Export of internal Abseil changes.

--
3d20ce6cd6541579abecaba169d4b8716d511272 by Jon Cohen <cohenjon@google.com>:

Only use LSAN for clang version >= 3.5.  This should fix https://github.com/abseil/abseil-cpp/issues/244

PiperOrigin-RevId: 234675129

--
e15bd4ec7a81aa95cc3d09fa1e0e81d58ae478fb by Conrad Parker <conradparker@google.com>:

Fix errors in apply() sample code

The following changes are made:
 * Make the example method public.
 * Give the two user functions different names to avoid confusion about
   whether apply() can select the correct overload of a function based
   on its tuple argument (it can't).
 * Pass tuple2 to the second example apply() invocation, instead of
   passing its contents individually.
 * Fix a s/tuple/tuple3/ typo in the third example apply() invocation.

PiperOrigin-RevId: 234223407

--
de0ed71e21bc76ddf9fe715fdbaef74cd0df95c7 by Abseil Team <absl-team@google.com>:

First test if a macro is defined to avoid -Wundef.

ABSL clients may need to compile their code with the -Wundef warning
flag. It will be helpful if ABSL header files can be compiled without
the -Wundef warning.

How to avoid the -Wundef warning: If a macro may be undefined, we need
to first test whether the macro is defined before testing its value. We
can't rely on the C preprocessor rule that an undefined macro has the
value 0L.

PiperOrigin-RevId: 234201123

--
fa484ad7dae0cac21140a96662809ecb0ec8eb5d by Abseil Team <absl-team@google.com>:

Internal change.

PiperOrigin-RevId: 234185697

--
d69b1baef681e27954b065375ecf9c2320463b2b by Samuel Benzaquen <sbenza@google.com>:

Mix pointers more thoroughly.
Some pointer alignments interact badly with the mixing constant. By mixing twice we reduce this problem.

PiperOrigin-RevId: 234178401

--
1041d0e474610f3a8fea0db90958857327d6da1c by Samuel Benzaquen <sbenza@google.com>:

Record rehashes in the hashtablez struct.
Only recording the probe length on insertion causes a huge overestimation of
the total probe length at any given time.

With natural growth, elements are inserted when the load factor is between
(max load/2, max load). However, after a rehash the majority of elements are
actually inserted when the load factor is less than max/2 and have a much lower
average probe length.

Also reset some values when the table is cleared.

PiperOrigin-RevId: 234013580

--
299205caf3c89c47339f7409bc831746602cea84 by Mark Barolak <mbar@google.com>:

Fix a sample code snippet that assumes `absl::string_view::const_iterator` is `const char*`.  This is generally true, however in C++17 builds, absl::string_view is an alias for std::string_view and on MSVC, the std::string_view::const_iterator is an object instead of just a pointer.

PiperOrigin-RevId: 233844595

--
af6c6370cf51a1e6c1469c79dfb2a486a4009136 by Abseil Team <absl-team@google.com>:

Internal change.

PiperOrigin-RevId: 233773470

--
6e59e4b8e2bb6101b448f0f32b0bea81fe399ccf by Abseil Team <absl-team@google.com>:

fix typo in {Starts|Ends}WithIgnoreCase comment in match.h

PiperOrigin-RevId: 233662951
GitOrigin-RevId: 3d20ce6cd6541579abecaba169d4b8716d511272
Change-Id: Ib9a29b1c38c6aedf5d9f3f7f00596e8d30e864dd

13 files changed, 170 insertions, 54 deletions

diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
index cc3d8b69b7a3..f6d28472ad2b 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -527,25 +527,26 @@ namespace container_internal {
 
 template <class K, class V>
 struct FlatHashMapPolicy {
-  using slot_type = container_internal::slot_type<K, V>;
+  using slot_policy = container_internal::map_slot_policy<K, V>;
+  using slot_type = typename slot_policy::slot_type;
   using key_type = K;
   using mapped_type = V;
   using init_type = std::pair</*non const*/ key_type, mapped_type>;
 
   template <class Allocator, class... Args>
   static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
-    slot_type::construct(alloc, slot, std::forward<Args>(args)...);
+    slot_policy::construct(alloc, slot, std::forward<Args>(args)...);
   }
 
