Export of internal Abseil changes.

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5dc8d7504b7c11710b19365a6582c288c8992366 by Derek Mauro <dmauro@google.com>:

Fix constexpr Span::last under MSVC and add Span constexpr tests.

PiperOrigin-RevId: 237515952

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5ea8c146e653bbc49ff7e698699478242df7de35 by Derek Mauro <dmauro@google.com>:

Implement Span::first and Span::last from C++20.
https://github.com/abseil/abseil-cpp/pull/274

PiperOrigin-RevId: 237494399

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08db3417f1d8fe4556255d57a2f0df51b09bdd9a by Derek Mauro <dmauro@google.com>:

HTTPS in more URLs.

PiperOrigin-RevId: 237486823

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83ec63a7f8e47b62af619546f9f7b3bf72e74e86 by Derek Mauro <dmauro@google.com>:

Changed HTTP URLs to HTTPS where possible.
https://github.com/abseil/abseil-cpp/pull/270

PiperOrigin-RevId: 237445310

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220bf279c14cb31efa239500d1a70e0ac0c32e3c by Abseil Team <absl-team@google.com>:

Support parsing decltype(nullptr) as a type.

PiperOrigin-RevId: 237336739

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ced234bbe78f5d495c3f6f6a9c2e0a95f7c080a5 by Gennadiy Rozental <rogeeff@google.com>:

Introduce internal interface for setting environment variable value in scope

PiperOrigin-RevId: 237275806

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1f1acb4e294af24d9f7598e85163d5e1d9958ae9 by Samuel Benzaquen <sbenza@google.com>:

Avoid using aliases in the SFINAE expressions to make it more compatible with MSVC.
Turn on the tests in MSVC.

PiperOrigin-RevId: 237261456

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06cf7de6250a0572ef90fa1176f742ca0451ce71 by Derek Mauro <dmauro@google.com>:

Fix unused variable warning.

PiperOrigin-RevId: 237108006
GitOrigin-RevId: 5dc8d7504b7c11710b19365a6582c288c8992366
Change-Id: Ife5182c80942945c4e8700844c8febb482d6ad82

3 files changed, 219 insertions, 0 deletions

diff --git a/absl/base/internal/scoped_set_env.cc b/absl/base/internal/scoped_set_env.cc
new file mode 100644
index 000000000000..9b1641240cf8
--- /dev/null
+++ b/absl/base/internal/scoped_set_env.cc
@@ -0,0 +1,79 @@
+// Copyright 2019 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/internal/scoped_set_env.h"
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+#include <cstdlib>
+
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+namespace base_internal {
+
+namespace {
+
+#ifdef _WIN32
+const int kMaxEnvVarValueSize = 1024;
+#endif
+
+void SetEnvVar(const char* name, const char* value) {
+#ifdef _WIN32
+  SetEnvironmentVariable(name, value);
+#else
+  if (value == nullptr) {
+    ::unsetenv(name);
+  } else {
+    ::setenv(name, value, 1);
+  }
+#endif
+}
+
+}  // namespace
+
+ScopedSetEnv::ScopedSetEnv(const char* var_name, const char* new_value)
+    : var_name_(var_name), was_unset_(false) {
+#ifdef _WIN32
+  char buf[kMaxEnvVarValueSize];
+  auto get_res = GetEnvironmentVariable(var_name_.c_str(), buf, sizeof(buf));
+  ABSL_INTERNAL_CHECK(get_res < sizeof(buf), "value exceeds buffer size");
+
+  if (get_res == 0) {
+    was_unset_ = (GetLastError() == ERROR_ENVVAR_NOT_FOUND);
+  } else {
+    old_value_.assign(buf, get_res);
+  }
+
+  SetEnvironmentVariable(var_name_.c_str(), new_value);
+#else
+  const char* val = ::getenv(var_name_.c_str());
+  if (val == nullptr) {
+    was_unset_ = true;
+  } else {
+    old_value_ = val;
+  }
+#endif
+
+  SetEnvVar(var_name_.c_str(), new_value);
+}
+
+ScopedSetEnv::~ScopedSetEnv() {
+  SetEnvVar(var_name_.c_str(), was_unset_ ? nullptr : old_value_.c_str());
+}
+
+}  // namespace base_internal
+}  // namespace absl
diff --git a/absl/base/internal/scoped_set_env.h b/absl/base/internal/scoped_set_env.h
new file mode 100644
index 000000000000..855b22fd41fe
--- /dev/null
+++ b/absl/base/internal/scoped_set_env.h
@@ -0,0 +1,41 @@
+//
+// Copyright 2019 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.
+//
+
+#ifndef ABSL_BASE_INTERNAL_SCOPED_SET_ENV_H_
+#define ABSL_BASE_INTERNAL_SCOPED_SET_ENV_H_
+
+#include <string>
+
+namespace absl {
+namespace base_internal {
+
+class ScopedSetEnv {
+ public:
+  ScopedSetEnv(const char* var_name, const char* new_value);
+  ~ScopedSetEnv();
+
+ private:
+  std::string var_name_;
+  std::string old_value_;
+
+  // True if the environment variable was initially not set.
+  bool was_unset_;
+};
+
+}  // namespace base_internal
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_SCOPED_SET_ENV_H_
diff --git a/absl/base/internal/scoped_set_env_test.cc b/absl/base/internal/scoped_set_env_test.cc
new file mode 100644
index 000000000000..4bd68c48b944
--- /dev/null
+++ b/absl/base/internal/scoped_set_env_test.cc
@@ -0,0 +1,99 @@
+// Copyright 2019 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.
+
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+#include "gtest/gtest.h"
+#include "absl/base/internal/scoped_set_env.h"
+
+namespace {
+
+using absl::base_internal::ScopedSetEnv;
+
+std::string GetEnvVar(const char* name) {
+#ifdef _WIN32
+  char buf[1024];
+  auto get_res = GetEnvironmentVariable(name, buf, sizeof(buf));
+  if (get_res == sizeof(buf)) {
+    return "TOO_BIG";
+  }
+
+  if (get_res == 0) {
+    return "UNSET";
+  }
+
+  return std::string(buf, get_res);
+#else
+  const char* val = ::getenv(name);
+  if (val == nullptr) {
+    return "UNSET";
+  }
+
+  return val;
+#endif
+}
+
+TEST(ScopedSetEnvTest, SetNonExistingVarToString) {
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "UNSET");
+
+  {
+    ScopedSetEnv scoped_set("SCOPED_SET_ENV_TEST_VAR", "value");
+
+    EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "value");
+  }
+
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "UNSET");
+}
+
+TEST(ScopedSetEnvTest, SetNonExistingVarToNull) {
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "UNSET");
+
+  {
+    ScopedSetEnv scoped_set("SCOPED_SET_ENV_TEST_VAR", nullptr);
+
+    EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "UNSET");
+  }
+
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "UNSET");
+}
+
+TEST(ScopedSetEnvTest, SetExistingVarToString) {
+  ScopedSetEnv scoped_set("SCOPED_SET_ENV_TEST_VAR", "value");
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "value");
+
+  {
+    ScopedSetEnv scoped_set("SCOPED_SET_ENV_TEST_VAR", "new_value");
+
+    EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "new_value");
+  }
+
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "value");
+}
+
+TEST(ScopedSetEnvTest, SetExistingVarToNull) {
+  ScopedSetEnv scoped_set("SCOPED_SET_ENV_TEST_VAR", "value");
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "value");
+
+  {
+    ScopedSetEnv scoped_set("SCOPED_SET_ENV_TEST_VAR", nullptr);
+
+    EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "UNSET");
+  }
+
+  EXPECT_EQ(GetEnvVar("SCOPED_SET_ENV_TEST_VAR"), "value");
+}
+
+}  // namespace