[glib/wip/atomic-take2: 2/2] atomic: Fix type check of g_atomic_pointer_compare_and_exchange()

[Sebastian Dröge <sdroege src gnome org>]

commit ed583f161ce7e7265057f390b17dc8dec21bae35
Author: Xavier Claessens <xavier claessens collabora com>
Date:   Tue Feb 2 13:17:33 2021 -0500

    atomic: Fix type check of g_atomic_pointer_compare_and_exchange()

 glib/gatomic.h | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)
---
diff --git a/glib/gatomic.h b/glib/gatomic.h
index e6eccfada..800ddb3aa 100644
--- a/glib/gatomic.h
+++ b/glib/gatomic.h
@@ -183,16 +183,24 @@ G_END_DECLS
     (guint) __atomic_fetch_xor ((atomic), (val), __ATOMIC_SEQ_CST);          \
   }))
 
-#if defined(glib_typeof)
+#if defined(glib_typeof) && defined(__cplusplus) && __cplusplus >= 201103L
+/* This is typesafe because we check we can assign oldval to the type of
+ * (*atomic). Unfortunately it can only be done in C++ because gcc/clang warn
+ * when atomic is volatile and not oldval, or when atomic is gsize* and oldval
+ * is NULL. Note that clang++ force us to be typesafe because it error if 2nd
+ * argument of __atomic_compare_exchange_n() has a different type than the
+ * first.
+ * https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1919
+ * https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1715#note_1024120. */
 #define g_atomic_pointer_compare_and_exchange(atomic, oldval, newval) \
   (G_GNUC_EXTENSION ({                                                       \
     G_STATIC_ASSERT (sizeof (oldval) == sizeof (gpointer));                  \
-    glib_typeof ((oldval)) gapcae_oldval = (oldval);                         \
+    glib_typeof (*(atomic)) gapcae_oldval = (oldval);                        \
     G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer));                 \
     (void) (0 ? (gpointer) *(atomic) : NULL);                                \
     __atomic_compare_exchange_n ((atomic), &gapcae_oldval, (newval), FALSE, __ATOMIC_SEQ_CST, 
__ATOMIC_SEQ_CST) ? TRUE : FALSE; \
   }))
-#else /* if !defined(glib_typeof) */
+#else /* if !(defined(glib_typeof) && defined(__cplusplus) && __cplusplus >= 201103L) */
 #define g_atomic_pointer_compare_and_exchange(atomic, oldval, newval) \
   (G_GNUC_EXTENSION ({                                                       \
     G_STATIC_ASSERT (sizeof (oldval) == sizeof (gpointer));                  \