   template <class Allocator>
   static void destroy(Allocator* alloc, slot_type* slot) {
-    slot_type::destroy(alloc, slot);
+    slot_policy::destroy(alloc, slot);
   }
 
   template <class Allocator>
   static void transfer(Allocator* alloc, slot_type* new_slot,
                        slot_type* old_slot) {
-    slot_type::transfer(alloc, new_slot, old_slot);
+    slot_policy::transfer(alloc, new_slot, old_slot);
   }
 
   template <class F, class... Args>
diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index 35b691ce5af3..3a3f97032cdf 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -311,7 +311,23 @@ struct IsLayoutCompatible {
 // kMutableKeys. For C++11, the relevant section of the standard is
 // https://timsong-cpp.github.io/cppwp/n3337/class.mem#19 (9.2.19)
 template <class K, class V>
-union slot_type {
+union map_slot_type {
+  map_slot_type() {}
+  ~map_slot_type() = delete;
+  using value_type = std::pair<const K, V>;
+  using mutable_value_type = std::pair<K, V>;
+
+  value_type value;
+  mutable_value_type mutable_value;
+  K key;
+};
+
+template <class K, class V>
+struct map_slot_policy {
+  using slot_type = map_slot_type<K, V>;
+  using value_type = std::pair<const K, V>;
+  using mutable_value_type = std::pair<K, V>;
+
  private:
   static void emplace(slot_type* slot) {
     // The construction of union doesn't do anything at runtime but it allows us
@@ -321,19 +337,17 @@ union slot_type {
   // If pair<const K, V> and pair<K, V> are layout-compatible, we can accept one
   // or the other via slot_type. We are also free to access the key via
   // slot_type::key in this case.
-  using kMutableKeys =
-      std::integral_constant<bool,
-                             memory_internal::IsLayoutCompatible<K, V>::value>;
+  using kMutableKeys = memory_internal::IsLayoutCompatible<K, V>;
 
  public:
-  slot_type() {}
-  ~slot_type() = delete;
-  using value_type = std::pair<const K, V>;
-  using mutable_value_type = std::pair<K, V>;
+  static value_type& element(slot_type* slot) { return slot->value; }
+  static const value_type& element(const slot_type* slot) {
+    return slot->value;
+  }
 
-  value_type value;
-  mutable_value_type mutable_value;
-  K key;
+  static const K& key(const slot_type* slot) {
+    return kMutableKeys::value ? slot->key : slot->value.first;
+  }
 
   template <class Allocator, class... Args>
   static void construct(Allocator* alloc, slot_type* slot, Args&&... args) {
diff --git a/absl/container/internal/hashtablez_sampler.h b/absl/container/internal/hashtablez_sampler.h
index 8b81653a57f6..e2d8f3643b86 100644
--- a/absl/container/internal/hashtablez_sampler.h
+++ b/absl/container/internal/hashtablez_sampler.h
@@ -33,6 +33,7 @@
 
 #include "absl/base/internal/per_thread_tls.h"
 #include "absl/base/optimization.h"
+#include "absl/container/internal/have_sse.h"
 #include "absl/synchronization/mutex.h"
 #include "absl/utility/utility.h"
 
@@ -82,10 +83,24 @@ struct HashtablezInfo {
   void* stack[kMaxStackDepth];
 };
 
+inline void RecordRehashSlow(HashtablezInfo* info, size_t total_probe_length) {
+#if SWISSTABLE_HAVE_SSE2
+  total_probe_length /= 16;
+#else
+  total_probe_length /= 8;
+#endif
+  info->total_probe_length.store(total_probe_length, std::memory_order_relaxed);
+  info->num_erases.store(0, std::memory_order_relaxed);
+}
+
 inline void RecordStorageChangedSlow(HashtablezInfo* info, size_t size,
                                      size_t capacity) {
   info->size.store(size, std::memory_order_relaxed);
   info->capacity.store(capacity, std::memory_order_relaxed);
+  if (size == 0) {
+    // This is a clear, reset the total/num_erases too.
+    RecordRehashSlow(info, 0);
+  }
 }
 
 void RecordInsertSlow(HashtablezInfo* info, size_t hash,
@@ -126,6 +141,11 @@ class HashtablezInfoHandle {
     RecordStorageChangedSlow(info_, size, capacity);
   }
 
+  inline void RecordRehash(size_t total_probe_length) {
+    if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
+    RecordRehashSlow(info_, total_probe_length);
+  }
+
   inline void RecordInsert(size_t hash, size_t distance_from_desired) {
     if (ABSL_PREDICT_TRUE(info_ == nullptr)) return;
     RecordInsertSlow(info_, hash, distance_from_desired);
diff --git a/absl/container/internal/hashtablez_sampler_test.cc b/absl/container/internal/hashtablez_sampler_test.cc
index f9ee941a015c..a6e4ac815749 100644
--- a/absl/container/internal/hashtablez_sampler_test.cc
+++ b/absl/container/internal/hashtablez_sampler_test.cc
@@ -145,6 +145,29 @@ TEST(HashtablezInfoTest, RecordErase) {
   EXPECT_EQ(info.num_erases.load(), 1);
 }
 
+TEST(HashtablezInfoTest, RecordRehash) {
+  HashtablezInfo info;
+  absl::MutexLock l(&info.init_mu);
+  info.PrepareForSampling();
+  RecordInsertSlow(&info, 0x1, 0);
+  RecordInsertSlow(&info, 0x2, kProbeLength);
+  RecordInsertSlow(&info, 0x4, kProbeLength);
+  RecordInsertSlow(&info, 0x8, 2 * kProbeLength);
+  EXPECT_EQ(info.size.load(), 4);
+  EXPECT_EQ(info.total_probe_length.load(), 4);
+
+  RecordEraseSlow(&info);
+  RecordEraseSlow(&info);
+  EXPECT_EQ(info.size.load(), 2);
+  EXPECT_EQ(info.total_probe_length.load(), 4);
+  EXPECT_EQ(info.num_erases.load(), 2);
+
+  RecordRehashSlow(&info, 3 * kProbeLength);
+  EXPECT_EQ(info.size.load(), 2);
+  EXPECT_EQ(info.total_probe_length.load(), 3);
+  EXPECT_EQ(info.num_erases.load(), 0);
+}
+
 TEST(HashtablezSamplerTest, SmallSampleParameter) {
   SetHashtablezEnabled(true);
   SetHashtablezSampleParameter(100);
diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index a5d0cae00bfb..33f8f8fac527 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -936,7 +936,7 @@ class raw_hash_set {
       reset_growth_left();
     }
     assert(empty());
-    infoz_.RecordStorageChanged(size_, capacity_);
+    infoz_.RecordStorageChanged(0, capacity_);
   }
 
   // This overload kicks in when the argument is an rvalue of insertable and
@@ -1226,7 +1226,7 @@ class raw_hash_set {
     if (n == 0 && capacity_ == 0) return;
     if (n == 0 && size_ == 0) {
       destroy_slots();
-      infoz_.RecordStorageChanged(size_, capacity_);
+      infoz_.RecordStorageChanged(0, 0);
       return;
     }
     // bitor is a faster way of doing `max` here. We will round up to the next
@@ -1483,11 +1483,14 @@ class raw_hash_set {
     capacity_ = new_capacity;
     initialize_slots();
 
+    size_t total_probe_length = 0;
     for (size_t i = 0; i != old_capacity; ++i) {
       if (IsFull(old_ctrl[i])) {
         size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
                                           PolicyTraits::element(old_slots + i));
-        size_t new_i = find_first_non_full(hash).offset;
+        auto target = find_first_non_full(hash);
+        size_t new_i = target.offset;
+        total_probe_length += target.probe_length;
         set_ctrl(new_i, H2(hash));
         PolicyTraits::transfer(&alloc_ref(), slots_ + new_i, old_slots + i);
       }
@@ -1499,6 +1502,7 @@ class raw_hash_set {
       Deallocate<Layout::Alignment()>(&alloc_ref(), old_ctrl,
                                       layout.AllocSize());
     }
+    infoz_.RecordRehash(total_probe_length);
   }
 
   void drop_deletes_without_resize() ABSL_ATTRIBUTE_NOINLINE {
@@ -1522,12 +1526,15 @@ class raw_hash_set {
     ConvertDeletedToEmptyAndFullToDeleted(ctrl_, capacity_);
     typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
         raw;
+    size_t total_probe_length = 0;
     slot_type* slot = reinterpret_cast<slot_type*>(&raw);
     for (size_t i = 0; i != capacity_; ++i) {
       if (!IsDeleted(ctrl_[i])) continue;
       size_t hash = PolicyTraits::apply(HashElement{hash_ref()},
                                         PolicyTraits::element(slots_ + i));
-      size_t new_i = find_first_non_full(hash).offset;
+      auto target = find_first_non_full(hash);
+      size_t new_i = target.offset;
+      total_probe_length += target.probe_length;
 
