructible.
+//
+// Usage as Global Static Variables
+//
+// NoDestructor<T> allows declaration of a global object with a non-trivial
+// constructor in static storage without needing to add a destructor.
+// However, such objects still need to worry about initialization order, so
+// such objects should be const initialized:
+//
+//    // Global or namespace scope.
+//    ABSL_CONST_INIT absl::NoDestructor<MyRegistry> reg{"foo", "bar", 8008};
+//
+// Note that if your object already has a trivial destructor, you don't need to
+// use NoDestructor<T>.
+//
+// Usage as Function Scope Static Variables
+//
+// Function static objects will be lazily initialized within static storage:
+//
+//    // Function scope.
+//    const std::string& MyString() {
+//      static const absl::NoDestructor<std::string> x("foo");
+//      return *x;
+//    }
+//
+// For function static variables, NoDestructor avoids heap allocation and can be
+// inlined in static storage, resulting in exactly-once, thread-safe
+// construction of an object, and very fast access thereafter (the cost is a few
+// extra cycles).
+//
+// Using NoDestructor<T> in this manner is generally better than other patterns
+// which require pointer chasing:
+//
+//   // Prefer using absl::NoDestructor<T> instead for the static variable.
+//   const std::string& MyString() {
+//     static const std::string* x = new std::string("foo");
+//     return *x;
+//   }
+//
+template <typename T>
+class NoDestructor {
+ public:
+  // Forwards arguments to the T's constructor: calls T(args...).
+  template <typename... Ts,
+            // Disable this overload when it might collide with copy/move.
+            typename std::enable_if<!std::is_same<void(std::decay_t<Ts>&...),
+                                                  void(NoDestructor&)>::value,
+                                    int>::type = 0>
+  explicit constexpr NoDestructor(Ts&&... args)
+      : impl_(std::forward<Ts>(args)...) {}
+
+  // Forwards copy and move construction for T. Enables usage like this:
+  //   static NoDestructor<std::array<string, 3>> x{{{"1", "2", "3"}}};
+  //   static NoDestructor<std::vector<int>> x{{1, 2, 3}};
+  explicit constexpr NoDestructor(const T& x) : impl_(x) {}
+  explicit constexpr NoDestructor(T&& x)
+      : impl_(std::move(x)) {}
+
+  // No copying.
+  NoDestructor(const NoDestructor&) = delete;
+  NoDestructor& operator=(const NoDestructor&) = delete;
+
+  // Pretend to be a smart pointer to T with deep constness.
+  // Never returns a null pointer.
+  T& operator*() { return *get(); }
+  T* operator->() { return get(); }
+  T* get() { return impl_.get(); }
+  const T& operator*() const { return *get(); }
+  const T* operator->() const { return get(); }
+  const T* get() const { return impl_.get(); }
+
+ private:
+  class DirectImpl {
+   public:
+    template <typename... Args>
+    explicit constexpr DirectImpl(Args&&... args)
+        : value_(std::forward<Args>(args)...) {}
+    const T* get() const { return &value_; }
+    T* get() { return &value_; }
+
+   private:
+    T value_;
+  };
+
+  class PlacementImpl {
+   public:
+    template <typename... Args>
+    explicit PlacementImpl(Args&&... args) {
+      new (&space_) T(std::forward<Args>(args)...);
+    }
+    const T* get() const {
+      return Launder(reinterpret_cast<const T*>(&space_));
+    }
+    T* get() { return Launder(reinterpret_cast<T*>(&space_)); }
+
+   private:
+    template <typename P>
+    static P* Launder(P* p) {
+#if defined(__cpp_lib_launder) && __cpp_lib_launder >= 201606L
+      return std::launder(p);
+#elif ABSL_HAVE_BUILTIN(__builtin_launder)
+      return __builtin_launder(p);
+#else
+      // When `std::launder` or equivalent are not available, we rely on
+      // undefined behavior, which works as intended on Abseil's officially
+      // supported platforms as of Q3 2023.
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#endif
+      return p;
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+#endif
+    }
+
+    alignas(T) unsigned char space_[sizeof(T)];
+  };
+
+  // If the object is trivially destructible we use a member directly to avoid
+  // potential once-init runtime initialization. It somewhat defeats the
+  // purpose of NoDestructor in this case, but this makes the class more
+  // friendly to generic code.
