Avoid errors in completion due to 'completion-regexp-list'

* doc/lispref/minibuf.texi (Basic Completion):
* src/minibuf.c (syms_of_minibuf) <completion-regexp-list>:
Document that global non-nil settings of 'completion-regexp-list'
are not safe.

* lisp/minibuffer.el (completion-pcm--merge-completions): Avoid
errors in 'try-completion' when PREFIX is nil.  (Bug#64351)

1 files changed, 6 insertions, 1 deletions

diff --git a/src/minibuf.c b/src/minibuf.c
index 6e54d8c3ba5..58adde1bf66 100644
--- a/src/minibuf.c
+++ b/src/minibuf.c
@@ -2471,7 +2471,12 @@ The basic completion functions only consider a completion acceptable
 if it matches all regular expressions in this list, with
 `case-fold-search' bound to the value of `completion-ignore-case'.
 See Info node `(elisp)Basic Completion', for a description of these
-functions.  */);
+functions.
+
+Do not set this variable to a non-nil value globally, as that is not
+safe and will probably cause errors in completion commands.  This
+variable should be only let-bound to non-nil values around calls to
+basic completion functions like `try-completion' and `all-completions'.  */);
   Vcompletion_regexp_list = Qnil;
 
   DEFVAR_BOOL ("minibuffer-allow-text-properties",