       // Verify if the old and new i fall within the same group wrt the hash.
       // If they do, we don't need to move the object as it falls already in the
@@ -1560,6 +1567,7 @@ class raw_hash_set {
       }
     }
     reset_growth_left();
+    infoz_.RecordRehash(total_probe_length);
   }
 
   void rehash_and_grow_if_necessary() {
diff --git a/absl/copts/AbseilConfigureCopts.cmake b/absl/copts/AbseilConfigureCopts.cmake
index 6fde7754d109..f68895d917fd 100644
--- a/absl/copts/AbseilConfigureCopts.cmake
+++ b/absl/copts/AbseilConfigureCopts.cmake
@@ -1,6 +1,9 @@
 # See absl/copts/copts.py and absl/copts/generate_copts.py
 include(GENERATED_AbseilCopts)
 
+set(ABSL_LSAN_LINKOPTS "")
+set(ABSL_HAVE_LSAN OFF)
+
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   set(ABSL_DEFAULT_COPTS "${GCC_FLAGS}")
   set(ABSL_TEST_COPTS "${GCC_FLAGS};${GCC_TEST_FLAGS}")
@@ -10,6 +13,14 @@ elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
   set(ABSL_DEFAULT_COPTS "${LLVM_FLAGS}")
   set(ABSL_TEST_COPTS "${LLVM_FLAGS};${LLVM_TEST_FLAGS}")
   set(ABSL_EXCEPTIONS_FLAG "${LLVM_EXCEPTIONS_FLAGS}")
+  if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+    # AppleClang doesn't have lsan
+    # https://developer.apple.com/documentation/code_diagnostics
+    if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 3.5)
+      set(ABSL_LSAN_LINKOPTS "-fsanitize=leak")
+      set(ABSL_HAVE_LSAN ON)
+    endif()
+  endif()
 elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   set(ABSL_DEFAULT_COPTS "${MSVC_FLAGS}")
   set(ABSL_TEST_COPTS "${MSVC_FLAGS};${MSVC_TEST_FLAGS}")
diff --git a/absl/debugging/CMakeLists.txt b/absl/debugging/CMakeLists.txt
index 4c1fc50871b6..345115217ce0 100644
--- a/absl/debugging/CMakeLists.txt
+++ b/absl/debugging/CMakeLists.txt
@@ -199,11 +199,6 @@ absl_cc_library(
   PUBLIC
 )
 
-# TODO(cohenjon) Move into the copts code.
-if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-  set(ABSL_LSAN_LINKOPTS "-fsanitize=leak")
-endif()
-
 absl_cc_library(
   NAME
     leak_check_api_enabled_for_testing
@@ -212,7 +207,7 @@ absl_cc_library(
   SRCS
     "leak_check.cc"
   COPTS
-    $<$<BOOL:${ABSL_USING_CLANG}>:-DLEAK_SANITIZER>
+    $<$<BOOL:${ABSL_HAVE_LSAN}>:-DLEAK_SANITIZER>
   TESTONLY
 )
 
@@ -234,7 +229,7 @@ absl_cc_test(
   SRCS
     "leak_check_test.cc"
   COPTS
-    "$<$<CXX_COMPILER_ID:Clang>:-DABSL_EXPECT_LEAK_SANITIZER>"
+    "$<$<BOOL:${ABSL_HAVE_LSAN}>:-DABSL_EXPECT_LEAK_SANITIZER>"
   LINKOPTS
     "${ABSL_LSAN_LINKOPTS}"
   DEPS
diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index a5af93ad67b4..2c9e46b76223 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -135,6 +135,36 @@ TEST(HashValueTest, Pointer) {
       std::make_tuple(&i, ptr, nullptr, ptr + 1, n)));
 }
 
