If a C23 compiler is detected, try asking for C17.

Branches before 16 can't be compiled with a C23 compiler (see
deprecation warnings silenced by commit f9a56e72, and non-back-patchable
changes made in 16 by commit 1c27d16e).  Test __STDC_VERSION__, and if
it's above C17 then try appending -std=gnu17.  The test is done with the
user's CFLAGS, so an acceptable language version can also be configured
manually that way.

This is done in branches 15 and older, back to 9.2, per policy of
keeping them buildable with modern tools.

Discussion: https://postgr.es/m/87o72eo9iu.fsf%40gentoo.org

diff --git a/configure b/configure
index 0c3e8784a1dc9eda5c2c73c1743b20bcfcdfd6e6..f4f4b2e0a9178292a514620cd6037fe19c81aed6 100755 (executable)
--- a/configure
+++ b/configure
@@ -4508,6 +4508,78 @@ else
   fi
 fi
 
+# We use C constructs that became invalid in C23.  Check if the compiler
+# reports a standard higher than C17, with the flags selected above (so the
+# user can control the language level explicitly to avoid the gcc/clang-only
+# fallback logic below if preferred).
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC reports a C standard higher than ISO C17" >&5
+$as_echo_n "checking whether $CC reports a C standard higher than ISO C17... " >&6; }
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+#if __STDC_VERSION__ > 201710L
+choke me
+#endif
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  POSTC17=no
+else
+  POSTC17=yes
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: ${POSTC17}" >&5
+$as_echo "${POSTC17}" >&6; }
+
+# If a too recent standard was detected with the user's CFLAGS, try asking for
+# C17 with GNU extensions explicitly.
+if test "$POSTC17" = yes; then
+  old_CFLAGS="$CFLAGS"
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC supports -std=gnu17" >&5
+$as_echo_n "checking whether $CC supports -std=gnu17... " >&6; }
+if ${pgac_cv_prog_cc_cflags__std_gnu17+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+CFLAGS="$pgac_save_CFLAGS -std=gnu17"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_prog_cc_cflags__std_gnu17=yes
+else
+  pgac_cv_prog_cc_cflags__std_gnu17=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$pgac_save_CFLAGS"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_cc_cflags__std_gnu17" >&5
+$as_echo "$pgac_cv_prog_cc_cflags__std_gnu17" >&6; }
+if test x"$pgac_cv_prog_cc_cflags__std_gnu17" = x"yes"; then
+  CFLAGS="$CFLAGS -std=gnu17"
+fi
+
+  if test "$CFLAGS" = "$old_CFLAGS"; then
+    as_fn_error $? "cannot proceed" "$LINENO" 5
+  fi
+fi
+
 # CFLAGS we determined above will be added back at the end
 user_CFLAGS=$CFLAGS
 CFLAGS=""

diff --git a/configure.in b/configure.in
index 06cb0e5e69f4a1a006d5f331899e85c85b52fabc..67c3cc2807cbcf894fa018e301178073c0588f1e 100644 (file)
--- a/configure.in
+++ b/configure.in
@@ -434,6 +434,26 @@ else
   fi
 fi
 
+# We use C constructs that became invalid in C23.  Check if the compiler
+# reports a standard higher than C17, with the flags selected above (so the
+# user can control the language level explicitly to avoid the gcc/clang-only
+# fallback logic below if preferred).
+AC_MSG_CHECKING([whether $CC reports a C standard higher than ISO C17])
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [@%:@if __STDC_VERSION__ > 201710L
+choke me
+@%:@endif])], [POSTC17=no], [POSTC17=yes])
+AC_MSG_RESULT(${POSTC17})
+
+# If a too recent standard was detected with the user's CFLAGS, try asking for
+# C17 with GNU extensions explicitly.
+if test "$POSTC17" = yes; then
+  old_CFLAGS="$CFLAGS"
+  PGAC_PROG_CC_CFLAGS_OPT([-std=gnu17])
+  if test "$CFLAGS" = "$old_CFLAGS"; then
+    AC_MSG_ERROR([cannot proceed])
+  fi
+fi
+
 # CFLAGS we determined above will be added back at the end
 user_CFLAGS=$CFLAGS
 CFLAGS=""