Fix 'replace-buffer-contents' in multibyte buffers

* src/editfns.c (buffer_chars_equal): Pass a byte position to
BUF_FETCH_CHAR_AS_MULTIBYTE, not a character position.
(Bug#31837)

* test/src/editfns-tests.el (replace-buffer-contents-bug31837):
New test.

src/editfns.c

@@ -3304,8 +3304,17 @@ buffer_chars_equal (struct context *ctx,
   eassert (pos_b >= BUF_BEGV (ctx->buffer_b));
   eassert (pos_b < BUF_ZV (ctx->buffer_b));
 
-  return BUF_FETCH_CHAR_AS_MULTIBYTE (ctx->buffer_a, pos_a)
-    == BUF_FETCH_CHAR_AS_MULTIBYTE (ctx->buffer_b, pos_b);
+  ptrdiff_t bpos_a =
+    NILP (BVAR (ctx->buffer_a, enable_multibyte_characters))
+    ? pos_a
+    : buf_charpos_to_bytepos (ctx->buffer_a, pos_a);
+  ptrdiff_t bpos_b =
+    NILP (BVAR (ctx->buffer_b, enable_multibyte_characters))
+    ? pos_b
+    : buf_charpos_to_bytepos (ctx->buffer_b, pos_b);
+
+  return BUF_FETCH_CHAR_AS_MULTIBYTE (ctx->buffer_a, bpos_a)
+    == BUF_FETCH_CHAR_AS_MULTIBYTE (ctx->buffer_b, bpos_b);
 }
 
 

test/src/editfns-tests.el

@@ -247,6 +247,17 @@
                  (buffer-string)
                  "foo bar baz qux"))))))
 
+(ert-deftest replace-buffer-contents-bug31837 ()
+  (switch-to-buffer "a")
+  (insert-char (char-from-name "SMILE"))
+  (insert "1234")
+  (switch-to-buffer "b")
+  (insert-char (char-from-name "SMILE"))
+  (insert "5678")
+  (replace-buffer-contents "a")
+  (should (equal (buffer-substring-no-properties (point-min) (point-max))
+                 (concat (string (char-from-name "SMILE")) "1234"))))
+
 (ert-deftest delete-region-undo-markers-1 ()
   "Make sure we don't end up with freed markers reachable from Lisp."
   ;; https://debbugs.gnu.org/cgi/bugreport.cgi?bug=30931#40