Export of internal Abseil changes

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dce086fa6e045bed433c664f95a6581b7ffb2d98 by Abseil Team <absl-team@google.com>:

fix spelling error "fulfil" -> "fulfill"

PiperOrigin-RevId: 339898016

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10faa4546b69d8a07f949bc03dd9671231232ad0 by Abseil Team <absl-team@google.com>:

Clarify comments on variant::emplace

PiperOrigin-RevId: 339897241

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0d02261a7a8b9c11e3a4e76dfedf4d95590e2419 by Derek Mauro <dmauro@google.com>:

Use thread_local for ThreadIdentity storage on Apple platforms
that support it

PiperOrigin-RevId: 339698124
GitOrigin-RevId: dce086fa6e045bed433c664f95a6581b7ffb2d98
Change-Id: I8c668b33c805fd5231ceb285652c80f312e5a33a

4 files changed, 18 insertions, 8 deletions

diff --git a/absl/algorithm/container.h b/absl/algorithm/container.h
index 09b91f74..6398438f 100644
--- a/absl/algorithm/container.h
+++ b/absl/algorithm/container.h
@@ -188,7 +188,7 @@ bool c_any_of(const C& c, Pred&& pred) {
 // c_none_of()
 //
 // Container-based version of the <algorithm> `std::none_of()` function to
-// test if no elements in a container fulfil a condition.
+// test if no elements in a container fulfill a condition.
 template <typename C, typename Pred>
 bool c_none_of(const C& c, Pred&& pred) {
   return std::none_of(container_algorithm_internal::c_begin(c),
diff --git a/absl/base/internal/thread_identity.cc b/absl/base/internal/thread_identity.cc
index d63a04ae..6ea010ed 100644
--- a/absl/base/internal/thread_identity.cc
+++ b/absl/base/internal/thread_identity.cc
@@ -23,6 +23,7 @@
 #include <cassert>
 #include <memory>
 
+#include "absl/base/attributes.h"
 #include "absl/base/call_once.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/spinlock.h"
@@ -53,9 +54,11 @@ void AllocateThreadIdentityKey(ThreadIdentityReclaimerFunction reclaimer) {
 // exist within a process (via dlopen() or similar), references to
 // thread_identity_ptr from each instance of the code will refer to
 // *different* instances of this ptr.
-#ifdef __GNUC__
+// Apple platforms have the visibility attribute, but issue a compile warning
+// that protected visibility is unsupported.
+#if ABSL_HAVE_ATTRIBUTE(visibility) && !defined(__APPLE__)
 __attribute__((visibility("protected")))
-#endif  // __GNUC__
+#endif  // ABSL_HAVE_ATTRIBUTE(visibility) && !defined(__APPLE__)
 #if ABSL_PER_THREAD_TLS
 // Prefer __thread to thread_local as benchmarks indicate it is a bit faster.
 ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr = nullptr;
diff --git a/absl/base/internal/thread_identity.h b/absl/base/internal/thread_identity.h
index c61d1feb..d2a65fd8 100644
--- a/absl/base/internal/thread_identity.h
+++ b/absl/base/internal/thread_identity.h
@@ -211,7 +211,9 @@ void ClearCurrentThreadIdentity();
 #define ABSL_THREAD_IDENTITY_MODE ABSL_FORCE_THREAD_IDENTITY_MODE
 #elif defined(_WIN32) && !defined(__MINGW32__)
 #define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_CPP11
-#elif ABSL_PER_THREAD_TLS && defined(__GOOGLE_GRTE_VERSION__) && \
+#elif defined(__APPLE__) && defined(ABSL_HAVE_THREAD_LOCAL)
+#define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_CPP11
+#elif ABSL_PER_THREAD_TLS && defined(__GOOGLE_GRTE_VERSION__) &&        \
     (__GOOGLE_GRTE_VERSION__ >= 20140228L)
 // Support for async-safe TLS was specifically added in GRTEv4.  It's not
 // present in the upstream eglibc.
diff --git a/absl/types/variant.h b/absl/types/variant.h
index 776d19a1..ac93464b 100644
--- a/absl/types/variant.h
+++ b/absl/types/variant.h
@@ -604,7 +604,10 @@ class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
 
   // emplace() Functions
 
-  // Constructs a value of the given alternative type T within the variant.
+  // Constructs a value of the given alternative type T within the variant. The
+  // existing value of the variant is destroyed first (provided that
+  // `absl::valueless_by_exception()` is false). Requires that T is unambiguous
+  // in the variant.
   //
   // Example:
   //
@@ -624,7 +627,9 @@ class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
   }
 
   // Constructs a value of the given alternative type T within the variant using
-  // an initializer list.
+  // an initializer list. The existing value of the variant is destroyed first
+  // (provided that `absl::valueless_by_exception()` is false). Requires that T
+  // is unambiguous in the variant.
   //
   // Example:
   //
@@ -643,7 +648,7 @@ class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
   }
 
   // Destroys the current value of the variant (provided that
-  // `absl::valueless_by_exception()` is false, and constructs a new value at
+  // `absl::valueless_by_exception()` is false) and constructs a new value at
   // the given index.
   //
   // Example:
@@ -662,7 +667,7 @@ class variant<T0, Tn...> : private variant_internal::VariantBase<T0, Tn...> {
   }
 
   // Destroys the current value of the variant (provided that
-  // `absl::valueless_by_exception()` is false, and constructs a new value at
+  // `absl::valueless_by_exception()` is false) and constructs a new value at
   // the given index using an initializer list and the provided arguments.
   //
   // Example: