fix an overly agressive optimization in Item_func_conv_charset

mysql-test/r/mdev316.result

@@ -5,3 +5,18 @@ select '' in ('',convert(0,time));
 select case '' when '' then 1 when convert(0,time) then 2 end;
 case '' when '' then 1 when convert(0,time) then 2 end
 1
+select convert(0x0030 using ucs2) div 1, concat(convert(0x0030 using ucs2)) div 1;
+convert(0x0030 using ucs2) div 1	concat(convert(0x0030 using ucs2)) div 1
+0	0
+select cast(convert(0x0030 using ucs2) as double), cast(concat(convert(0x0030 using ucs2)) as double);
+cast(convert(0x0030 using ucs2) as double)	cast(concat(convert(0x0030 using ucs2)) as double)
+0	0
+select cast(convert(0x0030 using ucs2) as decimal(5.2)), cast(concat(convert(0x0030 using ucs2)) as decimal(5.2));
+cast(convert(0x0030 using ucs2) as decimal(5.2))	cast(concat(convert(0x0030 using ucs2)) as decimal(5.2))
+0	0
+select cast(convert(_ucs2 0x0030 using latin1) as date), cast(concat(convert(_ucs2 0x0030 using latin1)) as date);
+cast(convert(_ucs2 0x0030 using latin1) as date)	cast(concat(convert(_ucs2 0x0030 using latin1)) as date)
+NULL	NULL
+Warnings:
+Warning	1292	Incorrect datetime value: '0'
+Warning	1292	Incorrect datetime value: '0'

mysql-test/t/mdev316.test

@@ -4,3 +4,11 @@
 set names swe7;
 select '' in ('',convert(0,time));
 select case '' when '' then 1 when convert(0,time) then 2 end;
+
+#
+# Excessive optimization of Item_func_conv_charset:
+#
+select convert(0x0030 using ucs2) div 1, concat(convert(0x0030 using ucs2)) div 1;
+select cast(convert(0x0030 using ucs2) as double), cast(concat(convert(0x0030 using ucs2)) as double);
+select cast(convert(0x0030 using ucs2) as decimal(5.2)), cast(concat(convert(0x0030 using ucs2)) as decimal(5.2));
+select cast(convert(_ucs2 0x0030 using latin1) as date), cast(concat(convert(_ucs2 0x0030 using latin1)) as date);

sql/item_strfunc.h

@@ -855,13 +855,29 @@ public:
   }
   String *val_str(String *);
   longlong val_int()
-  { return args[0]->val_int(); }
+  {
+    if (args[0]->result_type() == STRING_RESULT)
+      return Item_str_func::val_int();
+    return args[0]->val_int();
+  }
   double val_real()
-  { return args[0]->val_real(); }
+  {
+    if (args[0]->result_type() == STRING_RESULT)
+      return Item_str_func::val_real();
+    return args[0]->val_real();
+  }
   my_decimal *val_decimal(my_decimal *d)
-  { return args[0]->val_decimal(d); }
+  {
+    if (args[0]->result_type() == STRING_RESULT)
+      return Item_str_func::val_decimal(d);
+    return args[0]->val_decimal(d);
+  }
   bool get_date(MYSQL_TIME *ltime, ulonglong fuzzydate)
-  { return args[0]->get_date(ltime, fuzzydate); }
+  {
+    if (args[0]->result_type() == STRING_RESULT)
+      return Item_str_func::get_date(ltime, fuzzydate);
+    return args[0]->get_date(ltime, fuzzydate);
+  }
   void fix_length_and_dec();
   const char *func_name() const { return "convert"; }
   virtual void print(String *str, enum_query_type query_type);