+  std::conditional_t<std::is_trivially_destructible<T>::value, DirectImpl,
+                     PlacementImpl>
+      impl_;
+};
+
+#ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+// Provide 'Class Template Argument Deduction': the type of NoDestructor's T
+// will be the same type as the argument passed to NoDestructor's constructor.
+template <typename T>
+NoDestructor(T) -> NoDestructor<T>;
+#endif  // ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_NO_DESTRUCTOR_H_
diff --git a/absl/base/no_destructor_benchmark.cc b/absl/base/no_destructor_benchmark.cc
new file mode 100644
index 00000000..5fc88f1d
--- /dev/null
+++ b/absl/base/no_destructor_benchmark.cc
@@ -0,0 +1,165 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstdint>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/no_destructor.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+
+// Number of static-NoDestructor-in-a-function to exercise.
+// This must be low enough not to hit template instantiation limits
+// (happens around 1000).
+constexpr int kNumObjects = 1;  // set to 512 when doing benchmarks
+                                // 1 is faster to compile: just one templated
+                                // function instantiation
+
+// Size of individual objects to benchmark static-NoDestructor-in-a-function
+// usage with.
+constexpr int kObjSize = sizeof(void*)*1;
+
+// Simple object of kObjSize bytes (rounded to int).
+// We benchmark complete reading of its state via Verify().
+class BM_Blob {
+ public:
+  BM_Blob(int val) { for (auto& d : data_) d = val; }
+  BM_Blob() : BM_Blob(-1) {}
+  void Verify(int val) const {  // val must be the c-tor argument
+    for (auto& d : data_) ABSL_INTERNAL_CHECK(d == val, "");
+  }
+ private:
+  int data_[kObjSize / sizeof(int) > 0 ? kObjSize / sizeof(int) : 1];
+};
+
+// static-NoDestructor-in-a-function pattern instances.
+// We'll instantiate kNumObjects of them.
+template<int i>
+const BM_Blob& NoDestrBlobFunc() {
+  static absl::NoDestructor<BM_Blob> x(i);
+  return *x;
+}
+
+// static-heap-ptr-in-a-function pattern instances
+// We'll instantiate kNumObjects of them.
+template<int i>
+const BM_Blob& OnHeapBlobFunc() {
+  static BM_Blob* x = new BM_Blob(i);
+  return *x;
+}
+
+// Type for NoDestrBlobFunc or OnHeapBlobFunc.
+typedef const BM_Blob& (*FuncType)();
+
+// ========================================================================= //
+// Simple benchmarks that read a single BM_Blob over and over, hence
+// all they touch fits into L1 CPU cache:
+
+// Direct non-POD global variable (style guide violation) as a baseline.
+static BM_Blob direct_blob(0);
+
+void BM_Direct(benchmark::State& state) {
+  for (auto s : state) {
+    direct_blob.Verify(0);
+  }
+}
+BENCHMARK(BM_Direct);
+
+void BM_NoDestr(benchmark::State& state) {
+  for (auto s : state) {
+    NoDestrBlobFunc<0>().Verify(0);
+  }
+}
+BENCHMARK(BM_NoDestr);
+
+void BM_OnHeap(benchmark::State& state) {
+  for (auto s : state) {
+    OnHeapBlobFunc<0>().Verify(0);
+  }
+}
+BENCHMARK(BM_OnHeap);
+
+// ========================================================================= //
+// Benchmarks that read kNumObjects of BM_Blob over and over, hence with
+// appropriate values of sizeof(BM_Blob) and kNumObjects their working set
+// can exceed a given layer of CPU cache.
+
+// Type of benchmark to select between NoDestrBlobFunc and OnHeapBlobFunc.
+enum BM_Type { kNoDestr, kOnHeap, kDirect };
+
+// BlobFunc<n>(t, i) returns the i-th function of type t.
+// n must be larger than i (we'll use kNumObjects for n).
+template<int n>
+FuncType BlobFunc(BM_Type t, int i) {
+  if (i == n) {
+    switch (t) {
+      case kNoDestr:  return &NoDestrBlobFunc<n>;
+      case kOnHeap:   return &OnHeapBlobFunc<n>;
+      case kDirect:   return nullptr;
+    }
+  }
+  return BlobFunc<n-1>(t, i);
+}
+
+template<>
+FuncType BlobFunc<0>(BM_Type t, int i) {
+  ABSL_INTERNAL_CHECK(i == 0, "");
+  switch (t) {
+    case kNoDestr:  return &NoDestrBlobFunc<0>;
+    case kOnHeap:   return &OnHeapBlobFunc<0>;
+    case kDirect:   return nullptr;
+  }
+  return nullptr;
+}
+
+// Direct non-POD global variables (style guide violation) as a baseline.
