Return case common to all completions in try-completion

When completion-ignore-case is non-nil, if all completions share
a common prefix ignoring case, try-completion has always
returned that.  Now, if all completions also share a common
prefix including case, try-completion includes that common
prefix in its return value (bug#79377).

* lisp/minibuffer.el (completion-pcm--merge-completions): Always
use return value from try-completion, which may change case.
* src/minibuf.c (Ftry_completion): Return the common prefix
which changes case.
* test/lisp/minibuffer-tests.el (completion-pcm-bug4219)
(completion-substring-test-5): New tests.

lisp/minibuffer.el

@@ -4763,7 +4763,7 @@ the same set of elements."
                   ;; `prefix' only wants to include the fixed part before the
                   ;; wildcard, not the result of growing that fixed part.
                   (when (seq-some (lambda (elem) (eq elem 'prefix)) wildcards)
-                    (setq prefix fixed))
+                    (setq prefix (substring prefix 0 (length fixed))))
                   (push prefix res)
                   ;; Push all the wildcards in this stretch, to preserve `point' and
                   ;; `star' wildcards before ELEM.

src/minibuf.c

@@ -1820,12 +1820,6 @@ or from one of the possible completions.  */)
 
   if (NILP (bestmatch))
     return Qnil;		/* No completions found.  */
-  /* If we are ignoring case, and there is no exact match,
-     and no additional text was supplied,
-     don't change the case of what the user typed.  */
-  if (completion_ignore_case && bestmatchsize == SCHARS (string)
-      && SCHARS (bestmatch) > bestmatchsize)
-    return string;
 
   /* Return t if the supplied string is an exact match (counting case);
      it does not require any change to be made.  */

test/lisp/minibuffer-tests.el

@@ -345,7 +345,34 @@
   (should (equal
            (let ((completion-ignore-case t))
              (completion-pcm-try-completion "a" '("ABC" "ABD") nil 1))
-           '("AB" . 2))))
+           '("AB" . 2)))
+  ;; Even when the text isn't growing.
+  (should (equal
+           (let ((completion-ignore-case t))
+             (completion-pcm-try-completion "ab" '("ABC" "ABD") nil 2))
+           '("AB" . 2)))
+  ;; Or when point is in the middle of the region changing case.
+  (should (equal
+           (let ((completion-ignore-case t))
+             (completion-pcm-try-completion "ab" '("ABC" "ABD") nil 1))
+           '("AB" . 2)))
+  ;; Even when the existing minibuffer contents has mixed case.
+  (should (equal
+           (let ((completion-ignore-case t))
+             (completion-pcm-try-completion "Ab" '("ABC" "ABD") nil 1))
+           '("AB" . 2)))
+  ;; But not if the completions don't actually all have the same case.
+  (should (equal
+           (let ((completion-ignore-case t))
+             (completion-pcm-try-completion "Ab" '("abc" "ABD") nil 1))
+           '("Ab" . 2)))
+  ;; We don't change case if it doesn't match all of the completions, though.
+  (should (equal
+           (let ((completion-ignore-case t)) (try-completion "a" '("ax" "Ay")))
+           "a"))
+  (should (equal
+           (let ((completion-ignore-case t)) (try-completion "a" '("Ay" "ax")))
+           "a")))
 
 (ert-deftest completion-substring-test-1 ()
   ;; One third of a match!
@@ -409,6 +436,15 @@
   (should (equal
            (completion-pcm--merge-try '("a" prefix "b") '("axb" "ayb") "" "")
            '("ab" . 2)))
+  ;; Letter-casing from the completions on the common prefix is still applied.
+  (should (equal
+           (let ((completion-ignore-case t))
+             (completion-pcm--merge-try '("a" prefix "b") '("Axb" "Ayb") "" ""))
+           '("Ab" . 2)))
+  (should (equal
+           (let ((completion-ignore-case t))
+             (completion-pcm--merge-try '("a" prefix "b") '("AAxb" "AAyb") "" ""))
+           '("Ab" . 2)))
   ;; substring completion should successfully complete the entire string
   (should (equal
            (completion-substring-try-completion "b" '("ab" "ab") nil 0)