Problem

Using GObject and C, I am trying to create a subclass of the class GObject named "Bar" in module "Foo". However, the macro "G_DECLARE_FINAL_TYPE" (defined in gobject/gtype.h) expands incorrectly. Instead of expanding into FOO_BAR, it expands into nothing. So instead of expanding into:

... FooBar * FOO_BAR (gpointer ptr) { ...

It instead expands into:

... FooBar * (gpointer ptr) { ...

Error Message

I expect it to compile but instead gcc gives a syntax error:

In file included from /usr/include/glib-2.0/gobject/gobject.h:24,
                 from /usr/include/glib-2.0/gobject/gbinding.h:29,
                 from /usr/include/glib-2.0/glib-object.h:22,
                 from foo-bar.h:4,
                 from foo-bar.c:1:
/usr/include/glib-2.0/gobject/gtype.h:1407:77: error: expected ‘)’ before ‘ptr’
 1407 | USED static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {                       \
      |                                                                  ^~~

 foo-bar.h:10:1: note: in expansion of macro ‘G_DECLARE_FINAL_TYPE’
   10 | G_DECLARE_FINAL_TYPE (FooBar, foo_bar, FOO, BAR, GObject)
      | ^~~~~~~~~~~~~~~~~~~~

Definition of Macro

This definition comes from the glib 2.66.7 library. I did not define it myself. I include it for the sake of completeness.

#define G_DECLARE_FINAL_TYPE(ModuleObjName, module_obj_name, MODULE, OBJ_NAME, ParentName)  \
  GType module_obj_name##_get_type (void);                                                  \
  G_GNUC_BEGIN_IGNORE_DEPRECATIONS                                                          \
  typedef struct _##ModuleObjName ModuleObjName;                                            \ 
  typedef struct { ParentName##Class parent_class; } ModuleObjName##Class;                  \
                                                                                            \
  _GLIB_DEFINE_AUTOPTR_CHAINUP (ModuleObjName, ParentName)                                  \
  G_DEFINE_AUTOPTR_CLEANUP_FUNC (ModuleObjName##Class, g_type_class_unref)                  \
                                                                                            \
  G_GNUC_UNUSED static inline ModuleObjName * MODULE##_##OBJ_NAME (gpointer ptr) {          \
    return G_TYPE_CHECK_INSTANCE_CAST (ptr, module_obj_name##_get_type (), ModuleObjName); }\
  G_GNUC_UNUSED static inline gboolean MODULE##_IS_##OBJ_NAME (gpointer ptr) {              \
    return G_TYPE_CHECK_INSTANCE_TYPE (ptr, module_obj_name##_get_type ()); }               \
  G_GNUC_END_IGNORE_DEPRECATIONS

Partial Pre-Processor Output

I included the definition that contains the syntax error and formatted it for readability:

__attribute__((__unused__)) static inline FooBar * (gpointer ptr) {
  return (((FooBar*) g_type_check_instance_cast ((GTypeInstance*) (ptr), (foo_bar_get_type ()))));
}

What's odd is that the definition following does get pre-processed correctly:

__attribute__((__unused__)) static inline gboolean FOO_IS_BAR (gpointer ptr) {
  return ((__extension__ ({
    GTypeInstance *__inst = (GTypeInstance*) (ptr);
    GType __t = (foo_bar_get_type ());
    gboolean __r;
    if (!__inst) __r = (0);
    else if (__inst->g_class && __inst->g_class->g_type == __t) __r = (!(0));
    else __r = g_type_check_instance_is_a (__inst, __t); __r; }))); 
}

Full Minimal Example

I have included a minimal example that causes my error below.

foo-bar.h

#ifndef FOO_BAR
#define FOO_BAR

#include <glib-object.h>

G_BEGIN_DECLS

#define FOO_TYPE_BAR (foo_bar_get_type ())

G_DECLARE_FINAL_TYPE (FooBar, foo_bar, FOO, BAR, GObject)

G_END_DECLS

#endif /* FOO_BAR */

foo-bar.c

#include "foo-bar.h"

struct _FooBar
{
    GObject parent_instance;
};

G_DEFINE_TYPE (FooBar, foo_bar, G_TYPE_OBJECT);

static void foo_bar_class_init (FooBarClass *klass)
{

}

static void foo_bar_init (FooBar *app)
{

}

makefile

foo-bar.o: foo-bar.c
    gcc -c foo-bar.c $(shell pkg-config --cflags gobject-2.0) -o foo-bar.o
# included to debug the macro
foo-bar.pp: foo-bar.c
    gcc -E foo-bar.c $(shell pkg-config --cflags gobject-2.0) -o foo-bar.pp

Versions

glib 2.66.7
gcc 10.2.1
Linux (Fedora 33)

What I have tried

I have studied glib's documentation on how to create a new class and as far as I can tell I am doing it correctly. I have also looked up how token pasting and macro expansion work, and also tried googling my error. It must be something obvious, but I cannot figure out what. I also tried declaring a derivable class with private data, but while finding a minimal example I changed it to a final class without private data and still have the same error. I read about the ## operator here and still cannot tell what is going wrong.

GObject Compilation Error in Macro Expansion While Defining Class