Subject: bug#64537: mix lists and vectors when using backquote

From: Stefan Monnier <[email-masked]>
Date: Sat, 08 Jun 2024 19:45:45 -0400

>     `(... ,@[...])  ; by contrast: `(,@[] 1)  => (1)

AFAICT this is the result of an optimization:

    `(BAR ,@FOO)

is optimized to

    `(BAR . ,FOO)

I expect a fair bit of code "relies" on this optimization, so I'm not
sure we want to "fix" it.

>     `[,@'()]        ; by contrast: `[1 ,@'()] => [1]

This looks like a plain bug (probably linked to another optimization).

    `[,@(not t)]

doesn't suffer from this problem but

    `[,@'[]]

also suffers from it.

        Stefan