+TEST(HashValueTest, PointerAlignment) {
+  // We want to make sure that pointer alignment will not cause bits to be
+  // stuck.
+
+  constexpr size_t kTotalSize = 1 << 20;
+  std::unique_ptr<char[]> data(new char[kTotalSize]);
+  constexpr size_t kLog2NumValues = 5;
+  constexpr size_t kNumValues = 1 << kLog2NumValues;
+
+  for (size_t align = 1; align < kTotalSize / kNumValues;
+       align < 8 ? align += 1 : align < 1024 ? align += 8 : align += 32) {
+    SCOPED_TRACE(align);
+    ASSERT_LE(align * kNumValues, kTotalSize);
+
+    size_t bits_or = 0;
+    size_t bits_and = ~size_t{};
+
+    for (size_t i = 0; i < kNumValues; ++i) {
+      size_t hash = absl::Hash<void*>()(data.get() + i * align);
+      bits_or |= hash;
+      bits_and &= hash;
+    }
+
+    // Limit the scope to the bits we would be using for Swisstable.
+    constexpr size_t kMask = (1 << (kLog2NumValues + 7)) - 1;
+    size_t stuck_bits = (~bits_or | bits_and) & kMask;
+    EXPECT_EQ(stuck_bits, 0) << "0x" << std::hex << stuck_bits;
+  }
+}
+
 // TODO(EricWF): MSVC 15 has a bug that causes it to incorrectly evaluate the
 // SFINAE in internal/hash.h, causing this test to fail.
 #if !defined(_MSC_VER)
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index ba6d7468e1b8..bd07677a796d 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -264,7 +264,12 @@ H AbslHashValue(H hash_state, long double value) {
 // AbslHashValue() for hashing pointers
 template <typename H, typename T>
 H AbslHashValue(H hash_state, T* ptr) {
-  return hash_internal::hash_bytes(std::move(hash_state), ptr);
+  auto v = reinterpret_cast<uintptr_t>(ptr);
+  // Due to alignment, pointers tend to have low bits as zero, and the next few
+  // bits follow a pattern since they are also multiples of some base value.
+  // Mixing the pointer twice helps prevent stuck low bits for certain alignment
+  // values.
+  return H::combine(std::move(hash_state), v, v);
 }
 
 // AbslHashValue() for hashing nullptr_t
diff --git a/absl/memory/memory.h b/absl/memory/memory.h
index 8bf4fe82adbf..75506a742dc7 100644
--- a/absl/memory/memory.h
+++ b/absl/memory/memory.h
@@ -646,7 +646,7 @@ struct allocator_is_nothrow
     : memory_internal::ExtractOrT<memory_internal::GetIsNothrow, Alloc,
                                   std::false_type> {};
 
-#if ABSL_ALLOCATOR_NOTHROW
+#if defined(ABSL_ALLOCATOR_NOTHROW) && ABSL_ALLOCATOR_NOTHROW
 template <typename T>
 struct allocator_is_nothrow<std::allocator<T>> : std::true_type {};
 struct default_allocator_is_nothrow : std::true_type {};
diff --git a/absl/strings/match.h b/absl/strings/match.h
index 6e8ed10fc50f..3a4fefd9bef8 100644
--- a/absl/strings/match.h
+++ b/absl/strings/match.h
@@ -74,13 +74,13 @@ bool EqualsIgnoreCase(absl::string_view piece1, absl::string_view piece2);
 
 // StartsWithIgnoreCase()
 //
-// Returns whether a given ASCII string `text` starts with `starts_with`,
+// Returns whether a given ASCII string `text` starts with `prefix`,
 // ignoring case in the comparison.
 bool StartsWithIgnoreCase(absl::string_view text, absl::string_view prefix);
 
