Bug#15949 union + illegal mix of collations (IMPLICIT + COERCIBLE)

union.result, union.test:
  Adding test case.
item.cc:
  Allow safe character set conversion in UNION
  - string constant to column's charset
  - to unicode
  Thus, UNION now works the same with CONCAT (and other string functions)
  in respect of aggregating arguments with different character sets.

mysql-test/r/union.result

@@ -1185,6 +1185,16 @@ select concat('value is: ', @val) union select 'some text';
 concat('value is: ', @val)
 value is: 6
 some text
+select concat(_latin1'a', _ascii'b' collate ascii_bin);
+concat(_latin1'a', _ascii'b' collate ascii_bin)
+ab
+create table t1 (foo varchar(100)) collate ascii_bin;
+insert into t1 (foo) values ("foo");
+select foo from t1 union select 'bar' as foo from dual;
+foo
+foo
+bar
+drop table t1;
 CREATE TABLE t1 (
 a ENUM('','','') character set utf8 not null default '',
 b ENUM("one", "two") character set utf8,
@@ -1214,7 +1224,7 @@ Field	Type	Null	Key	Default	Extra
 a	char(1)				
 drop table t2;
 create table t2 select a from t1 union select c from t1;
-ERROR HY000: Illegal mix of collations (utf8_general_ci,IMPLICIT) and (latin1_swedish_ci,IMPLICIT) for operation 'UNION'
+drop table t2;
 create table t2 select a from t1 union select b from t1;
 show columns from t2;
 Field	Type	Null	Key	Default	Extra

mysql-test/t/union.test

@@ -710,6 +710,15 @@ drop table t1;
 set @val:=6;
 select concat('value is: ', @val) union select 'some text';
 
+#
+# Bug#15949 union + illegal mix of collations (IMPLICIT + COERCIBLE)
+#
+select concat(_latin1'a', _ascii'b' collate ascii_bin);
+create table t1 (foo varchar(100)) collate ascii_bin;
+insert into t1 (foo) values ("foo");
+select foo from t1 union select 'bar' as foo from dual;
+drop table t1;
+
 #
 # Enum merging test
 #
@@ -729,8 +738,8 @@ drop table t2;
 create table t2 select a from t1 union select a from t1;
 show columns from t2;
 drop table t2;
--- error 1267
 create table t2 select a from t1 union select c from t1;
+drop table t2;
 create table t2 select a from t1 union select b from t1;
 show columns from t2;
 drop table t2, t1;

sql/item.cc

@@ -3258,7 +3258,7 @@ bool Item_type_holder::join_types(THD *thd, Item *item)
     const char *old_cs, *old_derivation;
     old_cs= collation.collation->name;
     old_derivation= collation.derivation_name();
-    if (collation.aggregate(item->collation))
+    if (collation.aggregate(item->collation, MY_COLL_ALLOW_CONV))
     {
       my_error(ER_CANT_AGGREGATE_2COLLATIONS, MYF(0),
 	       old_cs, old_derivation,