bug#53012: 28.0.90; completion--nth-completion: Throw a meaningful error

[Daniel Mendler <mail@daniel-mendler.de>]

In the context of the GNU ELPA Vertico/Mct/Orderless/Embark ecosystem we
teach users to adjust the completion style. Unfortunately
completion--nth-completion is not very robust against misconfiguration.
I propose to throw a more meaningful error instead of the nil error.
Patch follows.

---

From da92169572e375facabb28b6d3c14cb2158ab1c9 Mon Sep 17 00:00:00 2001
From: Daniel Mendler <mail@daniel-mendler.de>
Date: Tue, 4 Jan 2022 21:08:04 +0100
Subject: [PATCH] completion--nth-completion: Throw meaningful error

If a configured completion style does not exist, throw a meaningful
error. Also make completion-styles-alist a defvar, since completion
styles like orderless extend it.
---
 lisp/minibuffer.el | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 1d596694e4..ab760a42d1 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -900,7 +900,7 @@ completion-auto-help
 the second failed attempt to complete."
   :type '(choice (const nil) (const t) (const lazy)))

-(defconst completion-styles-alist
+(defvar completion-styles-alist
   '((emacs21
      completion-emacs21-try-completion completion-emacs21-all-completions
      "Simple prefix-based completion.
@@ -1076,9 +1076,10 @@ completion--nth-completion
          (result-and-style
           (completion--some
            (lambda (style)
-             (let ((probe (funcall (nth n (assq style
-                                                completion-styles-alist))
-                                   string table pred point)))
+             (let ((probe (funcall
+                           (or (nth n (assq style completion-styles-alist))
+                               (error "Invalid completion style %s" style))
+                           string table pred point)))
                (and probe (cons probe style))))
            (completion--styles md)))
          (adjust-fn (get (cdr result-and-style)
'completion--adjust-metadata)))
-- 
2.20.1