MDEV-26646 SFORMAT Does not allow @variable use

If charset aggregation decides to convert arguments to some specific
charset and some arguments are numbers, they'll be wrapped into a
Item_func_conv_charset(), that changes result_type() to STRING_RESULT.

Fix: don't convert numbers, sformat needs their numerical value only.

mysql-test/main/func_sformat.result

@@ -437,6 +437,7 @@ t1	CREATE TABLE `t1` (
   `x` varchar(8) CHARACTER SET ucs2 DEFAULT NULL
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 drop table t1;
+set names latin1;
 #
 # ps parameters
 #
@@ -455,3 +456,15 @@ Warning	4183	SFORMAT error: invalid type specifier
 select sformat('{}', cast(1.1 as float));
 sformat('{}', cast(1.1 as float))
 1.1
+#
+# MDEV-26646 SFORMAT Does not allow @variable use
+#
+set names utf8;
+set @a=3.14;
+select sformat('{:f}', @a);
+sformat('{:f}', @a)
+3.140000
+set names latin1;
+#
+# End of 10.7 tests
+#

mysql-test/main/func_sformat.test

@@ -199,7 +199,7 @@ select hex(sformat(_ucs2 x'003D007B007D003D', _ucs2 x'0442043504410442'));
 create table t1 as select sformat(_ucs2 x'003D007B007D003D', _ucs2 x'0442043504410442') as x;
 show create table t1;
 drop table t1;
-
+set names latin1;
 
 echo #;
 echo # ps parameters;
@@ -213,3 +213,16 @@ echo # MDEV-26691 SFORMAT: Pass down FLOAT as FLOAT, without upcast to DOUBLE;
 echo #;
 
 select sformat('{}', cast(1.1 as float));
+
+echo #;
+echo # MDEV-26646 SFORMAT Does not allow @variable use;
+echo #;
+
+set names utf8;
+set @a=3.14;
+select sformat('{:f}', @a);
+set names latin1;
+
+echo #;
+echo # End of 10.7 tests;
+echo #;

sql/item_strfunc.cc

@@ -1336,12 +1336,14 @@ bool Item_func_sformat::fix_length_and_dec()
   if (c.collation->mbminlen > 1)
     c.collation= &my_charset_utf8mb4_bin;
 
-  if (Type_std_attributes::agg_item_set_converter(c, func_name_cstring(), args,
-                                                  arg_count, flags, 1))
-    return TRUE;
-
   for (uint i=0 ; i < arg_count ; i++)
+  {
     char_length+= args[i]->max_char_length();
+    if (args[i]->result_type() == STRING_RESULT &&
+        Type_std_attributes::agg_item_set_converter(c, func_name_cstring(),
+                                                    args+i, 1, flags, 1))
+      return TRUE;
+  }
 
   fix_char_length_ulonglong(char_length);
   return FALSE;