Changes imported from Abseil "staging" branch:

  - 0320844dcd8219965647f86db7c6c7296e40e41e Introduce internal macros for inline variable emulation s... by Matt Calabrese <calabrese@google.com>
  - 85197d0105805e6aab3a0a648351cb1582a61be0 Internal change. by Alex Strelnikov <strel@google.com>

GitOrigin-RevId: 0320844dcd8219965647f86db7c6c7296e40e41e
Change-Id: I7dce37f6839ff099462951059a9047c5c9377638

7 files changed, 301 insertions, 0 deletions

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 1778f4f8..9bb7bfa1 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -128,6 +128,7 @@ cc_library(
     name = "base_internal",
     hdrs = [
         "internal/identity.h",
+        "internal/inline_variable.h",
         "internal/invoke.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
@@ -258,6 +259,22 @@ cc_test(
 )
 
 cc_test(
+    name = "inline_variable_test",
+    size = "small",
+    srcs = [
+        "inline_variable_test.cc",
+        "inline_variable_test_a.cc",
+        "inline_variable_test_b.cc",
+        "internal/inline_variable_testing.h",
+    ],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":base_internal",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
     name = "invoke_test",
     size = "small",
     srcs = ["invoke_test.cc"],
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index d3066163..9d2de55f 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -37,6 +37,7 @@ list(APPEND BASE_INTERNAL_HEADERS
   "internal/exception_safety_testing.h"
   "internal/identity.h"
   "internal/invoke.h"
+  "internal/inline_variable.h"
   "internal/low_level_alloc.h"
   "internal/low_level_scheduling.h"
   "internal/malloc_extension.h"
@@ -214,6 +215,26 @@ absl_test(
 )
 
