Fix case-insensitive completion of buffer names

* test/src/minibuf-tests.el (test-try-completion-ignore-case):
New test, suggested by Stefan Monnier <monnier@iro.umontreal.ca>.

* src/minibuf.c (Ftry_completion): Don't treat strings that
are identical but for the case as if they were identical for
the purposes of not counting the same string twice.  This
fixes case-insensitive completion when all the candidates are
identical but for the letter-case.  (Bug#11339)

2 files changed, 21 insertions, 5 deletions

diff --git a/src/minibuf.c b/src/minibuf.c
index f6cf47f1f28..1e87c5044af 100644
--- a/src/minibuf.c
+++ b/src/minibuf.c
@@ -1323,13 +1323,13 @@ is used to further constrain the set of candidates.  */)
           else
             {
               compare = min (bestmatchsize, SCHARS (eltstring));
-              tem = Fcompare_strings (bestmatch, zero,
-                                      make_fixnum (compare),
-                                      eltstring, zero,
-                                      make_fixnum (compare),
+              Lisp_Object lcompare = make_fixnum (compare);
+              tem = Fcompare_strings (bestmatch, zero, lcompare,
+                                      eltstring, zero, lcompare,
                                       completion_ignore_case ? Qt : Qnil);
               matchsize = EQ (tem, Qt) ? compare : eabs (XFIXNUM (tem)) - 1;
 
+              Lisp_Object old_bestmatch = bestmatch;
               if (completion_ignore_case)
                 {
                   /* If this is an exact match except for case,
@@ -1363,7 +1363,12 @@ is used to further constrain the set of candidates.  */)
                     bestmatch = eltstring;
                 }
               if (bestmatchsize != SCHARS (eltstring)
-                  || bestmatchsize != matchsize)
+                  || bestmatchsize != matchsize
+                  || (completion_ignore_case
+                      && !EQ (Fcompare_strings (old_bestmatch, zero, lcompare,
+                                                eltstring, zero, lcompare,
+                                                Qnil),
+                              Qt)))
                 /* Don't count the same string multiple times.  */
                 matchcount += matchcount <= 1;
               bestmatchsize = matchsize;
diff --git a/test/src/minibuf-tests.el b/test/src/minibuf-tests.el
index 12b018b8228..c64892822d6 100644
--- a/test/src/minibuf-tests.el
+++ b/test/src/minibuf-tests.el
@@ -399,5 +399,16 @@
   (minibuf-tests--test-completion-regexp
    #'minibuf-tests--strings-to-symbol-hashtable))
 
+(ert-deftest test-try-completion-ignore-case ()
+  (let ((completion-ignore-case t))
+    (should (equal (try-completion "bar" '("bAr" "barfoo")) "bAr"))
+    (should (equal (try-completion "bar" '("bArfoo" "barbaz")) "bar"))
+    (should (equal (try-completion "bar" '("bArfoo" "barbaz"))
+                   (try-completion "bar" '("barbaz" "bArfoo"))))
+    ;; bug#11339
+    (should (equal (try-completion "baz" '("baz" "bAz")) "baz")) ;And not `t'!
+    (should (equal (try-completion "baz" '("bAz" "baz"))
+                   (try-completion "baz" '("baz" "bAz"))))))
+
 
 ;;; minibuf-tests.el ends here