master fc3caa4: * lisp/minibuffer.el (completion-table-with-quoting): Fix bug#47678

[Stefan Monnier]

branch: master
commit fc3caa45ef2dcbd5a1c8339f14696589b99888ce
Author: Stefan Monnier <monnier@iro.umontreal.ca>
Commit: Stefan Monnier <monnier@iro.umontreal.ca>

    * lisp/minibuffer.el (completion-table-with-quoting): Fix bug#47678
---
 lisp/minibuffer.el | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 5f59467..c900b0d 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -488,8 +488,17 @@ for use at QPOS."
              (qsuffix (cdr action))
              (ufull (if (zerop (length qsuffix)) ustring
                       (funcall unquote (concat string qsuffix))))
-             (_ (cl-assert (string-prefix-p ustring ufull)))
-             (usuffix (substring ufull (length ustring)))
+             ;; If (not (string-prefix-p ustring ufull)) we have a problem:
+             ;; the unquoting the qfull gives something "unrelated" to ustring.
+             ;; E.g. "~/" and "/" where "~//" gets unquoted to just "/" (see
+             ;; bug#47678).
+             ;; In that case we can't even tell if we're right before the
+             ;; "/" or right after it (aka if this "/" is from qstring or
+             ;; from qsuffix), which which usuffix to use is very unclear.
+             (usuffix (if (string-prefix-p ustring ufull)
+                          (substring ufull (length ustring))
+                        ;; FIXME: Maybe "" is preferable/safer?
+                        qsuffix))
              (boundaries (completion-boundaries ustring table pred usuffix))
              (qlboundary (car (funcall requote (car boundaries) string)))
              (qrboundary (if (zerop (cdr boundaries)) 0 ;Common case.