Export of internal Abseil changes

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4324716dc5384f03dcd7e36e8cca0e944e4dac74 by Evan Brown <ezb@google.com>:

Clarify comments about API differences from std::{set,map} in btree headers.

We note that the most important API differences are mentioned in the next paragraph and that other API differences are minor. An example of another minor API difference is the note about std::launder in the comment for `extract`.

Motivation: readers shouldn't feel like they need to read the entire header file to understand why b-tree containers are not "drop-in replacements" for STL containers.
PiperOrigin-RevId: 408703780

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7e8da4f14afd25d11713eee6b743ba31605332bf by Derek Mauro <dmauro@google.com>:

Remove the test for absl::base_internal::NominalCPUFrequency() from OSS code

This is an internal-only function that should never by called by OSS code.
By its nature fails on unsupported platforms.
Google code has tests for this function on supported internal platforms.

Fixes #1053

PiperOrigin-RevId: 408692861

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37bad2e003b17e3f82d6fd5d90ae183553c018b0 by Abseil Team <absl-team@google.com>:

To avoid triggering -Wmissing-variable-declarations warnings, ABSL_FLAG now
declares the Flag before defining it.

PiperOrigin-RevId: 408673990
GitOrigin-RevId: 4324716dc5384f03dcd7e36e8cca0e944e4dac74
Change-Id: I8c8ea73c4a4e38729c5bfdfa3fefb5d593e0536c

4 files changed, 9 insertions, 26 deletions

diff --git a/absl/base/internal/sysinfo_test.cc b/absl/base/internal/sysinfo_test.cc
index 5f9e45f6..f305b6c5 100644
--- a/absl/base/internal/sysinfo_test.cc
+++ b/absl/base/internal/sysinfo_test.cc
@@ -37,29 +37,6 @@ TEST(SysinfoTest, NumCPUs) {
       << "NumCPUs() should not have the default value of 0";
 }
 
-// Ensure that NominalCPUFrequency returns a reasonable value, or 1.00 on
-// platforms where the CPU frequency is not available through sysfs.
-//
-// POWER is particularly problematic here; some Linux kernels expose the CPU
-// frequency, while others do not. Since we can't predict a priori what a given
-// machine is going to do, just disable this test on POWER on Linux.
-#if !(defined(__linux) && (defined(__ppc64__) || defined(__PPC64__)))
-TEST(SysinfoTest, NominalCPUFrequency) {
-  // Linux only exposes the CPU frequency on certain architectures, and
-  // Emscripten doesn't expose it at all.
-#if defined(__linux__) &&                                                  \
-        (defined(__aarch64__) || defined(__hppa__) || defined(__mips__) || \
-         defined(__riscv) || defined(__s390x__)) ||                        \
-    defined(__EMSCRIPTEN__)
-  EXPECT_EQ(NominalCPUFrequency(), 1.0)
-      << "CPU frequency detection was fixed! Please update unittest.";
-#else
-  EXPECT_GE(NominalCPUFrequency(), 1000.0)
-      << "NominalCPUFrequency() did not return a reasonable value";
-#endif
-}
-#endif
-
 TEST(SysinfoTest, GetTID) {
   EXPECT_EQ(GetTID(), GetTID());  // Basic compile and equality test.
 #ifdef __native_client__
diff --git a/absl/container/btree_map.h b/absl/container/btree_map.h
index f0a8d4a6..96ebcb77 100644
--- a/absl/container/btree_map.h
+++ b/absl/container/btree_map.h
@@ -35,14 +35,17 @@
 //
 // However, these types should not be considered drop-in replacements for
 // `std::map` and `std::multimap` as there are some API differences, which are
-// noted in this header file.
+// noted in this header file. The most consequential differences with respect to
+// migrating to b-tree from the STL types are listed in the next paragraph.
+// Other API differences are minor.
 //
 // Importantly, insertions and deletions may invalidate outstanding iterators,
 // pointers, and references to elements. Such invalidations are typically only
 // an issue if insertion and deletion operations are interleaved with the use of
 // more than one iterator, pointer, or reference simultaneously. For this
 // reason, `insert()` and `erase()` return a valid iterator at the current
-// position.
+// position. Another important difference is that key-types must be
+// copy-constructible.
 
 #ifndef ABSL_CONTAINER_BTREE_MAP_H_
 #define ABSL_CONTAINER_BTREE_MAP_H_
diff --git a/absl/container/btree_set.h b/absl/container/btree_set.h
index 89739006..f587ca22 100644
--- a/absl/container/btree_set.h
+++ b/absl/container/btree_set.h
@@ -35,7 +35,9 @@
 //
 // However, these types should not be considered drop-in replacements for
 // `std::set` and `std::multiset` as there are some API differences, which are
-// noted in this header file.
+// noted in this header file. The most consequential differences with respect to
+// migrating to b-tree from the STL types are listed in the next paragraph.
+// Other API differences are minor.
 //
 // Importantly, insertions and deletions may invalidate outstanding iterators,
 // pointers, and references to elements. Such invalidations are typically only
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index a724ccc9..50106082 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -163,6 +163,7 @@ ABSL_NAMESPACE_END
 // Note: do not construct objects of type `absl::Flag<T>` directly. Only use the
 // `ABSL_FLAG()` macro for such construction.
 #define ABSL_FLAG(Type, name, default_value, help) \
+  extern ::absl::Flag<Type> FLAGS_##name;          \
   ABSL_FLAG_IMPL(Type, name, default_value, help)
 
 // ABSL_FLAG().OnUpdate()