 
+# test inline_variable_test
+list(APPEND INLINE_VARIABLE_TEST_SRC
+  "internal/inline_variable_testing.h"
+  "inline_variable_test.cc"
+  "inline_variable_test_a.cc"
+  "inline_variable_test_b.cc"
+)
+
+set(INLINE_VARIABLE_TEST_PUBLIC_LIBRARIES absl::base)
+
+absl_test(
+  TARGET
+    inline_variable_test
+  SOURCES
+    ${INLINE_VARIABLE_TEST_SRC}
+  PUBLIC_LIBRARIES
+    ${INLINE_VARIABLE_TEST_PUBLIC_LIBRARIES}
+)
+
+
 # test spinlock_test_common
 set(SPINLOCK_TEST_COMMON_SRC "spinlock_test_common.cc")
 set(SPINLOCK_TEST_COMMON_PUBLIC_LIBRARIES absl::base absl::synchronization)
diff --git a/absl/base/inline_variable_test.cc b/absl/base/inline_variable_test.cc
new file mode 100644
index 00000000..5499189a
--- /dev/null
+++ b/absl/base/inline_variable_test.cc
@@ -0,0 +1,62 @@
+// Copyright 2017 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
+//
+//      http://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 <type_traits>
+
+#include "absl/base/internal/inline_variable.h"
+#include "absl/base/internal/inline_variable_testing.h"
+
+#include "gtest/gtest.h"
+
+namespace absl {
+namespace inline_variable_testing_internal {
+namespace {
+
+TEST(InlineVariableTest, Constexpr) {
+  static_assert(inline_variable_foo.value == 5, "");
+  static_assert(other_inline_variable_foo.value == 5, "");
+  static_assert(inline_variable_int == 5, "");
+  static_assert(other_inline_variable_int == 5, "");
+}
+
+TEST(InlineVariableTest, DefaultConstructedIdentityEquality) {
+  EXPECT_EQ(get_foo_a().value, 5);
+  EXPECT_EQ(get_foo_b().value, 5);
+  EXPECT_EQ(&get_foo_a(), &get_foo_b());
+}
+
+TEST(InlineVariableTest, DefaultConstructedIdentityInequality) {
+  EXPECT_NE(&inline_variable_foo, &other_inline_variable_foo);
+}
+
+TEST(InlineVariableTest, InitializedIdentityEquality) {
+  EXPECT_EQ(get_int_a(), 5);
+  EXPECT_EQ(get_int_b(), 5);
+  EXPECT_EQ(&get_int_a(), &get_int_b());
+}
+
+TEST(InlineVariableTest, InitializedIdentityInequality) {
+  EXPECT_NE(&inline_variable_int, &other_inline_variable_int);
+}
+
+TEST(InlineVariableTest, FunPtrType) {
+  static_assert(
+      std::is_same<void(*)(),
+                   std::decay<decltype(inline_variable_fun_ptr)>::type>::value,
+      "");
+}
+
+}  // namespace
+}  // namespace inline_variable_testing_internal
+}  // namespace absl
diff --git a/absl/base/inline_variable_test_a.cc b/absl/base/inline_variable_test_a.cc
new file mode 100644
index 00000000..a3bf3b68
--- /dev/null
+++ b/absl/base/inline_variable_test_a.cc
@@ -0,0 +1,25 @@
+// Copyright 2017 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
+//
+//      http://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/inline_variable_testing.h"
+
+namespace absl {
+namespace inline_variable_testing_internal {
+
+const Foo& get_foo_a() { return inline_variable_foo; }
+
+const int& get_int_a() { return inline_variable_int; }
+
+}  // namespace inline_variable_testing_internal
+}  // namespace absl
diff --git a/absl/base/inline_variable_test_b.cc b/absl/base/inline_variable_test_b.cc
new file mode 100644
index 00000000..b4b9393a
--- /dev/null
+++ b/absl/base/inline_variable_test_b.cc
@@ -0,0 +1,25 @@
+// Copyright 2017 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
+//
+//      http://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/inline_variable_testing.h"
+
+namespace absl {
+namespace inline_variable_testing_internal {
+
+const Foo& get_foo_b() { return inline_variable_foo; }
+
+const int& get_int_b() { return inline_variable_int; }
+
+}  // namespace inline_variable_testing_internal
+}  // namespace absl
diff --git a/absl/base/internal/inline_variable.h b/absl/base/internal/inline_variable.h
new file mode 100644
index 00000000..f7bb8c56
--- /dev/null
+++ b/absl/base/internal/inline_variable.h
@@ -0,0 +1,107 @@
+// Copyright 2017 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
+//
+//      http://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_INLINE_VARIABLE_EMULATION_H_
+#define ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
+
+#include <type_traits>
+
+#include "absl/base/internal/identity.h"
+
+// File:
+//   This file define a macro that allows the creation of or emulation of C++17
+//   inline variables based on whether or not the feature is supported.
+
+////////////////////////////////////////////////////////////////////////////////
+// Macro: ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init)
+//
+// Description:
+//   Expands to the equivalent of an inline constexpr instance of the specified
+//   `type` and `name`, initialized to the value `init`. If the compiler being
+//   used is detected as supporting actual inline variables as a language
+//   feature, then the macro expands to an actual inline variable definition.
+//
+// Requires:
+//   `type` is a type that is usable in an extern variable declaration.
+//
+// Requires: `name` is a valid identifier
+//
+// Requires:
+//   `init` is an expression that can be used in the following definition:
+//     constexpr type name = init;
+//
+// Usage:
+//
+//   // Equivalent to: `inline constexpr size_t variant_npos = -1;`
+//   ABSL_INTERNAL_INLINE_CONSTEXPR(size_t, variant_npos, -1);
+//
+// Differences in implementation:
+//   For a direct, language-level inline variable, decltype(name) will be the
+//   type that was specified along with const qualification, whereas for
+//   emulated inline variables, decltype(name) may be different (in practice
+//   it will likely be a reference type).
+////////////////////////////////////////////////////////////////////////////////
+
+#ifdef __cpp_inline_variables
+
+// Clang's -Wmissing-variable-declarations option erroneously warned that
+// inline constexpr objects need to be pre-declared. This has now been fixed,
+// but we will need to support this workaround for people building with older
+// versions of clang.
+//
+// Bug: https://bugs.llvm.org/show_bug.cgi?id=35862
+//
+// Note:
+//   identity_t is used here so that the const and name are in the
+//   appropriate place for pointer types, reference types, function pointer
+//   types, etc..
+#if defined(__clang__)
+#define ABSL_INTERNAL_EXTERN_DECL(type, name) \
+  extern const ::absl::internal::identity_t<type> name;
+#else  // Otherwise, just define the macro to do nothing.
+#define ABSL_INTERNAL_EXTERN_DECL(type, name)
+#endif  // defined(__clang__)
+
+// See above comment at top of file for details.
+#define ABSL_INTERNAL_INLINE_CONSTEXPR(type, name, init) \
+  ABSL_INTERNAL_EXTERN_DECL(type, name)                  \
+  inline constexpr ::absl::internal::identity_t<type> name = init
+
+#else
+
+// See above comment at top of file for details.
+//
+// Note:
+//   identity_t is used here so that the const and name are in the
+//   appropriate place for pointer types, reference types, function pointer
+//   types, etc..
+#define ABSL_INTERNAL_INLINE_CONSTEXPR(var_type, name, init)                  \
+  template <class /*AbslInternalDummy*/ = void>                               \
+  struct AbslInternalInlineVariableHolder##name {                             \
+    static constexpr ::absl::internal::identity_t<var_type> kInstance = init; \
+  };                                                                          \
+                                                                              \
+  template <class AbslInternalDummy>                                          \
+  constexpr ::absl::internal::identity_t<var_type>                            \
+      AbslInternalInlineVariableHolder##name<AbslInternalDummy>::kInstance;   \
+                                                                              \
+  static constexpr const ::absl::internal::identity_t<var_type>&              \
+      name = /* NOLINT */                                                     \
+      AbslInternalInlineVariableHolder##name<>::kInstance;                    \
+  static_assert(sizeof(void (*)(decltype(name))) != 0,                        \
+                "Silence unused variable warnings.")
+
+#endif  // __cpp_inline_variables
+
+#endif  // ABSL_BASE_INTERNAL_INLINE_VARIABLE_EMULATION_H_
diff --git a/absl/base/internal/inline_variable_testing.h b/absl/base/internal/inline_variable_testing.h
new file mode 100644
index 00000000..a0dd2bb2
--- /dev/null
+++ b/absl/base/internal/inline_variable_testing.h
@@ -0,0 +1,44 @@
+// Copyright 2017 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
+//
+//      http://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_INLINE_VARIABLE_TESTING_H_
+#define ABSL_BASE_INLINE_VARIABLE_TESTING_H_
+
+#include "absl/base/internal/inline_variable.h"
+
+namespace absl {
+namespace inline_variable_testing_internal {
+
+struct Foo {
+  int value = 5;
+};
+
+ABSL_INTERNAL_INLINE_CONSTEXPR(Foo, inline_variable_foo, {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(Foo, other_inline_variable_foo, {});
+
+ABSL_INTERNAL_INLINE_CONSTEXPR(int, inline_variable_int, 5);
+ABSL_INTERNAL_INLINE_CONSTEXPR(int, other_inline_variable_int, 5);
+
+ABSL_INTERNAL_INLINE_CONSTEXPR(void(*)(), inline_variable_fun_ptr, nullptr);
+
+const Foo& get_foo_a();
+const Foo& get_foo_b();
+
+const int& get_int_a();
+const int& get_int_b();
+
+}  // namespace inline_variable_testing_internal
+}  // namespace absl
+
+#endif  // ABSL_BASE_INLINE_VARIABLE_TESTING_H_