On Sun, 22 Mar 2020 10:01:34 +0100 Joost Kremers <joostkremers@fastmail.fm> wrote:

> `cl-concatenate` seems to return the wrong result:
>
> ```
> ELISP> (cl-concatenate 'list '(a b c) '(d e f))
> ((a b c)  (d e f))
> ```
>
> In Emacs 26 the return value was `(a b c d e f)`, which I assume should still
> be the return value in Emacs 27, given that Common Lisp hasn't changed. :-)

Similarly, (cl-concatenate 'vector '[a b c] '[d e f]) returns `[[a b c]
[d e f]]' and worse, (cl-concatenate 'string "abc" "def") raises the
error: Wrong type argument: characterp, "abc".  This is because
cl-concatenate is now defined in terms of seq-concatenate, which is
defined by cl-defgeneric, which adds an extra pair of parens around the
SEQUENCES argument.  Hence, the following patch restores the correct
pre-27 behavior: