MDEV-31319 Assertion const_item_cache == true failed in Item_func::fix_fields

On create table tmp as select ... we exited Item_func::fix_fields()
with error. fix_fields_if_needed('foo' or 'bar') failed and we
returned true, but already changed const_item_cache. So the item is in
inconsistent state: fixed == false and const_item_cache == false.

Now we cleanup the item before the return if Item_func::fix_fields()
fails to process.

mysql-test/suite/vcol/r/vcol_syntax.result

@@ -197,3 +197,22 @@ Warnings:
 Warning	1292	Truncated incorrect DECIMAL value: 'x'
 Warning	1292	Truncated incorrect DECIMAL value: 'test'
 drop table t1;
+#
+# MDEV-31319 Assertion const_item_cache == true failed in Item_func::fix_fields
+#
+create table t (f1 int, f2 int, fv int generated always as (case user() when 'foo' or 'bar' then f1 else f2 end) virtual);
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'foo'
+Warning	1292	Truncated incorrect DOUBLE value: 'bar'
+select * from t;
+f1	f2	fv
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'foo'
+Warning	1292	Truncated incorrect DOUBLE value: 'bar'
+create table tmp as select * from information_schema.tables where table_name = 't';
+select * from t;
+f1	f2	fv
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'foo'
+Warning	1292	Truncated incorrect DOUBLE value: 'bar'
+drop table t, tmp;

mysql-test/suite/vcol/t/vcol_syntax.test

@@ -162,3 +162,14 @@ create table t1 (a int , b date as (1 in ('x' ,(database ()) ))) ;
 select b from t1;
 select a from t1 order by 'x' = b;
 drop table t1;
+
+--echo #
+--echo # MDEV-31319 Assertion const_item_cache == true failed in Item_func::fix_fields
+--echo #
+create table t (f1 int, f2 int, fv int generated always as (case user() when 'foo' or 'bar' then f1 else f2 end) virtual);
+select * from t;
+create table tmp as select * from information_schema.tables where table_name = 't';
+select * from t;
+
+# cleanup
+drop table t, tmp;

sql/item_func.cc

@@ -353,7 +353,10 @@ Item_func::fix_fields(THD *thd, Item **ref)
 	We shouldn't call fix_fields() twice, so check 'fixed' field first
       */
       if ((*arg)->fix_fields_if_needed(thd, arg))
+      {
+        cleanup();
 	return TRUE;				/* purecov: inspected */
+      }
       item= *arg;
 
       if (item->maybe_null)
@@ -369,9 +372,15 @@ Item_func::fix_fields(THD *thd, Item **ref)
     }
   }
   if (check_arguments())
+  {
+    cleanup();
     return true;
+  }
   if (fix_length_and_dec())
+  {
+    cleanup();
     return TRUE;
+  }
   fixed= 1;
   return FALSE;
 }