Is there a way to use __fl_attr() for class methods like Fl_Text_Buffer::printf() and Fl_Text_Buffer::vprintf()?

I assumed something like this would work, but it fails with weird errors:

 #include "Fl_Export.H"
+#include "fl_attr.h"
 
 class Fl_Text_Undo_Action;
 
@@ -289,8 +290,8 @@ public:
    */
   void append(const char* t) { insert(length(), t); }
 
-  void vprintf(const char *fmt, va_list ap);
-  void printf(const char* fmt, ...);
+  void vprintf(const char *fmt, va_list ap) __fl_attr((__format__(__printf__, 1, 0)));
+  void printf(const char* fmt, ...)         __fl_attr((__format__(__printf__, 1, 2)));
 

This seems like the same usage in fl_ask.h, it's just a method here
instead of a function.

Is there maybe some special syntax needed since these are methods in a class?
The GNU documentation seems unclear on this, and the error messages are as well:

/usr/local/src/fltk-1.4.x.git/FL/fl_attr.h:53:40: error: 'format' attribute argument 2 value '1' refers to parameter type 'Fl_Text_Buffer*'
   53 | #  define __fl_attr(x) __attribute__ (x)
      |                                        ^
/usr/local/src/fltk-1.4.x.git/FL/Fl_Text_Buffer.H:293:45: note: in expansion of macro '__fl_attr'
  293 |   void vprintf(const char *fmt, va_list ap) __fl_attr((__format__(__printf__, 1, 0)));
      |                                             ^~~~~~~~~
/usr/local/src/fltk-1.4.x.git/FL/fl_attr.h:53:40: error: 'format' attribute argument 2 value '1' refers to parameter type 'Fl_Text_Buffer*'
   53 | #  define __fl_attr(x) __attribute__ (x)
      |                                        ^
/usr/local/src/fltk-1.4.x.git/FL/Fl_Text_Buffer.H:294:45: note: in expansion of macro '__fl_attr'
  294 |   void printf(const char* fmt, ...)         __fl_attr((__format__(__printf__, 1, 2)));
      |                                             ^~~~~~~~~

I'm sure that "refers to parameter type 'Fl_Text_Buffer*' should be giving me a hint
but I'm not sure what to make of it.

I have a vague memory that non-static class methods might secretly have 'this' somehow
involved as the first argument (perhaps similar to python?), and perhaps with this low level
compiler stuff, __attribute__ and __format__() need to know about this offset for the two
argument index numbers?

I ask because I figure I might need this for the new Fl_Terminal widget I'm working on.

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