[glib/wip/pwithnall/2625-clang-cxx] gatomic: Add a C++ variant of g_atomic_int_compare_and_exchange()

[Philip Withnall <pwithnall src gnome org>]

commit 36127fadc4fcfc504e58c1777e7ff144fd9830b8
Author: Philip Withnall <pwithnall endlessos org>
Date:   Fri Apr 1 13:47:19 2022 +0100

    gatomic: Add a C++ variant of g_atomic_int_compare_and_exchange()
    
    The C++ variant implements type safety differently, to avoid warnings
    from C++ compilers about:
    ```
    ../../../gnome-commander-1.14.2/src/intviewer/searcher.cc:303:5: error: cannot initialize a parameter of 
type 'gint *' (aka 'int *') with an rvalue of type 'void *'
        g_atomic_int_compare_and_exchange ((gint*)&src->priv->progress_value, oldval, (gint)d);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
/mnt/b/yoe/master/build/tmp/work/cortexa72-yoe-linux/gnome-commander/1.14.2-r0/recipe-sysroot/usr/include/glib-2.0/glib/gatomic.h:160:44:
 note: expanded from macro 'g_atomic_int_compare_and_exchange'
        __atomic_compare_exchange_n ((atomic), (void *) (&(gaicae_oldval)), (newval), FALSE, 
__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST) ? TRUE : FALSE; \
                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~
    ```
    
    This complements the existing C++ variant for
    `g_atomic_pointer_compare_and_exchange()`, and fixes a regression on C++
    from https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2114.
    
    With the addition of the unit tests in the previous commit, this is
    effectively tested by the FreeBSD and macOS CI jobs, as they use
    `clang++` in C++ mode. `g++` doesn’t seem to emit a warning about this.
    
    Signed-off-by: Philip Withnall <pwithnall endlessos org>
    
    Fixes: #2625

 glib/gatomic.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)
---
diff --git a/glib/gatomic.h b/glib/gatomic.h
index 5eba1dbc7c..8b2b880c8d 100644
--- a/glib/gatomic.h
+++ b/glib/gatomic.h
@@ -152,6 +152,17 @@ G_END_DECLS
     (void) (0 ? *(atomic) ^ *(atomic) : 1);                                  \
     __atomic_fetch_sub ((atomic), 1, __ATOMIC_SEQ_CST) == 1;                 \
   }))
+#if defined(glib_typeof) && defined(__cplusplus) && __cplusplus >= 201103L
+/* See comments below about equivalent g_atomic_pointer_compare_and_exchange()
+ * shenanigans for type-safety when compiling in C++ mode. */
+#define g_atomic_int_compare_and_exchange(atomic, oldval, newval) \
+  (G_GNUC_EXTENSION ({                                                       \
+    glib_typeof (*(atomic)) gaicae_oldval = (oldval);                        \
+    G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint));                     \
+    (void) (0 ? *(atomic) ^ (newval) ^ (oldval) : 1);                        \
+    __atomic_compare_exchange_n ((atomic), &gaicae_oldval, (newval), FALSE, __ATOMIC_SEQ_CST, 
__ATOMIC_SEQ_CST) ? TRUE : FALSE; \
+  }))
+#else /* if !(defined(glib_typeof) && defined(__cplusplus) && __cplusplus >= 201103L) */
 #define g_atomic_int_compare_and_exchange(atomic, oldval, newval) \
   (G_GNUC_EXTENSION ({                                                       \
     gint gaicae_oldval = (oldval);                                           \
@@ -159,6 +170,7 @@ G_END_DECLS
     (void) (0 ? *(atomic) ^ (newval) ^ (oldval) : 1);                        \
     __atomic_compare_exchange_n ((atomic), (void *) (&(gaicae_oldval)), (newval), FALSE, __ATOMIC_SEQ_CST, 
__ATOMIC_SEQ_CST) ? TRUE : FALSE; \
   }))
+#endif /* defined(glib_typeof) */
 #define g_atomic_int_add(atomic, val) \
   (G_GNUC_EXTENSION ({                                                       \
     G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint));                     \