[libc++] Mark some variables as inline, per the spec

[ldionne]

Summary

This is really what D110243 should have done. These variables are
inline in the spec. We unfortunately need to support pre-C++17 since
some of these inline variables are in C++17 parts of the library that
we provide pre-C++17 as an extension.

[Mordante]

Thanks for working on this. LGTM, but I would like to bikeshed regarding the name.

libcxx/include/__config

889

I wonder should we use this instead? And just constexpr in the #else. For me that makes it easier to understand what the macro does.

[ldionne]

libcxx/include/__config

889

The problem is that we still want those variables to be constexpr even in pre C++17.

[Mordante]

libcxx/include/__config

889

Yes therefore I suggested "And just constexpr in the #else". With our attributes we often write the "ideal" version for a certain C++ version and a work-around for older versions.

[ldionne]

Address comments.

[Mordante]

LGTM!

libcxx/include/__config

891

I really like to comment!

[ldionne]

It turns out that this is more complicated than it seems. I am going to abandon this in favour of the stack of patches starting at D145422.

libcxx/include/__config

 #  endif
 
 #  if _LIBCPP_STD_VER > 17 || !defined(_LIBCPP_DISABLE_NODISCARD_EXT)
 #    define _LIBCPP_NODISCARD_AFTER_CXX17 _LIBCPP_NODISCARD
 #  else
 #    define _LIBCPP_NODISCARD_AFTER_CXX17
 #  endif
 
+#  if _LIBCPP_STD_VER >= 17
+#    define _LIBCPP_INLINE_CONSTEXPR_VAR inline constexpr
+#  else
+#    define _LIBCPP_INLINE_CONSTEXPR_VAR constexpr // This technically leads to ODR violations, but they are benign.
+#  endif
+
 #  if __has_attribute(__no_destroy__)
 #    define _LIBCPP_NO_DESTROY __attribute__((__no_destroy__))
 #  else
 #    define _LIBCPP_NO_DESTROY
 #  endif
 
 #  ifndef _LIBCPP_HAS_NO_ASAN
     extern "C" _LIBCPP_FUNC_VIS void

libcxx/include/__functional/bind.h

 _LIBCPP_FUNC_VIS extern const __ph<4>   _4;
 _LIBCPP_FUNC_VIS extern const __ph<5>   _5;
 _LIBCPP_FUNC_VIS extern const __ph<6>   _6;
 _LIBCPP_FUNC_VIS extern const __ph<7>   _7;
 _LIBCPP_FUNC_VIS extern const __ph<8>   _8;
 _LIBCPP_FUNC_VIS extern const __ph<9>   _9;
 _LIBCPP_FUNC_VIS extern const __ph<10> _10;
 #else
-/* inline */ constexpr __ph<1>   _1{};
-/* inline */ constexpr __ph<2>   _2{};
-/* inline */ constexpr __ph<3>   _3{};
-/* inline */ constexpr __ph<4>   _4{};
-/* inline */ constexpr __ph<5>   _5{};
-/* inline */ constexpr __ph<6>   _6{};
-/* inline */ constexpr __ph<7>   _7{};
-/* inline */ constexpr __ph<8>   _8{};
-/* inline */ constexpr __ph<9>   _9{};
-/* inline */ constexpr __ph<10> _10{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<1>   _1{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<2>   _2{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<3>   _3{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<4>   _4{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<5>   _5{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<6>   _6{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<7>   _7{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<8>   _8{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<9>   _9{};
+_LIBCPP_INLINE_CONSTEXPR_VAR __ph<10> _10{};
 #endif // defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 
 } // namespace placeholders
 
 template<int _Np>
 struct is_placeholder<placeholders::__ph<_Np> >
     : public integral_constant<int, _Np> {};
 

libcxx/include/__memory/allocator_arg_t.h

 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct _LIBCPP_TEMPLATE_VIS allocator_arg_t { explicit allocator_arg_t() = default; };
 
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 extern _LIBCPP_EXPORTED_FROM_ABI const allocator_arg_t allocator_arg;
 #else
-/* inline */ constexpr allocator_arg_t allocator_arg = allocator_arg_t();
+_LIBCPP_INLINE_CONSTEXPR_VAR allocator_arg_t allocator_arg = allocator_arg_t();
 #endif
 
 #ifndef _LIBCPP_CXX03_LANG
 
 // allocator construction
 
 template <class _Tp, class _Alloc, class ..._Args>
 struct __uses_alloc_ctor_imp

libcxx/include/__mutex_base

 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 
 extern _LIBCPP_EXPORTED_FROM_ABI const defer_lock_t  defer_lock;
 extern _LIBCPP_EXPORTED_FROM_ABI const try_to_lock_t try_to_lock;
 extern _LIBCPP_EXPORTED_FROM_ABI const adopt_lock_t  adopt_lock;
 
 #else
 
-/* inline */ constexpr defer_lock_t  defer_lock  = defer_lock_t();
-/* inline */ constexpr try_to_lock_t try_to_lock = try_to_lock_t();
-/* inline */ constexpr adopt_lock_t  adopt_lock  = adopt_lock_t();
+_LIBCPP_INLINE_CONSTEXPR_VAR defer_lock_t  defer_lock  = defer_lock_t();
+_LIBCPP_INLINE_CONSTEXPR_VAR try_to_lock_t try_to_lock = try_to_lock_t();
+_LIBCPP_INLINE_CONSTEXPR_VAR adopt_lock_t  adopt_lock  = adopt_lock_t();
 
 #endif
 
 template <class _Mutex>
 class _LIBCPP_TEMPLATE_VIS _LIBCPP_THREAD_SAFETY_ANNOTATION(scoped_lockable)
 lock_guard
 {
 public:

libcxx/include/__utility/piecewise_construct.h

 #endif
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 struct _LIBCPP_TEMPLATE_VIS piecewise_construct_t { explicit piecewise_construct_t() = default; };
 #if defined(_LIBCPP_CXX03_LANG) || defined(_LIBCPP_BUILDING_LIBRARY)
 extern _LIBCPP_EXPORTED_FROM_ABI const piecewise_construct_t piecewise_construct;// = piecewise_construct_t();
 #else
-/* inline */ constexpr piecewise_construct_t piecewise_construct = piecewise_construct_t();
+_LIBCPP_INLINE_CONSTEXPR_VAR piecewise_construct_t piecewise_construct = piecewise_construct_t();
 #endif
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___UTILITY_PIECEWISE_CONSTRUCT_H