bug#39600: [PATCH] Fix handling of non-exclusive non-prefix completion functions

[Amai Kinono]

# What does this fix?

Currently, with non-exclusive completion functions, Emacs will do
`try-completion` on the current text, and decide whether to try next
completion function based on that. This makes completion functions that
can do non-prefix completions fails when the current text only occurs in
the middle of the candidates. This is a problem I found in a FIXME in
the code.

# How does this work?

I use `completion-all-completions` instead. As far as I know, this
respects the `completion-styles`.

Here's a simple test. Eval this:

```
(require 'thingatpt)
(defun my-completion-at-point ()
  (let* ((symbol (thing-at-point 'symbol))
         (bounds (bounds-of-thing-at-point 'symbol)))
    (list (car bounds) (cdr bounds)
          '("gnuemacs" "xemacs" "uemacs")
          :exclusive 'no)))
(setq completion-at-point-functions
      '(my-completion-at-point elisp-completion-at-point))
(setq completion-styles '(substring))
```

Now type "emacs" and press `C-M-i`, the candidates defined in
`my-completion-at-point` will show up correctly. Now change
`completion-styles` to `(basic)`, and complete "emacs" again, it
fallbacks to `elisp-completion-at-point`.

# The patch

See the attachment. Since I don't know much about email, I'm not sure if
gmail's attachment is permanent, so I'll also paste the diff and commit
log here:

## Diff

--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -2244,18 +2244,11 @@ completion--capf-wrapper
           (unless (member fun completion--capf-safe-funs)
             (push fun completion--capf-safe-funs))
           (and (eq 'no (plist-get (nthcdr 3 res) :exclusive))
-               ;; FIXME: Here we'd need to decide whether there are
-               ;; valid completions against the current text.  But this depends
-               ;; on the actual completion UI (e.g. with the default completion
-               ;; it depends on completion-style) ;-(
-               ;; We approximate this result by checking whether prefix
-               ;; completion might work, which means that non-prefix completion
-               ;; will not work (or not right) for completion functions that
-               ;; are non-exclusive.
-               (null (try-completion (buffer-substring-no-properties
-                                      (car res) (point))
-                                     (nth 2 res)
-                                     (plist-get (nthcdr 3 res) :predicate)))
+               (null (completion-all-completions
+                      (buffer-substring-no-properties (car res) (point))
+                      (nth 2 res)
+                      (plist-get (nthcdr 3 res) :predicate)
+                      (- (point) (car res))))
                (setq res nil)))
          ((not (or (listp res) (functionp res)))
           (unless (member fun completion--capf-misbehave-funs)

## Commit Log

Fix handling for non-exclusive non-prefix completion functions

* lisp/minibuffer.el (completion--capf-wrapper):
use completion-all-completions to do the test.

0001-Fix-handling-for-non-exclusive-non-prefix-completion.patch
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