Correctly include fixed strings before a prefix wildcard in PCM

In 03ac16ece40ba3e3ba805d6a61cc457d84bf3792 I fixed a bug with the
PCM implementation of substring completion, relating to the handling
of PCM wildcards.
However, this fix was incomplete.  This change completes the fix by
also including a fixed string if it appears before a 'prefix'
wildcard, even if 'try-completion' doesn't discover that fixed
string grows to a unique completion.
I discovered this bug while working on enhancements to PCM
completion related to 'completion-pcm-leading-wildcard'.
* lisp/minibuffer.el (completion-pcm--merge-completions): Include
fixed strings before 'prefix wildcard.  (Bug#72819)
* test/lisp/minibuffer-tests.el (completion-substring-test-5): Add a
test for this behavior.

2 files changed, 21 insertions, 11 deletions

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index bef565378ea..216385d9cdf 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -4384,18 +4384,21 @@ the same set of elements."
                      (unique (or (and (eq prefix t) (setq prefix fixed))
                                  (and (stringp prefix)
                                       (eq t (try-completion prefix comps))))))
-                ;; if the common prefix is unique, it also is a common
-                ;; suffix, so we should add it for `prefix' elements
-                (unless (or (and (eq elem 'prefix) (not unique))
-                            (equal prefix ""))
-                  (push prefix res))
                 ;; If there's only one completion, `elem' is not useful
                 ;; any more: it can only match the empty string.
                 ;; FIXME: in some cases, it may be necessary to turn an
                 ;; `any' into a `star' because the surrounding context has
                 ;; changed such that string->pattern wouldn't add an `any'
                 ;; here any more.
-                (unless unique
+                (if unique
+                    ;; if the common prefix is unique, it also is a common
+                    ;; suffix, so we should add it for `prefix' elements
+                    (push prefix res)
+                  ;; `prefix' only wants to include the fixed part before the
+                  ;; wildcard, not the result of growing that fixed part.
+                  (when (eq elem 'prefix)
+                    (setq prefix fixed))
+                  (push prefix res)
                   (push elem res)
                   ;; Extract common suffix additionally to common prefix.
                   ;; Don't do it for `any' since it could lead to a merged
diff --git a/test/lisp/minibuffer-tests.el b/test/lisp/minibuffer-tests.el
index df36bce4634..38c2b8c4552 100644
--- a/test/lisp/minibuffer-tests.el
+++ b/test/lisp/minibuffer-tests.el
@@ -306,13 +306,20 @@
            6)))
 
 (ert-deftest completion-substring-test-5 ()
-  ;; merge-completions needs to work correctly when
+  ;; Normally a `prefix' wildcard ignores the common prefix to its
+  ;; left, since it only grows the common suffix; but if that common
+  ;; prefix is also a common suffix, it should be included.
   (should (equal
-           (completion-pcm--merge-completions '("ab" "sab") '(prefix "b"))
-           '("b" "a" prefix)))
+           (completion-pcm--merge-try '(prefix "b") '("ab" "sab") "" "")
+           '("ab" . 2)))
   (should (equal
-           (completion-pcm--merge-completions '("ab" "ab") '(prefix "b"))
-           '("b" "a")))
+           (completion-pcm--merge-try '(prefix "b") '("ab" "ab") "" "")
+           '("ab" . 2)))
+  ;; When there's a fixed string before `prefix', that fixed string
+  ;; should always be included.
+  (should (equal
+           (completion-pcm--merge-try '("a" prefix "b") '("axb" "ayb") "" "")
+           '("ab" . 2)))
   ;; substring completion should successfully complete the entire string
   (should (equal
            (completion-substring-try-completion "b" '("ab" "ab") nil 0)