 // EndsWithIgnoreCase()
 //
-// Returns whether a given ASCII string `text` ends with `ends_with`, ignoring
+// Returns whether a given ASCII string `text` ends with `suffix`, ignoring
 // case in the comparison.
 bool EndsWithIgnoreCase(absl::string_view text, absl::string_view suffix);
 
diff --git a/absl/strings/str_split.h b/absl/strings/str_split.h
index 485f24354d1a..86effd300a83 100644
--- a/absl/strings/str_split.h
+++ b/absl/strings/str_split.h
@@ -72,22 +72,23 @@ namespace absl {
 //   - `MaxSplits`
 //
 //
-// A Delimiter's Find() member function will be passed the input text that is to
-// be split and the position to begin searching for the next delimiter in the
-// input text. The returned absl::string_view should refer to the next
-// occurrence (after pos) of the represented delimiter; this returned
-// absl::string_view represents the next location where the input string should
-// be broken. The returned absl::string_view may be zero-length if the Delimiter
-// does not represent a part of the string (e.g., a fixed-length delimiter). If
-// no delimiter is found in the given text, a zero-length absl::string_view
-// referring to text.end() should be returned (e.g.,
-// absl::string_view(text.end(), 0)). It is important that the returned
-// absl::string_view always be within the bounds of input text given as an
+// A Delimiter's `Find()` member function will be passed an input `text` that is
+// to be split and a position (`pos`) to begin searching for the next delimiter
+// in `text`. The returned absl::string_view should refer to the next occurrence
+// (after `pos`) of the represented delimiter; this returned absl::string_view
+// represents the next location where the input `text` should be broken.
+//
+// The returned absl::string_view may be zero-length if the Delimiter does not
+// represent a part of the string (e.g., a fixed-length delimiter). If no
+// delimiter is found in the input `text`, a zero-length absl::string_view
+// referring to `text.end()` should be returned (e.g.,
+// `text.substr(text.size())`). It is important that the returned
+// absl::string_view always be within the bounds of the input `text` given as an
 // argument--it must not refer to a string that is physically located outside of
 // the given string.
 //
 // The following example is a simple Delimiter object that is created with a
-// single char and will look for that char in the text passed to the Find()
+// single char and will look for that char in the text passed to the `Find()`
 // function:
 //
 //   struct SimpleDelimiter {
diff --git a/absl/utility/utility.h b/absl/utility/utility.h
index aef4baa02dcd..104ec82939d6 100644
--- a/absl/utility/utility.h
+++ b/absl/utility/utility.h
@@ -234,25 +234,33 @@ auto apply_helper(Functor&& functor, Tuple&& t, index_sequence<Indexes...>)
 //
 // Example:
 //
-//   class Foo{void Bar(int);};
-//   void user_function(int, string);
-//   void user_function(std::unique_ptr<Foo>);
+//   class Foo {
+//    public:
+//     void Bar(int);
+//   };
+//   void user_function1(int, string);
+//   void user_function2(std::unique_ptr<Foo>);
+//   auto user_lambda = [](int, int) {};
 //
 //   int main()
 //   {
 //       std::tuple<int, string> tuple1(42, "bar");
-//       // Invokes the user function overload on int, string.
-//       absl::apply(&user_function, tuple1);
+//       // Invokes the first user function on int, string.
+//       absl::apply(&user_function1, tuple1);
 //
-//       auto foo = absl::make_unique<Foo>();
-//       std::tuple<Foo*, int> tuple2(foo.get(), 42);
-//       // Invokes the method Bar on foo with one argument 42.
-//       absl::apply(&Foo::Bar, foo.get(), 42);
-//
-//       std::tuple<std::unique_ptr<Foo>> tuple3(absl::make_unique<Foo>());
+//       std::tuple<std::unique_ptr<Foo>> tuple2(absl::make_unique<Foo>());
 //       // Invokes the user function that takes ownership of the unique
 //       // pointer.
-//       absl::apply(&user_function, std::move(tuple));
+//       absl::apply(&user_function2, std::move(tuple2));
+//
+//       auto foo = absl::make_unique<Foo>();
+//       std::tuple<Foo*, int> tuple3(foo.get(), 42);
+//       // Invokes the method Bar on foo with one argument, 42.
+//       absl::apply(&Foo::Bar, tuple3);
+//
+//       std::tuple<int, int> tuple4(8, 9);
+//       // Invokes a lambda.
+//       absl::apply(user_lambda, tuple4);
 //   }
 template <typename Functor, typename Tuple>
 auto apply(Functor&& functor, Tuple&& t)