bug #20835 (Subqueries: literal string with =any fails)

We create Item_cache_* object for each operand for each left operand of
a subquery predicate. We also create Item_func_conv_charset for each string
constant that needs charset conversion. So here we have Item_cache wrapped
into Item_func_conv_charset.
When Item_func_conv_charset wraps an constant Item it gets it's value
in constructor. The problem is that Item_cache is ready to be used only
at execution time, which is too late.
The fix makes Item_cache wrapping constant to get ready at fix_fields() time.

mysql-test/r/subselect.result

@@ -3592,3 +3592,9 @@ FROM t1) t;
 COUNT(*)
 3000
 DROP TABLE t1,t2;
+CREATE TABLE t1 (s1 char(1));
+INSERT INTO t1 VALUES ('a');
+SELECT * FROM t1 WHERE _utf8'a' = ANY (SELECT s1 FROM t1);
+s1
+a
+DROP TABLE t1;

mysql-test/t/subselect.test

@@ -2496,3 +2496,11 @@ SELECT SQL_NO_CACHE COUNT(*)
           FROM t1) t;
 
 DROP TABLE t1,t2;
+
+#
+# Bug#20835 (literal string with =any values)
+#
+CREATE TABLE t1 (s1 char(1));
+INSERT INTO t1 VALUES ('a');
+SELECT * FROM t1 WHERE _utf8'a' = ANY (SELECT s1 FROM t1);
+DROP TABLE t1;

sql/item_cmpfunc.cc

@@ -797,7 +797,8 @@ bool Item_in_optimizer::fix_left(THD *thd, Item **ref)
   }
   not_null_tables_cache= args[0]->not_null_tables();
   with_sum_func= args[0]->with_sum_func;
-  const_item_cache= args[0]->const_item();
+  if ((const_item_cache= args[0]->const_item()))
+    cache->store(args[0]);
   return 0;
 }