On byte compilation

[Petteri Hintsanen <petterih@iki.fi>]

Hello,

I'm converting some old code to use new bindat-type introduced in
Emacs 28.1.  I'd like to keep the legacy forms for backwards
compatibility with older emacsen.  But there is a problem with byte
compilation.

Toy example:

  (defconst foo-bindat-spec
    (eval-when-compile
      (if (fboundp 'bindat-type)
          (bindat-type  ;; new form
            (id uint 32))
        '((id u32))))  ;; legacy form
    "Bindat spec for foo.")

What I try to accomplish here is to assign to foo-bindat-spec either the
result of (bindat-type ...) or list '((id u32)), depending on whether
bindat-type macro is available or not.  However, Emacs' 27.2 byte
compiler complains:

  foo.el:5:14:Warning: reference to free variable ‘uint’

even though (fboundp 'bindat-type) evaluates to nil with that Emacs
version.  The resulting elc seems to be ok (?) despite the warning:

  #@22 Bindat spec for foo.
  (defconst foo-bindat-spec '((id u32)) (#$ . 408))

If I relocate eval-when-compile like this

  (defconst foo-bindat-spec
    (if (eval-when-compile (fboundp 'bindat-type))
        (bindat-type  ;; new form
          (id uint 32))
      '((id u32)))  ;; legacy form
    "Bindat spec for foo.")

then the byte compiler does not complain.  Could someone please explain
why?

Thanks,
Petteri