MDEV-18968 Both (WHERE 0.1) and (WHERE NOT 0.1) return empty set

mysql-test/r/func_group.result

@@ -1979,8 +1979,8 @@ FROM t2);
 MIN(t2.pk)
 NULL
 Warnings:
-Warning	1292	Truncated incorrect INTEGER value: 'j'
-Warning	1292	Truncated incorrect INTEGER value: 'j'
+Warning	1292	Truncated incorrect DOUBLE value: 'j'
+Warning	1292	Truncated incorrect DOUBLE value: 'j'
 
 EXPLAIN
 SELECT MIN(t2.pk)
@@ -1993,8 +1993,8 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE
 2	MATERIALIZED	t2	ALL	NULL	NULL	NULL	NULL	2	
 Warnings:
-Warning	1292	Truncated incorrect INTEGER value: 'j'
-Warning	1292	Truncated incorrect INTEGER value: 'j'
+Warning	1292	Truncated incorrect DOUBLE value: 'j'
+Warning	1292	Truncated incorrect DOUBLE value: 'j'
 
 #
 # 2) Test that subquery materialization is setup for query with

mysql-test/r/type_decimal.result

@@ -1019,3 +1019,23 @@ cast('-0.0' as decimal(5,1)) < 0
 #
 # End of 5.5 tests
 #
+#
+# Start of 10.1 tests
+#
+#
+# MDEV-18968 Both (WHERE 0.1) and (WHERE NOT 0.1) return empty set
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (10);
+SELECT CASE WHEN 0.1 THEN 'TRUE' ELSE 'FALSE' END FROM t1;
+CASE WHEN 0.1 THEN 'TRUE' ELSE 'FALSE' END
+TRUE
+SELECT * FROM t1 WHERE 0.1;
+a
+10
+SELECT * FROM t1 WHERE NOT 0.1;
+a
+DROP TABLE t1;
+#
+# End of 10.1 tests
+#

mysql-test/t/type_decimal.test

@@ -612,3 +612,24 @@ select cast('-0.0' as decimal(5,1)) < 0;
 --echo #
 --echo # End of 5.5 tests
 --echo #
+
+
+--echo #
+--echo # Start of 10.1 tests
+--echo #
+
+--echo #
+--echo # MDEV-18968 Both (WHERE 0.1) and (WHERE NOT 0.1) return empty set
+--echo #
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (10);
+SELECT CASE WHEN 0.1 THEN 'TRUE' ELSE 'FALSE' END FROM t1;
+SELECT * FROM t1 WHERE 0.1;
+SELECT * FROM t1 WHERE NOT 0.1;
+DROP TABLE t1;
+
+
+--echo #
+--echo # End of 10.1 tests
+--echo #

sql/item.h

@@ -1000,6 +1000,13 @@ public:
   virtual bool val_bool();
   virtual String *val_nodeset(String*) { return 0; }
 
+  bool eval_const_cond()
+  {
+    DBUG_ASSERT(const_item());
+    DBUG_ASSERT(!is_expensive());
+    return val_bool();
+  }
+
   /*
     save_val() is method of val_* family which stores value in the given
     field.

sql/item_cmpfunc.cc

@@ -4667,7 +4667,7 @@ Item_cond::fix_fields(THD *thd, Item **ref)
     if (item->const_item() && !item->with_param &&
         !item->is_expensive() && !cond_has_datetime_is_null(item))
     {
-      if (item->val_int() == is_and_cond && top_level())
+      if (item->eval_const_cond() == is_and_cond && top_level())
       {
         /* 
           a. This is "... AND true_cond AND ..."

sql/item_func.cc

@@ -68,18 +68,6 @@ bool check_reserved_words(LEX_STRING *name)
 }
 
 
-/**
-  @return
-    TRUE if item is a constant
-*/
-
-bool
-eval_const_cond(COND *cond)
-{
-  return ((Item_func*) cond)->val_int() ? TRUE : FALSE;
-}
-
-
 /**
    Test if the sum of arguments overflows the ulonglong range.
 */

sql/item_func.h

@@ -2279,7 +2279,6 @@ extern enum_field_types agg_field_type(Item **items, uint nitems,
 Item *find_date_time_item(Item **args, uint nargs, uint col);
 double my_double_round(double value, longlong dec, bool dec_unsigned,
                        bool truncate);
-bool eval_const_cond(COND *cond);
 
 extern bool volatile  mqh_used;
 

sql/sql_select.cc

@@ -15644,7 +15644,7 @@ Item::remove_eq_conds(THD *thd, Item::cond_result *cond_value, bool top_level_ar
 {
   if (const_item() && !is_expensive())
   {
-    *cond_value= eval_const_cond(this) ? Item::COND_TRUE : Item::COND_FALSE;
+    *cond_value= eval_const_cond() ? Item::COND_TRUE : Item::COND_FALSE;
     return (COND*) 0;
   }
   *cond_value= Item::COND_OK;
@@ -15658,7 +15658,7 @@ Item_bool_func2::remove_eq_conds(THD *thd, Item::cond_result *cond_value,
 {
   if (const_item() && !is_expensive())
   {
-    *cond_value= eval_const_cond(this) ? Item::COND_TRUE : Item::COND_FALSE;
+    *cond_value= eval_const_cond() ? Item::COND_TRUE : Item::COND_FALSE;
     return (COND*) 0;
   }
   if ((*cond_value= eq_cmp_result()) != Item::COND_OK)