+static BM_Blob direct_blobs[kNumObjects];
+
+// Helper that cheaply maps benchmark iteration to randomish index in
+// [0, kNumObjects).
+int RandIdx(int i) {
+  // int64 is to avoid overflow and generating negative return values:
+  return (static_cast<int64_t>(i) * 13) % kNumObjects;
+}
+
+// Generic benchmark working with kNumObjects for any of the possible BM_Type.
+template <BM_Type t>
+void BM_Many(benchmark::State& state) {
+  FuncType funcs[kNumObjects];
+  for (int i = 0; i < kNumObjects; ++i) {
+    funcs[i] = BlobFunc<kNumObjects-1>(t, i);
+  }
+  if (t == kDirect) {
+    for (auto s : state) {
+      int idx = RandIdx(state.iterations());
+      direct_blobs[idx].Verify(-1);
+    }
+  } else {
+    for (auto s : state) {
+      int idx = RandIdx(state.iterations());
+      funcs[idx]().Verify(idx);
+    }
+  }
+}
+
+void BM_DirectMany(benchmark::State& state) { BM_Many<kDirect>(state); }
+void BM_NoDestrMany(benchmark::State& state) { BM_Many<kNoDestr>(state); }
+void BM_OnHeapMany(benchmark::State& state) { BM_Many<kOnHeap>(state); }
+
+BENCHMARK(BM_DirectMany);
+BENCHMARK(BM_NoDestrMany);
+BENCHMARK(BM_OnHeapMany);
+
+}  // namespace
diff --git a/absl/base/no_destructor_test.cc b/absl/base/no_destructor_test.cc
new file mode 100644
index 00000000..71693c7e
--- /dev/null
+++ b/absl/base/no_destructor_test.cc
@@ -0,0 +1,209 @@
+// Copyright 2023 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/no_destructor.h"
+
+#include <array>
+#include <initializer_list>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace {
+
+struct Blob {
+  Blob() : val(42) {}
+  Blob(int x, int y) : val(x + y) {}
+  Blob(std::initializer_list<int> xs) {
+    val = 0;
+    for (auto& x : xs) val += x;
+  }
+
+  Blob(const Blob& /*b*/) = delete;
+  Blob(Blob&& b) noexcept : val(b.val) {
+    b.moved_out = true;
+  }  // moving is fine
+
+  // no crash: NoDestructor indeed does not destruct (the moved-out Blob
+  // temporaries do get destroyed though)
+  ~Blob() { ABSL_INTERNAL_CHECK(moved_out, "~Blob"); }
+
+  int val;
+  bool moved_out = false;
+};
+
+struct TypeWithDeletedDestructor {
+  ~TypeWithDeletedDestructor() = delete;
+};
+
+TEST(NoDestructorTest, DestructorNeverCalled) {
+  absl::NoDestructor<TypeWithDeletedDestructor> a;
+  (void)a;
+}
+
+TEST(NoDestructorTest, Noncopyable) {
+  using T = absl::NoDestructor<int>;
+
+  EXPECT_FALSE((std::is_constructible<T, T>::value));
+  EXPECT_FALSE((std::is_constructible<T, const T>::value));
+  EXPECT_FALSE((std::is_constructible<T, T&>::value));
+  EXPECT_FALSE((std::is_constructible<T, const T&>::value));
+
+  EXPECT_FALSE((std::is_assignable<T&, T>::value));
+  EXPECT_FALSE((std::is_assignable<T&, const T>::value));
+  EXPECT_FALSE((std::is_assignable<T&, T&>::value));
+  EXPECT_FALSE((std::is_assignable<T&, const T&>::value));
+}
+
+TEST(NoDestructorTest, Interface) {
+  EXPECT_TRUE(std::is_trivially_destructible<absl::NoDestructor<Blob>>::value);
+  EXPECT_TRUE(
+      std::is_trivially_destructible<absl::NoDestructor<const Blob>>::value);
+  {
+    absl::NoDestructor<Blob> b;  // default c-tor
+    // access: *, ->, get()
+    EXPECT_EQ(42, (*b).val);
+    (*b).val = 55;
+    EXPECT_EQ(55, b->val);
+    b->val = 66;
+    EXPECT_EQ(66, b.get()->val);
+    b.get()->val = 42;  // NOLINT
+    EXPECT_EQ(42, (*b).val);
+  }
+  {
