Reversing additional change suggested by Serg

under terms of bug#28875 for better performance.
The change appeared to require more changes in item_cmpfunc.cc,
which is dangerous in 5.0.
    
Conversion between a latin1 column and an ascii string constant
stopped to work.

mysql-test/r/ctype_recoding.result

@@ -187,6 +187,14 @@ select * from t1 where a=_latin1'
 ERROR HY000: Illegal mix of collations (cp1251_general_ci,IMPLICIT) and (latin1_swedish_ci,COERCIBLE) for operation '='
 drop table t1;
 set names latin1;
+set names ascii;
+create table t1 (a char(1) character set latin1);
+insert into t1 values ('a');
+select * from t1 where a='a';
+a
+a
+drop table t1;
+set names latin1;
 create table t1 (a char(10) character set utf8 collate utf8_bin);
 insert into t1 values ('       xxx');
 select * from t1 where a=lpad('xxx',10,' ');

mysql-test/t/ctype_recoding.test

@@ -152,6 +152,16 @@ select * from t1 where a=_latin1'
 drop table t1;
 set names latin1;
 
+#
+# Test the same with ascii
+#
+set names ascii;
+create table t1 (a char(1) character set latin1);
+insert into t1 values ('a');
+select * from t1 where a='a';
+drop table t1;
+set names latin1;
+
 #
 # Bug#10446 Illegal mix of collations
 #

sql/sql_string.cc

@@ -263,8 +263,6 @@ bool String::needs_conversion(uint32 arg_length,
       (to_cs == &my_charset_bin) || 
       (to_cs == from_cs) ||
       my_charset_same(from_cs, to_cs) ||
-      (my_charset_is_ascii_based(to_cs) &&
-       my_charset_is_8bit_pure_ascii(from_cs)) ||
       ((from_cs == &my_charset_bin) &&
        (!(*offset=(arg_length % to_cs->mbminlen)))))
     return FALSE;