Bug#14146 CHAR(...USING ...) and CONVERT(CHAR(...) USING...) produce different results

ctype_utf8.result, ctype_utf8.test:
  Adding test case.
item_strfunc.cc:
item_strfunc.h:
  Moving the well formed checking code into a method,
  to reuse in several Item_func_xxx. Reusing the new
  method in Item_func_char and Item_func_charset_conv.

mysql-test/r/ctype_utf8.result

@@ -1095,6 +1095,11 @@ char(0xff,0x8f using utf8)
 
 Warnings:
 Warning	1300	Invalid utf8 character string: 'FF8F'
+select convert(char(0xff,0x8f) using utf8);
+convert(char(0xff,0x8f) using utf8)
+
+Warnings:
+Warning	1300	Invalid utf8 character string: 'FF8F'
 set sql_mode=traditional;
 select char(0xff,0x8f using utf8);
 char(0xff,0x8f using utf8)
@@ -1116,6 +1121,11 @@ char(2557 using utf8)
 NULL
 Warnings:
 Error	1300	Invalid utf8 character string: 'FD'
+select convert(char(0xff,0x8f) using utf8);
+convert(char(0xff,0x8f) using utf8)
+NULL
+Warnings:
+Error	1300	Invalid utf8 character string: 'FF8F'
 select hex(convert(char(2557 using latin1) using utf8));
 hex(convert(char(2557 using latin1) using utf8))
 09C3BD

mysql-test/t/ctype_utf8.test

@@ -884,7 +884,9 @@ SELECT DISTINCT id FROM t1 ORDER BY id;
 DROP TABLE t1;
 
 #
-#  Bugs#10504: Character set does not support traditional mode
+#  Bug#10504: Character set does not support traditional mode
+#  Bug#14146: CHAR(...USING ...) and CONVERT(CHAR(...) USING...)
+#             produce different results
 #
 set names utf8;
 # correct value
@@ -894,12 +896,14 @@ select char(0xd18f using utf8);
 select char(53647 using utf8);
 # incorrect value: return with warning
 select char(0xff,0x8f using utf8);
+select convert(char(0xff,0x8f) using utf8);
 # incorrect value in strict mode: return NULL with "Error" level warning
 set sql_mode=traditional;
 select char(0xff,0x8f using utf8);
 select char(195 using utf8);
 select char(196 using utf8);
 select char(2557 using utf8);
+select convert(char(0xff,0x8f) using utf8);
 
 #
 # Check convert + char + using

sql/item_strfunc.cc

@@ -48,6 +48,38 @@ static void my_coll_agg_error(DTCollation &c1, DTCollation &c2,
 }
 
 
+String *Item_str_func::check_well_formed_result(String *str)
+{
+  /* Check whether we got a well-formed string */
+  CHARSET_INFO *cs= str->charset();
+  int well_formed_error;
+  uint wlen= cs->cset->well_formed_len(cs,
+                                       str->ptr(), str->ptr() + str->length(),
+                                       str->length(), &well_formed_error);
+  if (wlen < str->length())
+  {
+    THD *thd= current_thd;
+    char hexbuf[7];
+    enum MYSQL_ERROR::enum_warning_level level;
+    uint diff= str->length() - wlen;
+    set_if_smaller(diff, 3);
+    octet2hex(hexbuf, str->ptr() + wlen, diff);
+    if (thd->variables.sql_mode &
+        (MODE_STRICT_TRANS_TABLES | MODE_STRICT_ALL_TABLES))
+    {
+      level= MYSQL_ERROR::WARN_LEVEL_ERROR;
+      null_value= 1;
+      str= 0;
+    }
+    else
+      level= MYSQL_ERROR::WARN_LEVEL_WARN;
+    push_warning_printf(thd, level, ER_INVALID_CHARACTER_STRING,
+                        ER(ER_INVALID_CHARACTER_STRING), cs->csname, hexbuf);
+  }
+  return str;
+}
+
+
 double Item_str_func::val_real()
 {
   DBUG_ASSERT(fixed == 1);
@@ -1984,34 +2016,7 @@ String *Item_func_char::val_str(String *str)
   }
   str->set_charset(collation.collation);
   str->realloc(str->length());			// Add end 0 (for Purify)
-
-  /* Check whether we got a well-formed string */
-  CHARSET_INFO *cs= collation.collation;
-  int well_formed_error;
-  uint wlen= cs->cset->well_formed_len(cs,
-                                       str->ptr(), str->ptr() + str->length(),
-                                       str->length(), &well_formed_error);
-  if (wlen < str->length())
-  {
-    THD *thd= current_thd;
-    char hexbuf[7];
-    enum MYSQL_ERROR::enum_warning_level level;
-    uint diff= str->length() - wlen;
-    set_if_smaller(diff, 3);
-    octet2hex(hexbuf, str->ptr() + wlen, diff);
-    if (thd->variables.sql_mode &
-        (MODE_STRICT_TRANS_TABLES | MODE_STRICT_ALL_TABLES))
-    {
-      level= MYSQL_ERROR::WARN_LEVEL_ERROR;
-      null_value= 1;
-      str= 0;
-    }
-    else
-      level= MYSQL_ERROR::WARN_LEVEL_WARN;
-    push_warning_printf(thd, level, ER_INVALID_CHARACTER_STRING,
-                        ER(ER_INVALID_CHARACTER_STRING), cs->csname, hexbuf);
-  }
-  return str;
+  return check_well_formed_result(str);
 }
 
 
@@ -2320,7 +2325,7 @@ String *Item_func_conv_charset::val_str(String *str)
   }
   null_value= str_value.copy(arg->ptr(),arg->length(),arg->charset(),
                              conv_charset, &dummy_errors);
-  return null_value ? 0 : &str_value;
+  return null_value ? 0 : check_well_formed_result(&str_value);
 }
 
 void Item_func_conv_charset::fix_length_and_dec()

sql/item_strfunc.h

@@ -35,6 +35,7 @@ public:
   double val_real();
   enum Item_result result_type () const { return STRING_RESULT; }
   void left_right_max_length();
+  String *check_well_formed_result(String *str);
 };
 
 class Item_func_md5 :public Item_str_func