+    absl::NoDestructor<const Blob> b(70, 7);  // regular c-tor, const
+    EXPECT_EQ(77, (*b).val);
+    EXPECT_EQ(77, b->val);
+    EXPECT_EQ(77, b.get()->val);
+  }
+  {
+    const absl::NoDestructor<Blob> b{
+        {20, 28, 40}};  // init-list c-tor, deep const
+    // This only works in clang, not in gcc:
+    // const absl::NoDestructor<Blob> b({20, 28, 40});
+    EXPECT_EQ(88, (*b).val);
+    EXPECT_EQ(88, b->val);
+    EXPECT_EQ(88, b.get()->val);
+  }
+}
+
+TEST(NoDestructorTest, SfinaeRegressionAbstractArg) {
+  struct Abstract {
+    virtual ~Abstract() = default;
+    virtual int foo() const = 0;
+  };
+
+  struct Concrete : Abstract {
+    int foo() const override { return 17; }
+  };
+
+  struct UsesAbstractInConstructor {
+    explicit UsesAbstractInConstructor(const Abstract& abstract)
+        : i(abstract.foo()) {}
+    int i;
+  };
+
+  Concrete input;
+  absl::NoDestructor<UsesAbstractInConstructor> foo1(input);
+  EXPECT_EQ(foo1->i, 17);
+  absl::NoDestructor<UsesAbstractInConstructor> foo2(
+      static_cast<const Abstract&>(input));
+  EXPECT_EQ(foo2->i, 17);
+}
+
+// ========================================================================= //
+
+std::string* Str0() {
+  static absl::NoDestructor<std::string> x;
+  return x.get();
+}
+
+extern const std::string& Str2();
+
+const char* Str1() {
+  static absl::NoDestructor<std::string> x(Str2() + "_Str1");
+  return x->c_str();
+}
+
+const std::string& Str2() {
+  static absl::NoDestructor<std::string> x("Str2");
+  return *x;
+}
+
+const std::string& Str2Copy() {
+  // Exercise copy construction
+  static absl::NoDestructor<std::string> x(Str2());
+  return *x;
+}
+
+typedef std::array<std::string, 3> MyArray;
+const MyArray& Array() {
+  static absl::NoDestructor<MyArray> x{{{"foo", "bar", "baz"}}};
+  // This only works in clang, not in gcc:
+  // static absl::NoDestructor<MyArray> x({{"foo", "bar", "baz"}});
+  return *x;
+}
+
+typedef std::vector<int> MyVector;
+const MyVector& Vector() {
+  static absl::NoDestructor<MyVector> x{{1, 2, 3}};
+  return *x;
+}
+
+const int& Int() {
+  static absl::NoDestructor<int> x;
+  return *x;
+}
+
+TEST(NoDestructorTest, StaticPattern) {
+  EXPECT_TRUE(
+      std::is_trivially_destructible<absl::NoDestructor<std::string>>::value);
+  EXPECT_TRUE(
+      std::is_trivially_destructible<absl::NoDestructor<MyArray>>::value);
+  EXPECT_TRUE(
+      std::is_trivially_destructible<absl::NoDestructor<MyVector>>::value);
+  EXPECT_TRUE(std::is_trivially_destructible<absl::NoDestructor<int>>::value);
+
+  EXPECT_EQ(*Str0(), "");
+  Str0()->append("foo");
+  EXPECT_EQ(*Str0(), "foo");
+
+  EXPECT_EQ(std::string(Str1()), "Str2_Str1");
+
+  EXPECT_EQ(Str2(), "Str2");
+  EXPECT_EQ(Str2Copy(), "Str2");
+
+  EXPECT_THAT(Array(), testing::ElementsAre("foo", "bar", "baz"));
+
+  EXPECT_THAT(Vector(), testing::ElementsAre(1, 2, 3));
+
+  EXPECT_EQ(0, Int());  // should get zero-initialized
+}
+
+#ifdef ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION
+// This would fail to compile if Class Template Argument Deduction was not
+// provided for absl::NoDestructor.
+TEST(NoDestructorTest, ClassTemplateArgumentDeduction) {
+  absl::NoDestructor i(1);
+  static_assert(std::is_same<decltype(i), absl::NoDestructor<int>>::value,
+                "Expected deduced type to be int.");
+}
+#endif
+
+}  // namespace