From 54edfc613947d93a3250b374c81df9d335de2a96 Mon Sep 17 00:00:00 2001
From: Thomas Haller <thaller@redhat.com>
Date: Mon, 22 Mar 2021 13:10:02 +0100
Subject: [PATCH] libnm: improve detection for C11 _Generic() support

Older gcc doesn't like this:

  ../src/libnm-core-impl/nm-setting-wired.c:132:49: error: controlling expression type 'const char *const [31]' not compatible with any generic association type
             && (nm_utils_strv_find_binary_search(valid_s390_opts,
                                                  ^~~~~~~~~~~~~~~
  ../src/libnm-glib-aux/nm-shared-utils.h:2033:60: note: expanded from macro 'nm_utils_strv_find_binary_search'
          const char *const *const _strv   = NM_CAST_STRV_CC(strv);     \
                                                             ^
  ../src/libnm-glib-aux/nm-macros-internal.h:706:21: note: expanded from macro 'NM_CAST_STRV_CC'
          (_Generic ((value), \
                      ^
---
 configure.ac                            |  2 +-
 meson.build                             | 14 ++++++++++----
 src/libnm-glib-aux/nm-macros-internal.h |  9 ++++++---
 3 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/configure.ac b/configure.ac
index de6e272360..fe0568afe6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1177,7 +1177,7 @@ if test -n "$sanitizers"; then
 fi
 
 AC_MSG_CHECKING([CC support C11 _Generic()])
-AC_COMPILE_IFELSE([AC_LANG_SOURCE([[int foo(void); int foo() { int a = 0; int b = _Generic (a, int: 4); return b + a; }]],
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[int foo(void); static const char *const buf[1] = { "a" }; int foo() { int a = 0; int b = _Generic (a, int: 4) + _Generic(buf, const char *const*: 5); return b + a; }]],
                                   [[foo();]])],
                   [cc_support_generic=1],
                   [cc_support_generic=0])
diff --git a/meson.build b/meson.build
index 7e2bf7ee93..f24718b052 100644
--- a/meson.build
+++ b/meson.build
@@ -785,11 +785,17 @@ config_h.set('NM_MORE_ASSERTS', more_asserts)
 more_logging = get_option('more_logging')
 config_h.set10('NM_MORE_LOGGING', more_logging)
 
-generic_support_src = 'int main() { int a = 0; int b = _Generic (a, int: 4); return b + a; };'
-config_h.set10('_NM_CC_SUPPORT_GENERIC', cc.compiles(generic_support_src))
+config_h.set10('_NM_CC_SUPPORT_GENERIC',
+  cc.compiles(
+    'int foo(void); static const char *const buf[1] = { "a" }; int foo() { int a = 0; int b = _Generic (a, int: 4) + _Generic(buf, const char *const*: 5); return b + a; }'
+  )
+)
 
-auto_support_src = 'int main() { int a = 0; __auto_type b = a; return b + a; };'
-config_h.set10('_NM_CC_SUPPORT_AUTO_TYPE', cc.compiles(auto_support_src))
+config_h.set10('_NM_CC_SUPPORT_AUTO_TYPE',
+  cc.compiles(
+    'int main() { int a = 0; __auto_type b = a; return b + a; };'
+  )
+)
 
 # Vala bindings
 vapi_opt = get_option('vapi')
diff --git a/src/libnm-glib-aux/nm-macros-internal.h b/src/libnm-glib-aux/nm-macros-internal.h
index ce89e235c1..d31599f630 100644
--- a/src/libnm-glib-aux/nm-macros-internal.h
+++ b/src/libnm-glib-aux/nm-macros-internal.h
@@ -522,9 +522,12 @@ NM_G_ERROR_MSG(GError *error)
     /* In the meantime, NetworkManager requires C11 and _Generic() should always be available.
      * However, shared/nm-utils may also be used in VPN/applet, which possibly did not yet
      * bump the C standard requirement. Leave this for the moment, but eventually we can
-     * drop it. */
-    #if (defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9))) \
-        || (defined(__clang__))
+     * drop it.
+     *
+     * Technically, gcc 4.9 already has some support for _Generic(). But there seems
+     * to be issues with propagating "const char *[5]" to "const char **". Only assume
+     * we have _Generic() since gcc 5. */
+    #if (defined(__GNUC__) && __GNUC__ >= 5) || (defined(__clang__))
         #define _NM_CC_SUPPORT_GENERIC 1
     #else
         #define _NM_CC_SUPPORT_GENERIC 0