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Newlib shares large parts of <sys/cdefs.h> with FreeBSD and received this bug report: https://sourceware.org/pipermail/newlib/2023/020400.html As an extension, GCC and clang offer C99-style restricted pointers in C++ mode: https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html We notice that this extension is broken when including newlib headers: restricted pointers are treated as ordinary pointers. We traced this to the following section of newlib/libc/include/sys/cdefs.h: /* * GCC 2.95 provides `__restrict' as an extension to C90 to support the * C99-specific `restrict' type qualifier. We happen to use `__restrict' as * a way to define the `restrict' type qualifier without disturbing older * software that is unaware of C99 keywords. */ #if !(__GNUC__ == 2 && __GNUC_MINOR__ == 95) #if !defined(__STDC_VERSION__) || __STDC_VERSION__ < 199901 #define __restrict #else #define __restrict restrict #endif #endif While the GCC __restrict extension was indeed introduced in GCC 2.95, it is not limited to this version; the extension is also not limited to C90: https://gcc.gnu.org/gcc-2.95/c++features.html Rewrite the logic in the header so that __restrict is kept alone when available. PR: 272723 MFC after: 1 week