A user variable are now always have IMPLICIT coercibility,

independently from the expression it is initialized from.
In other words, this change treats a user variable like
a table with one column and one record. Discussed with 
PeterG, Serg and Lars. This change also simplifies replication
allowing not to replicate variables' coercibility.

mysql-test/r/user_var.result

@@ -123,7 +123,7 @@ drop table t1;
 set @a=_latin2'test';
 select charset(@a),collation(@a),coercibility(@a);
 charset(@a)	collation(@a)	coercibility(@a)
-latin2	latin2_general_ci	3
+latin2	latin2_general_ci	2
 select @a=_latin2'TEST';
 @a=_latin2'TEST'
 1
@@ -133,12 +133,13 @@ select @a=_latin2'TEST' collate latin2_bin;
 set @a=_latin2'test' collate latin2_general_ci;
 select charset(@a),collation(@a),coercibility(@a);
 charset(@a)	collation(@a)	coercibility(@a)
-latin2	latin2_general_ci	0
+latin2	latin2_general_ci	2
 select @a=_latin2'TEST';
 @a=_latin2'TEST'
 1
 select @a=_latin2'TEST' collate latin2_bin;
-ERROR HY000: Illegal mix of collations (latin2_general_ci,EXPLICIT) and (latin2_bin,EXPLICIT) for operation '='
+@a=_latin2'TEST' collate latin2_bin
+0
 select charset(@a:=_latin2'test');
 charset(@a:=_latin2'test')
 latin2
@@ -147,21 +148,22 @@ collation(@a:=_latin2'test')
 latin2_general_ci
 select coercibility(@a:=_latin2'test');
 coercibility(@a:=_latin2'test')
-3
+2
 select collation(@a:=_latin2'test' collate latin2_bin);
 collation(@a:=_latin2'test' collate latin2_bin)
 latin2_bin
 select coercibility(@a:=_latin2'test' collate latin2_bin);
 coercibility(@a:=_latin2'test' collate latin2_bin)
-0
+2
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST';
 (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST'
 0
 select charset(@a),collation(@a),coercibility(@a);
 charset(@a)	collation(@a)	coercibility(@a)
-latin2	latin2_bin	0
+latin2	latin2_bin	2
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST' collate latin2_general_ci;
-ERROR HY000: Illegal mix of collations (latin2_bin,EXPLICIT) and (latin2_general_ci,EXPLICIT) for operation '='
+(@a:=_latin2'test' collate latin2_bin) = _latin2'TEST' collate latin2_general_ci
+1
 create table t1 (a varchar(50));
 reset master;
 SET TIMESTAMP=10000;

mysql-test/t/user_var.test

@@ -84,7 +84,6 @@ select @a=_latin2'TEST' collate latin2_bin;
 set @a=_latin2'test' collate latin2_general_ci;
 select charset(@a),collation(@a),coercibility(@a);
 select @a=_latin2'TEST';
---error 1267
 select @a=_latin2'TEST' collate latin2_bin;
 
 #
@@ -97,7 +96,6 @@ select collation(@a:=_latin2'test' collate latin2_bin);
 select coercibility(@a:=_latin2'test' collate latin2_bin);
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST';
 select charset(@a),collation(@a),coercibility(@a);
---error 1267
 select (@a:=_latin2'test' collate latin2_bin) = _latin2'TEST' collate latin2_general_ci;
 
 # Check that user variables are binlogged correctly (BUG#3875)

sql/item_func.cc

@@ -2361,7 +2361,7 @@ static user_var_entry *get_variable(HASH *hash, LEX_STRING &name,
     entry->value=0;
     entry->length=0;
     entry->update_query_id=0;
-    entry->collation.set(NULL, DERIVATION_NONE);
+    entry->collation.set(NULL, DERIVATION_IMPLICIT);
     /*
       If we are here, we were called from a SET or a query which sets a
       variable. Imagine it is this:
@@ -2419,8 +2419,8 @@ bool Item_func_set_user_var::fix_fields(THD *thd, TABLE_LIST *tables,
     and the variable has previously been initialized.
   */
   if (!entry->collation.collation || !args[0]->null_value)
-    entry->collation.set(args[0]->collation);
-  collation.set(entry->collation);
+    entry->collation.set(args[0]->collation.collation, DERIVATION_IMPLICIT);
+  collation.set(entry->collation.collation, DERIVATION_IMPLICIT);
   cached_result_type= args[0]->result_type();
   return 0;
 }
@@ -2432,7 +2432,7 @@ Item_func_set_user_var::fix_length_and_dec()
   maybe_null=args[0]->maybe_null;
   max_length=args[0]->max_length;
   decimals=args[0]->decimals;
-  collation.set(args[0]->collation);
+  collation.set(args[0]->collation.collation, DERIVATION_IMPLICIT);
 }
 
 
@@ -2659,7 +2659,7 @@ Item_func_set_user_var::update()
       res= update_hash((void*) save_result.vstr->ptr(),
 		       save_result.vstr->length(), STRING_RESULT,
 		       save_result.vstr->charset(),
-		       args[0]->collation.derivation);
+		       DERIVATION_IMPLICIT);
     break;
   }
   case ROW_RESULT: