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, 15 insertions, 8 deletions

diff --git a/absl/strings/internal/cord_rep_btree.cc b/absl/strings/internal/cord_rep_btree.cc
index 2b592b47..cacbf3da 100644
--- a/absl/strings/internal/cord_rep_btree.cc
+++ b/absl/strings/internal/cord_rep_btree.cc
@@ -33,7 +33,9 @@ namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace cord_internal {
 
-constexpr size_t CordRepBtree::kMaxCapacity;  // NOLINT: needed for c++ < c++17
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr size_t CordRepBtree::kMaxCapacity;
+#endif
 
 namespace {
 
diff --git a/absl/strings/internal/cord_rep_ring.cc b/absl/strings/internal/cord_rep_ring.cc
index db1f63fa..af2fc768 100644
--- a/absl/strings/internal/cord_rep_ring.cc
+++ b/absl/strings/internal/cord_rep_ring.cc
@@ -129,7 +129,9 @@ class CordRepRing::Filler {
   index_type pos_;
 };
 
-constexpr size_t CordRepRing::kMaxCapacity; // NOLINT: needed for c++11
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+constexpr size_t CordRepRing::kMaxCapacity;
+#endif
 
 bool CordRepRing::IsValid(std::ostream& output) const {
   if (capacity_ == 0) {
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
index c891d0ed..dac3fd8b 100644
--- a/absl/strings/internal/cordz_info.cc
+++ b/absl/strings/internal/cordz_info.cc
@@ -34,7 +34,9 @@ namespace cord_internal {
 
 using ::absl::base_internal::SpinLockHolder;
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 constexpr int CordzInfo::kMaxStackDepth;
+#endif
 
 ABSL_CONST_INIT CordzInfo::List CordzInfo::global_list_{absl::kConstInit};
 
diff --git a/absl/strings/internal/str_format/extension.cc b/absl/strings/internal/str_format/extension.cc
index 484f6ebf..f93153d5 100644
--- a/absl/strings/internal/str_format/extension.cc
+++ b/absl/strings/internal/str_format/extension.cc
@@ -33,6 +33,8 @@ std::string FlagsToString(Flags v) {
   return s;
 }
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+
 #define ABSL_INTERNAL_X_VAL(id) \
   constexpr absl::FormatConversionChar FormatConversionCharInternal::id;
 ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL, )
@@ -45,17 +47,14 @@ constexpr absl::FormatConversionChar FormatConversionCharInternal::kNone;
 ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
 #undef ABSL_INTERNAL_CHAR_SET_CASE
 
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kStar;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kIntegral;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kFloating;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kNumeric;
-// NOLINTNEXTLINE(readability-redundant-declaration)
 constexpr FormatConversionCharSet FormatConversionCharSetInternal::kPointer;
 
+#endif  // ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
+
 bool FormatSinkImpl::PutPaddedString(string_view value, int width,
                                      int precision, bool left) {
   size_t space_remaining = 0;
diff --git a/absl/strings/internal/string_constant.h b/absl/strings/internal/string_constant.h
index b358efdd..f68b17d7 100644
--- a/absl/strings/internal/string_constant.h
+++ b/absl/strings/internal/string_constant.h
@@ -50,8 +50,10 @@ struct StringConstant {
                 "The input string_view must point to constant data.");
 };
 
+#ifdef ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
 template <typename T>
-constexpr absl::string_view StringConstant<T>::value;  // NOLINT
+constexpr absl::string_view StringConstant<T>::value;
+#endif
 
 // Factory function for `StringConstant` instances.
 // It supports callables that have a constexpr default constructor and a