Add availability markup

Author: vogelsgesang

libcxx/include/__configuration/availability.h

@@ -276,6 +276,12 @@
 #define _LIBCPP_AVAILABILITY_HAS_PMR _LIBCPP_INTRODUCED_IN_LLVM_16
 #define _LIBCPP_AVAILABILITY_PMR
 
+// This macro controls the availability of exception_ptr::__{inc,dec}rement_refcount
+// which are used in the inline implementations of exception_ptr's move constructor,
+// assignment operator, and destructor (see libcxx/include/__exception/exception_ptr.h).
+#define _LIBCPP_AVAILABILITY_HAS_INCREMENT_DECREMENT_REFCOUNT_EXCEPTION_PTR _LIBCPP_INTRODUCED_IN_LLVM_21
+// No attribute, since we've have a fallback implementation in the headers
+
 // These macros controls the availability of __cxa_init_primary_exception
 // in the built library, which std::make_exception_ptr might use
 // (see libcxx/include/__exception/exception_ptr.h).

libcxx/include/__exception/exception_ptr.h

@@ -79,11 +79,11 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
   _LIBCPP_HIDE_FROM_ABI exception_ptr(nullptr_t) _NOEXCEPT : __ptr_() {}
 
 // These symbols are still exported from the library to prevent ABI breakage.
-#  ifdef _LIBCPP_BUILDING_LIBRARY
+#  if defined(_LIBCPP_BUILDING_LIBRARY) || !_LIBCPP_AVAILABILITY_HAS_INCREMENT_DECREMENT_REFCOUNT_EXCEPTION_PTR
   exception_ptr(const exception_ptr&) _NOEXCEPT;
   exception_ptr& operator=(const exception_ptr&) _NOEXCEPT;
   ~exception_ptr() _NOEXCEPT;
-#  else  // _LIBCPP_BUILDING_LIBRARY
+#  else  // defined(_LIBCPP_BUILDING_LIBRARY) || !_LIBCPP_AVAILABILITY_HAS_INCREMENT_DECREMENT_REFCOUNT_EXCEPTION_PTR
   _LIBCPP_HIDE_FROM_ABI exception_ptr(const exception_ptr& __other) _NOEXCEPT : __ptr_(__other.__ptr_) {
     if (__ptr_)
       __increment_refcount(__ptr_);
@@ -102,7 +102,7 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
     if (__ptr_)
       __decrement_refcount(__ptr_);
   }
-#  endif // _LIBCPP_BUILDING_LIBRARY
+#  endif // defined(_LIBCPP_BUILDING_LIBRARY) || !_LIBCPP_AVAILABILITY_HAS_INCREMENT_DECREMENT_REFCOUNT_EXCEPTION_PTR
 
   _LIBCPP_HIDE_FROM_ABI exception_ptr(exception_ptr&& __other) _NOEXCEPT : __ptr_(__other.__ptr_) {
     __other.__ptr_ = nullptr;