gmem.h: Use __typeof__() in the g_clear_pointer() macro

Type punning is used on the existing implementation, which hides errors
such as:

  GSList *list = NULL;
  g_clear_pointer (&list, g_error_free);

Let's use __typeof__ to cast the passed-in pointer before it's passed to
the free function so it trips -Wincompatible-pointer-types if it's wrong.

Fixes #1425

2 files changed, 28 insertions, 0 deletions

diff --git a/glib/gmem.h b/glib/gmem.h
index 5cccb045c..302809478 100644
--- a/glib/gmem.h
+++ b/glib/gmem.h
@@ -110,6 +110,16 @@ gpointer g_try_realloc_n  (gpointer	 mem,
 			   gsize	 n_blocks,
 			   gsize	 n_block_bytes) G_GNUC_WARN_UNUSED_RESULT;
 
+#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 8)) && !defined(__cplusplus) && GLIB_VERSION_MAX_ALLOWED >= GLIB_VERSION_2_58
+#define g_clear_pointer(pp, destroy)                                           \
+  G_STMT_START {                                                               \
+    G_STATIC_ASSERT (sizeof *(pp) == sizeof (gpointer));                       \
+    __typeof__(*(pp)) _ptr = *(pp);                                            \
+    *(pp) = NULL;                                                              \
+    if (_ptr)                                                                  \
+      (destroy) (_ptr);                                                        \
+  } G_STMT_END
+#else /* __GNUC__ */
 #define g_clear_pointer(pp, destroy) \
   G_STMT_START {                                                               \
     G_STATIC_ASSERT (sizeof *(pp) == sizeof (gpointer));                       \
@@ -127,6 +137,7 @@ gpointer g_try_realloc_n  (gpointer	 mem,
         _destroy (_p);                                                         \
       }                                                                        \
   } G_STMT_END
+#endif /* __GNUC__ */
 
 /**
  * g_steal_pointer:
diff --git a/glib/tests/utils.c b/glib/tests/utils.c
index 7f8edd8da..95c6138e4 100644
--- a/glib/tests/utils.c
+++ b/glib/tests/utils.c
@@ -517,6 +517,22 @@ test_clear_pointer (void)
   g_assert (a == NULL);
 }
 
+/* Test that g_clear_pointer() works with a GDestroyNotify which contains a cast.
+ * See https://gitlab.gnome.org/GNOME/glib/issues/1425 */
+static void
+test_clear_pointer_cast (void)
+{
+  GHashTable *hash_table = NULL;
+
+  hash_table = g_hash_table_new (g_str_hash, g_str_equal);
+
+  g_assert_nonnull (hash_table);
+
+  g_clear_pointer (&hash_table, (void (*) (GHashTable *)) g_hash_table_destroy);
+
+  g_assert_null (hash_table);
+}
+
 static int obj_count;
 
 static void
@@ -656,6 +672,7 @@ main (int   argc,
   g_test_add_func ("/utils/specialdir", test_special_dir);
   g_test_add_func ("/utils/specialdir/desktop", test_desktop_special_dir);
   g_test_add_func ("/utils/clear-pointer", test_clear_pointer);
+  g_test_add_func ("/utils/clear-pointer-cast", test_clear_pointer_cast);
   g_test_add_func ("/utils/take-pointer", test_take_pointer);
   g_test_add_func ("/utils/clear-source", test_clear_source);
   g_test_add_func ("/utils/misc-mem", test_misc_mem);