[network-manager-sstp] Copying change from c1dc75cf by Thomas Haller to network-manager-pptp project

[Eivind Næss <eivnaes src gnome org>]

commit 7108b4d15132dbc658ef82740cbd94c81f5d7735
Author: Eivind Næss <eivnaes yahoo com>
Date:   Sat Mar 19 12:47:38 2022 -0700

    Copying change from c1dc75cf by Thomas Haller to network-manager-pptp project

 m4/compiler_warnings.m4 | 115 ++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 96 insertions(+), 19 deletions(-)
---
diff --git a/m4/compiler_warnings.m4 b/m4/compiler_warnings.m4
index 95ad3fb..0023c36 100644
--- a/m4/compiler_warnings.m4
+++ b/m4/compiler_warnings.m4
@@ -1,33 +1,110 @@
+AC_DEFUN([_NM_COMPILER_FLAG], [
+       CFLAGS_SAVED="$CFLAGS"
+       CFLAGS="$CFLAGS $GLIB_CFLAGS -Werror $1"
+       AC_MSG_CHECKING([whether $1 works as expected])
+
+       AC_COMPILE_IFELSE([AC_LANG_SOURCE([[]])], [
+               AC_COMPILE_IFELSE([AC_LANG_SOURCE([[$2]])], [
+                       AC_MSG_RESULT(yes)
+                       CFLAGS="$CFLAGS_SAVED"
+                       $3
+               ],[
+                       AC_MSG_RESULT(no)
+                       CFLAGS="$CFLAGS_SAVED"
+                       $4
+               ])
+       ],[
+               AC_MSG_RESULT(not supported)
+               CFLAGS="$CFLAGS_SAVED"
+       ])
+])
+
+dnl Check whether a particular compiler flag is supported,
+dnl add it to CFLAGS if it is
+AC_DEFUN([NM_COMPILER_FLAG], [
+        _NM_COMPILER_FLAG([$1], [], [
+               CFLAGS="$CFLAGS $1"
+               $2
+       ], [$3])
+])
+
+dnl Check whether a particular warning is not emitted with code provided,
+dnl disable it in CFLAGS if the check fails.
+AC_DEFUN([NM_COMPILER_WARNING], [
+        _NM_COMPILER_FLAG([-W$1], [$2], [CFLAGS="$CFLAGS -W$1"], [CFLAGS="$CFLAGS -Wno-$1"])
+])
+
 AC_DEFUN([NM_COMPILER_WARNINGS],
 [AC_ARG_ENABLE(more-warnings,
        AS_HELP_STRING([--enable-more-warnings], [Possible values: no/yes/error]),
-       set_more_warnings="$enableval",set_more_warnings=error)
+       set_more_warnings="$enableval",set_more_warnings=$1)
 AC_MSG_CHECKING(for more warnings)
 if test "$GCC" = "yes" -a "$set_more_warnings" != "no"; then
        AC_MSG_RESULT(yes)
-       CFLAGS="-Wall -std=gnu89 $CFLAGS"
+
+       dnl This is enabled in clang by default, makes little sense,
+       dnl and causes the build to abort with -Werror.
+       CFLAGS_SAVED="$CFLAGS"
+       CFLAGS="$CFLAGS -Qunused-arguments"
+       AC_COMPILE_IFELSE([AC_LANG_SOURCE([])], [], CFLAGS="$CFLAGS_SAVED")
+       unset CFLAGS_SAVED
+
+       dnl clang only warns about unknown warnings, unless
+       dnl called with "-Werror=unknown-warning-option"
+       dnl Test if the compiler supports that, and if it does
+       dnl attach it to the CFLAGS.
+       NM_COMPILER_WARNING([unknown-warning-option], [])
+
+       CFLAGS_MORE_WARNINGS="-Wall -std=gnu99"
+
+       if test "x$set_more_warnings" = xerror; then
+               CFLAGS_MORE_WARNINGS="$CFLAGS_MORE_WARNINGS -Werror"
+       fi
 
        for option in -Wshadow -Wmissing-declarations -Wmissing-prototypes \
-                     -Wdeclaration-after-statement -Wstrict-prototypes \
+                     -Wdeclaration-after-statement -Wformat-security \
                      -Wfloat-equal -Wno-unused-parameter -Wno-sign-compare \
-                     -fno-strict-aliasing -Wno-unused-but-set-variable; do
-               SAVE_CFLAGS="$CFLAGS"
-               CFLAGS="$CFLAGS $option"
-               AC_MSG_CHECKING([whether gcc understands $option])
-               AC_TRY_COMPILE([], [],
-                       has_option=yes,
-                       has_option=no,)
-               if test $has_option = no; then
-                       CFLAGS="$SAVE_CFLAGS"
-               fi
-               AC_MSG_RESULT($has_option)
-               unset has_option
-               unset SAVE_CFLAGS
+                     -Wno-duplicate-decl-specifier \
+                     -Wstrict-prototypes \
+                     -Wno-unused-but-set-variable \
+                     -Wno-format-y2k \
+                     -Wundef -Wimplicit-function-declaration \
+                     -Wpointer-arith -Winit-self -Wformat-nonliteral \
+                     -Wmissing-include-dirs -Wno-pragmas; do
+               dnl GCC 4.4 does not warn when checking for -Wno-* flags 
(https://gcc.gnu.org/wiki/FAQ#wnowarning)
+                _NM_COMPILER_FLAG([$(printf '%s' "$option" | sed 's/^-Wno-/-W/')], [],
+                                 [CFLAGS_MORE_WARNINGS="$CFLAGS_MORE_WARNINGS $option"], [])
        done
        unset option
-       if test "x$set_more_warnings" = xerror; then
-               CFLAGS="$CFLAGS -Werror"
-       fi
+
+       dnl Disable warnings triggered by known compiler problems
+
+       dnl https://bugzilla.gnome.org/show_bug.cgi?id=745821
+       NM_COMPILER_WARNING([unknown-attributes], [#include <glib.h>])
+
+       dnl https://bugzilla.gnome.org/show_bug.cgi?id=744473
+       NM_COMPILER_WARNING([typedef-redefinition], [#include <gio/gio.h>])
+
+       dnl https://llvm.org/bugs/show_bug.cgi?id=21614
+       NM_COMPILER_WARNING([array-bounds],
+               [#include <string.h>]
+               [void f () { strcmp ("something", "0"); }]
+       )
+
+       dnl https://llvm.org/bugs/show_bug.cgi?id=22949
+       NM_COMPILER_WARNING([parentheses-equality],
+               [#include <sys/wait.h>]
+               [void f () { if (WIFCONTINUED(0)) return; }]
+       )
+
+       dnl systemd-dhcp's log_internal macro and our handle_warn are sometimes
+       dnl used in void context,u sometimes in int. Makes clang unhappy.
+       NM_COMPILER_WARNING([unused-value],
+               [#define yolo ({ (666 + 666); })]
+               [int f () { int i = yolo; yolo; return i; }]
+       )
+
+       CFLAGS="$CFLAGS_MORE_WARNINGS $CFLAGS"
 else
        AC_MSG_RESULT(no)
 fi