[libcxx] [llvm] [libc++] Deprecate and remove meaningless `<cxxx>` headers (PR #111615)

[A. Jiang via llvm-commits]

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@@ -23,4 +23,17 @@
 #  pragma GCC system_header
 #endif
 
+#if _LIBCPP_STD_VER >= 17
+#  if _LIBCPP_STD_VER >= 20
+#    define _DEPRECATION_ANNOTATION _LIBCPP_DEPRECATED_("removed in C++20")
+#  else
+#    define _DEPRECATION_ANNOTATION _LIBCPP_DEPRECATED
+#  endif
+
+using __standard_header_ccomplex _DEPRECATION_ANNOTATION = void;
+using __use_standard_header_ccomplex                     = __standard_header_ccomplex;
+
+#  undef _DEPRECATION_ANNOTATION
+#endif
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frederick-vs-ja wrote:

Perhaps we need to switch back to the `[[deprecated]]`-based method...

https://github.com/llvm/llvm-project/pull/111615