* src/coding.c (decode_coding_gap): Remove `chars` argument.

* src/json.c (Fjson_insert):
* src/fileio.c (Finsert_file_contents):
* src/coding.h (decode_coding_gap): Adjust accordingly.

src/coding.c

@@ -7892,23 +7892,26 @@ coding_restore_undo_list (Lisp_Object arg)
   bset_undo_list (buf, undo_list);
 }
 
+/* Decode the *last* BYTES of the gap and insert them at point.  */
 void
-decode_coding_gap (struct coding_system *coding,
-		   ptrdiff_t chars, ptrdiff_t bytes)
+decode_coding_gap (struct coding_system *coding, ptrdiff_t bytes)
 {
   ptrdiff_t count = SPECPDL_INDEX ();
   Lisp_Object attrs;
 
+  eassert (GPT_BYTE == PT_BYTE);
+
   coding->src_object = Fcurrent_buffer ();
-  coding->src_chars = chars;
+  coding->src_chars = bytes;
   coding->src_bytes = bytes;
-  coding->src_pos = -chars;
+  coding->src_pos = -bytes;
   coding->src_pos_byte = -bytes;
-  coding->src_multibyte = chars < bytes;
+  coding->src_multibyte = false;
   coding->dst_object = coding->src_object;
   coding->dst_pos = PT;
   coding->dst_pos_byte = PT_BYTE;
-  coding->dst_multibyte = ! NILP (BVAR (current_buffer, enable_multibyte_characters));
+  eassert (coding->dst_multibyte
+           == !NILP (BVAR (current_buffer, enable_multibyte_characters)));
 
   coding->head_ascii = -1;
   coding->detected_utf8_bytes = coding->detected_utf8_chars = -1;
@@ -7922,7 +7925,7 @@ decode_coding_gap (struct coding_system *coding,
       && NILP (CODING_ATTR_POST_READ (attrs))
       && NILP (get_translation_table (attrs, 0, NULL)))
     {
-      chars = coding->head_ascii;
+      ptrdiff_t chars = coding->head_ascii;
       if (chars < 0)
 	chars = check_ascii (coding);
       if (chars != bytes)

src/coding.h

@@ -697,8 +697,7 @@ extern Lisp_Object coding_inherit_eol_type (Lisp_Object, Lisp_Object);
 extern Lisp_Object complement_process_encoding_system (Lisp_Object);
 extern Lisp_Object make_string_from_utf8 (const char *, ptrdiff_t);
 
-extern void decode_coding_gap (struct coding_system *,
-			       ptrdiff_t, ptrdiff_t);
+extern void decode_coding_gap (struct coding_system *, ptrdiff_t);
 extern void decode_coding_object (struct coding_system *,
                                   Lisp_Object, ptrdiff_t, ptrdiff_t,
                                   ptrdiff_t, ptrdiff_t, Lisp_Object);

src/fileio.c

@@ -4538,7 +4538,7 @@ by calling `format-decode', which see.  */)
       Z_BYTE -= inserted;
       ZV -= inserted;
       Z -= inserted;
-      decode_coding_gap (&coding, inserted, inserted);
+      decode_coding_gap (&coding, inserted);
       inserted = coding.produced_char;
       coding_system = CODING_ID_NAME (coding.id);
     }

src/json.c

@@ -769,7 +769,7 @@ usage: (json-insert OBJECT &rest ARGS)  */)
 	  Z_BYTE -= inserted_bytes;
 	  ZV -= inserted_bytes;
 	  Z -= inserted_bytes;
-	  decode_coding_gap (&coding, inserted_bytes, inserted_bytes);
+	  decode_coding_gap (&coding, inserted_bytes);
 	  inserted = coding.produced_char;
 	}
       else