sql_yacc.yy:

  _charset cast changes coercibility to implicit
item_strfunc.cc:
  Collation cast change coercibility to EXPLICIT

sql/item_strfunc.cc

@@ -2201,6 +2201,7 @@ bool Item_func_set_collation::fix_fields(THD *thd,struct st_table_list *tables,
   const_item_cache=args[0]->const_item();
   fix_length_and_dec();
   fixed= 1;
+  coercibility= COER_EXPLICIT;
   return 0;
 }
 

sql/sql_yacc.yy

@@ -3778,7 +3778,7 @@ text_literal:
 	{ $$ = new Item_string($1.str,$1.length,
 			       YYTHD->variables.thd_charset); }
 	| UNDERSCORE_CHARSET TEXT_STRING
-	  { $$ = new Item_string($2.str,$2.length,Lex->charset,Item::COER_EXPLICIT); }
+	  { $$ = new Item_string($2.str,$2.length,Lex->charset,Item::COER_IMPLICIT); }
 	| text_literal TEXT_STRING
 	  { ((Item_string*) $1)->append($2.str,$2.length); };