I've been annoyed at the use of lists to represent function values for
a while now.  For a reason I cannot fathom, I even managed to reproduce
that very same mistake in Emacs-24 with the `(closure ...)` value for
statically scoped interpreted function values.

That was a major blunder.

In any case, I'm playing around with a "fix", making lambda evaluate
(when interpreted) not to (lambda ...) or (closure ...) but to
a self-evaluating value that can be more reliably distinguished.
In the patch below, I just reused the #[...] byte-code objects for that,
putting the function's body where the bytecode string goes and the
function's captured environment where the "constant vector" goes.

It's got several rough edges (most importantly that
`byte-code-function-p` returns non-nil for interpreted function values),
but it seems to work OK so far.

You can't use the patch as-is because it's written against my local
branch, with various local hacks, some of which (partly) remove support
for lexical-binding==nil.
But hopefully, it's readable enough for you to form an opinion.

WDYT?


        Stefan