Expand FIXME near definition of fboundp

This expansion of the FIXME is so that future developers are aware of
the potential problems of aliasing fboundp to symbol-function without
taking backwards compatibility into account.

* src/data.c (fboundp): Note potential backwards compatibility issues in
FIXME.

Copyright-paperwork-exempt: yes

1 files changed, 6 insertions, 1 deletions

diff --git a/src/data.c b/src/data.c
index 2574cbbd764..d14c84e0472 100644
--- a/src/data.c
+++ b/src/data.c
@@ -629,7 +629,12 @@ global value outside of any lexical scope.  */)
   return (EQ (valcontents, Qunbound) ? Qnil : Qt);
 }
 
-/* FIXME: Make it an alias for function-symbol!  */
+/* FIXME: It has been previously suggested to make this function an
+   alias for symbol-function, but upon discussion at Debbug#23957,
+   there is a risk breaking backward compatiblity, as some users of
+   fboundp may expect `t' in particular, rather than any true
+   value.  An alias is still welcome so long as the compatibility
+   issues are addressed.  */
 DEFUN ("fboundp", Ffboundp, Sfboundp, 1, 1, 0,
        doc: /* Return t if SYMBOL's function definition is not void.  */)
   (register Lisp_Object symbol)