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

1 files changed, 42 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_