4 files changed, 18 insertions, 31 deletions

diff --git a/src/buffer.c b/src/buffer.c
index ab477481912..3b5078a175b 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2264,7 +2264,7 @@ validate_region (register Lisp_Object *b, register Lisp_Object *e)
 /* Advance BYTE_POS up to a character boundary
    and return the adjusted position.  */
 
-static ptrdiff_t
+ptrdiff_t
 advance_to_char_boundary (ptrdiff_t byte_pos)
 {
   int c;
@@ -2702,6 +2702,9 @@ current buffer is cleared.  */)
 
       /* Do this last, so it can calculate the new correspondences
          between chars and bytes.  */
+      /* FIXME: Is it worth the trouble, really?  Couldn't we just throw
+         away all the text-properties instead of trying to guess how
+         to adjust them?  AFAICT the result is not reliable anyway.  */
       set_intervals_multibyte (1);
     }
 
diff --git a/src/buffer.h b/src/buffer.h
index f42c3e97b97..2080a6f40b7 100644
--- a/src/buffer.h
+++ b/src/buffer.h
@@ -327,6 +327,10 @@ extern void enlarge_buffer_text (struct buffer *, ptrdiff_t);
 #define BYTE_TO_CHAR(bytepos)                   \
   (buf_bytepos_to_charpos (current_buffer, bytepos))
 
+/* For those very rare cases where you may have a "random" pointer into
+   the middle of a multibyte char, this moves to the next boundary.  */
+extern ptrdiff_t advance_to_char_boundary (ptrdiff_t byte_pos);
+
 /* Convert PTR, the address of a byte in the buffer, into a byte position.  */
 
 #define PTR_BYTE_POS(ptr) \
diff --git a/src/intervals.c b/src/intervals.c
index 8f39c45762f..34829ab050d 100644
--- a/src/intervals.c
+++ b/src/intervals.c
@@ -2335,22 +2335,10 @@ set_intervals_multibyte_1 (INTERVAL i, bool multi_flag,
       if (multi_flag)
         {
           ptrdiff_t temp;
-          left_end_byte = start_byte + LEFT_TOTAL_LENGTH (i);
+          left_end_byte
+            = advance_to_char_boundary (start_byte + LEFT_TOTAL_LENGTH (i));
           left_end = BYTE_TO_CHAR (left_end_byte);
-
-          temp = CHAR_TO_BYTE (left_end);
-
-          /* If LEFT_END_BYTE is in the middle of a character,
-             adjust it and LEFT_END to a char boundary.  */
-          if (left_end_byte > temp)
-            {
-              left_end_byte = temp;
-            }
-          if (left_end_byte < temp)
-            {
-              left_end--;
-              left_end_byte = CHAR_TO_BYTE (left_end);
-            }
+          eassert (CHAR_TO_BYTE (left_end) == left_end_byte);
         }
       else
         {
@@ -2369,22 +2357,10 @@ set_intervals_multibyte_1 (INTERVAL i, bool multi_flag,
         {
           ptrdiff_t temp;
 
-          right_start_byte = end_byte - RIGHT_TOTAL_LENGTH (i);
+          right_start_byte
+            = advance_to_char_boundary (end_byte - RIGHT_TOTAL_LENGTH (i));
           right_start = BYTE_TO_CHAR (right_start_byte);
-
-          /* If RIGHT_START_BYTE is in the middle of a character,
-             adjust it and RIGHT_START to a char boundary.  */
-          temp = CHAR_TO_BYTE (right_start);
-
-          if (right_start_byte < temp)
-            {
-              right_start_byte = temp;
-            }
-          if (right_start_byte > temp)
-            {
-              right_start++;
-              right_start_byte = CHAR_TO_BYTE (right_start);
-            }
+          eassert (CHAR_TO_BYTE (right_start) == right_start_byte);
         }
       else
         {
diff --git a/src/marker.c b/src/marker.c
index b58051a8c2b..0b2e1bf5c6b 100644
--- a/src/marker.c
+++ b/src/marker.c
@@ -332,6 +332,10 @@ buf_bytepos_to_charpos (struct buffer *b, ptrdiff_t bytepos)
   if (best_above == best_above_byte)
     return bytepos;
 
+  /* Check bytepos is not in the middle of a character. */
+  eassert (bytepos >= BUF_Z_BYTE (b)
+           || CHAR_HEAD_P (BUF_FETCH_BYTE (b, bytepos)));
+
   best_below = BEG;
   best_below_byte = BEG_BYTE;