Bug #31884: Assertion + crash in subquery in the SELECT clause.

Item_in_subselect's only externally callable method is val_bool().
However the nullability in the wrapper class (Item_in_optimizer) is 
established by calling the "forbidden" method val_int().

Fixed to use the correct method (val_bool() ) to establish nullability 
of Item_in_subselect in Item_in_optimizer.

mysql-test/r/subselect.result

@@ -4139,4 +4139,15 @@ SELECT (SELECT SUM(t1.a) FROM t2 WHERE a=1) FROM t1;
 (SELECT SUM(t1.a) FROM t2 WHERE a=1)
 3
 DROP TABLE t1,t2;
+CREATE TABLE t1 (a1 INT, a2 INT);
+CREATE TABLE t2 (b1 INT, b2 INT);
+INSERT INTO t1 VALUES (100, 200);
+INSERT INTO t1 VALUES (101, 201);
+INSERT INTO t2 VALUES (101, 201);
+INSERT INTO t2 VALUES (103, 203);
+SELECT ((a1,a2) IN (SELECT * FROM t2 WHERE b2 > 0)) IS NULL FROM t1;
+((a1,a2) IN (SELECT * FROM t2 WHERE b2 > 0)) IS NULL
+0
+0
+DROP TABLE t1, t2;
 End of 5.0 tests.

mysql-test/t/subselect.test

@@ -2987,4 +2987,19 @@ SELECT (SELECT SUM(t1.a) FROM t2 WHERE a!=0) FROM t1;
 SELECT (SELECT SUM(t1.a) FROM t2 WHERE a=1) FROM t1;
 DROP TABLE t1,t2;
 
+#
+# Bug #31884: Assertion + crash in subquery in the SELECT clause.
+#
+
+CREATE TABLE t1 (a1 INT, a2 INT);
+CREATE TABLE t2 (b1 INT, b2 INT);
+
+INSERT INTO t1 VALUES (100, 200);
+INSERT INTO t1 VALUES (101, 201);
+INSERT INTO t2 VALUES (101, 201);
+INSERT INTO t2 VALUES (103, 203);
+
+SELECT ((a1,a2) IN (SELECT * FROM t2 WHERE b2 > 0)) IS NULL FROM t1;
+DROP TABLE t1, t2;
+
 --echo End of 5.0 tests.

sql/item_subselect.h

@@ -306,6 +306,7 @@ public:
   double val_real();
   String *val_str(String*);
   my_decimal *val_decimal(my_decimal *);
+  void update_null_value () { (void) val_bool(); }
   bool val_bool();
   void top_level_item() { abort_on_null=1; }
   inline bool is_top_level_item() { return abort_on_null; }