Fix C++17 constexpr storage deprecation warnings

This change introduces the symbol
ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
to guard redundant declarations of static constexpr data
members that are needed prior to C++17.

This change also introduces the symbol
ABSL_INTERNAL_CPLUSPLUS_LANG, which is supposed to be set
to the same value as __cplusplus, except it uses _MSVC_LANG
on MSVC so that the value is correct on MSVC.

Neither of these new symbols should be used outside of Abseil.

Fixes #1191

PiperOrigin-RevId: 453923908
Change-Id: I1316c52c19fa0c168b93cced0c817e4cb7c9c862

5 files changed, 53 insertions, 1 deletions

diff --git a/absl/base/config.h b/absl/base/config.h
index 58097b0d..94f7fcb5 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -56,6 +56,25 @@
 #include <cstddef>
 #endif  // __cplusplus
 
+// ABSL_INTERNAL_CPLUSPLUS_LANG
+//
+// MSVC does not set the value of __cplusplus correctly, but instead uses
+// _MSVC_LANG as a stand-in.
+// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+//
+// However, there are reports that MSVC even sets _MSVC_LANG incorrectly at
+// times, for example:
+// https://github.com/microsoft/vscode-cpptools/issues/1770
+// https://reviews.llvm.org/D70996
+//
+// For this reason, this symbol is considered INTERNAL and code outside of
+// Abseil must not use it.
+#if defined(_MSVC_LANG)
+#define ABSL_INTERNAL_CPLUSPLUS_LANG _MSVC_LANG
+#elif defined(__cplusplus)
+#define ABSL_INTERNAL_CPLUSPLUS_LANG __cplusplus
+#endif
+
 #if defined(__APPLE__)
 // Included for TARGET_OS_IPHONE, __IPHONE_OS_VERSION_MIN_REQUIRED,
 // __IPHONE_8_0.
@@ -807,6 +826,29 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_HAVE_CLASS_TEMPLATE_ARGUMENT_DEDUCTION 1
 #endif
 
+// ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+//
+// Prior to C++17, static constexpr variables defined in classes required a
+// separate definition outside of the class body, for example:
+//
+// class Foo {
+//   static constexpr int kBar = 0;
+// };
+// constexpr int Foo::kBar;
+//
+// In C++17, these variables defined in classes are considered inline variables,
+// and the extra declaration is redundant. Since some compilers warn on the
+// extra declarations, ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL can be used
+// conditionally ignore them:
+//
+// #ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+// constexpr int Foo::kBar;
+// #endif
+#if defined(ABSL_INTERNAL_CPLUSPLUS_LANG) && \
+    ABSL_INTERNAL_CPLUSPLUS_LANG < 201703L
+#define ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL 1
+#endif
+
 // `ABSL_INTERNAL_HAS_RTTI` determines whether abseil is being compiled with
 // RTTI support.
 #ifdef ABSL_INTERNAL_HAS_RTTI
@@ -868,5 +910,4 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
 #define ABSL_INTERNAL_HAVE_ARM_NEON 1
 #endif
 
-
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/exception_safety_testing_test.cc b/absl/base/exception_safety_testing_test.cc
index a59be29e..a87fd6a9 100644
--- a/absl/base/exception_safety_testing_test.cc
+++ b/absl/base/exception_safety_testing_test.cc
@@ -701,7 +701,10 @@ struct BasicGuaranteeWithExtraContracts : public NonNegative {
 
   static constexpr int kExceptionSentinel = 9999;
 };
+
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int BasicGuaranteeWithExtraContracts::kExceptionSentinel;
+#endif
 
 TEST(ExceptionCheckTest, BasicGuaranteeWithExtraContracts) {
   auto tester_with_val =
diff --git a/absl/base/internal/cycleclock.cc b/absl/base/internal/cycleclock.cc
index f6e64242..902e3f5e 100644
--- a/absl/base/internal/cycleclock.cc
+++ b/absl/base/internal/cycleclock.cc
@@ -26,6 +26,7 @@
 #include <chrono>  // NOLINT(build/c++11)
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/unscaledcycleclock.h"
 
 namespace absl {
@@ -34,8 +35,10 @@ namespace base_internal {
 
 #if ABSL_USE_UNSCALED_CYCLECLOCK
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int32_t CycleClock::kShift;
 constexpr double CycleClock::kFrequencyScale;
+#endif
 
 ABSL_CONST_INIT std::atomic<CycleClockSourceFunc>
     CycleClock::cycle_clock_source_{nullptr};
diff --git a/absl/base/internal/fast_type_id.h b/absl/base/internal/fast_type_id.h
index 3db59e83..a547b3a8 100644
--- a/absl/base/internal/fast_type_id.h
+++ b/absl/base/internal/fast_type_id.h
@@ -28,8 +28,10 @@ struct FastTypeTag {
   constexpr static char dummy_var = 0;
 };
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 template <typename Type>
 constexpr char FastTypeTag<Type>::dummy_var;
+#endif
 
 // FastTypeId<Type>() evaluates at compile/link-time to a unique pointer for the
 // passed-in type. These are meant to be good match for keys into maps or
diff --git a/absl/base/internal/spinlock.cc b/absl/base/internal/spinlock.cc
index 35c0696a..9b5ed6e4 100644
--- a/absl/base/internal/spinlock.cc
+++ b/absl/base/internal/spinlock.cc
@@ -19,6 +19,7 @@
 #include <limits>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/internal/cycleclock.h"
 #include "absl/base/internal/spinlock_wait.h"
@@ -66,12 +67,14 @@ void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
   submit_profile_data.Store(fn);
 }
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 // Static member variable definitions.
 constexpr uint32_t SpinLock::kSpinLockHeld;
 constexpr uint32_t SpinLock::kSpinLockCooperative;
 constexpr uint32_t SpinLock::kSpinLockDisabledScheduling;
 constexpr uint32_t SpinLock::kSpinLockSleeper;
 constexpr uint32_t SpinLock::kWaitTimeMask;
+#endif
 
 // Uncommon constructors.
 SpinLock::SpinLock(base_internal::SchedulingMode mode)