-Wdiscarded-qualifiers warning in _ctypes_test.c my_strchr() under FORTIFY_SOURCE=3

[utkonos]

Bug report

Bug description:

Building 3.14.5 with gcc 15.2 and --enable-slower-safety produces this warning:

    default: ./Modules/_ctypes/_ctypes_test.c: In function ‘my_strchr’:
    default: ./Modules/_ctypes/_ctypes_test.c:451:12: warning: return discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
    default:   451 |     return strchr(s, c);
    default:       |            ^~~~~~

Build environment: Ubuntu 26.04 LTS, gcc 15.2.0, configure with:

./configure --enable-optimizations --with-lto --enable-bolt --enable-safety --enable-slower-safety --with-system-expat CXX=/usr/bin/g++ CFLAGS="-flto=auto" LDFLAGS="-flto=auto"

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

• gh-149707: Fix compiler warning in _ctypes_test on strchr() #149791
• [3.15] gh-149707: Fix compiler warning in _ctypes_test on strchr() (GH-149791) #149862
• [3.14] gh-149707: Fix compiler warning in _ctypes_test on strchr() (GH-149791) #149863
• [3.13] gh-149707: Fix compiler warning in _ctypes_test on strchr() (GH-149791) #149864
• [3.15] gh-149707: Fix compiler warning in _ctypes_test on strchr() (#149791) #149865
• [3.14] gh-149707: Fix compiler warning in _ctypes_test on strchr() (#149791) #149866
• [3.13] gh-149707: Fix compiler warning in _ctypes_test on strchr() (#149791) #149867

[vstinner]

On Linux with the glibc, when using C23, strchr(text, ch) return type is const char* if text type is const char*.

See for example the glibc commit which introduced C23 support in string/string.h.

The string.h code is now:

extern char *strchr (const char *__s, int __c)
     __THROW __attribute_pure__ __nonnull ((1));
# if __GLIBC_USE (ISOC23) && defined __glibc_const_generic && !defined _LIBC
#  define strchr(S, C)                                         \
  __glibc_const_generic (S, const char *, strchr (S, C))
# endif