diff --git a/mysql-test/r/distinct.result b/mysql-test/r/distinct.result
index 86ab2141e2d..32151305698 100644
--- a/mysql-test/r/distinct.result
+++ b/mysql-test/r/distinct.result
@@ -607,3 +607,40 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 SELECT DISTINCT a,a FROM t1 WHERE b < 12 ORDER BY a;
 a	a
 DROP TABLE t1;
+CREATE TABLE t1 (a CHAR(1));
+INSERT INTO t1 VALUES('A'), (0);
+SELECT a FROM t1 WHERE a=0;
+a
+A
+0
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'A'
+SELECT DISTINCT a FROM t1 WHERE a=0;
+a
+A
+0
+Warnings:
+Warning	1292	Truncated incorrect DOUBLE value: 'A'
+DROP TABLE t1;
+CREATE TABLE t1 (a DATE);
+INSERT INTO t1 VALUES ('1972-07-29'), ('1972-02-06');
+EXPLAIN SELECT (SELECT DISTINCT a FROM t1 WHERE a = '2002-08-03');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+EXPLAIN SELECT (SELECT DISTINCT ADDDATE(a,1) FROM t1
+WHERE ADDDATE(a,1) = '2002-08-03');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where; Using temporary
+CREATE TABLE t2 (a CHAR(5) CHARACTER SET latin1 COLLATE latin1_general_ci);
+INSERT INTO t2 VALUES (0xf6);
+INSERT INTO t2 VALUES ('oe');
+SELECT COUNT(*) FROM (SELECT DISTINCT a FROM t2) dt;
+COUNT(*)
+2
+SELECT COUNT(*) FROM 
+(SELECT DISTINCT a FROM t2 WHERE a='oe' COLLATE latin1_german2_ci) dt;
+COUNT(*)
+2
+DROP TABLE t1, t2;
diff --git a/mysql-test/t/distinct.test b/mysql-test/t/distinct.test
index b2cc42cc0ff..8734b940241 100644
--- a/mysql-test/t/distinct.test
+++ b/mysql-test/t/distinct.test
@@ -504,3 +504,24 @@ DROP TABLE t1;
 #DROP TABLE t1;
 #DROP TABLE t2;
 
+#
+# Bug #15881: cast problems
+#
+CREATE TABLE t1 (a CHAR(1)); INSERT INTO t1 VALUES('A'), (0);
+SELECT a FROM t1 WHERE a=0;
+SELECT DISTINCT a FROM t1 WHERE a=0;
+DROP TABLE t1;
+CREATE TABLE t1 (a DATE);
+INSERT INTO t1 VALUES ('1972-07-29'), ('1972-02-06');
+EXPLAIN SELECT (SELECT DISTINCT a FROM t1 WHERE a = '2002-08-03');
+EXPLAIN SELECT (SELECT DISTINCT ADDDATE(a,1) FROM t1
+                WHERE ADDDATE(a,1) = '2002-08-03');
+CREATE TABLE t2 (a CHAR(5) CHARACTER SET latin1 COLLATE latin1_general_ci);
+INSERT INTO t2 VALUES (0xf6);
+INSERT INTO t2 VALUES ('oe');
+
+SELECT COUNT(*) FROM (SELECT DISTINCT a FROM t2) dt;
+SELECT COUNT(*) FROM 
+  (SELECT DISTINCT a FROM t2 WHERE a='oe' COLLATE latin1_german2_ci) dt;
+
+DROP TABLE t1, t2;
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 30514f09493..0765b97c4d0 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -8432,6 +8432,46 @@ remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value)
   return cond;					// Point at next and level
 }
 
+/* 
+  Check if equality can be used in removing components of GROUP BY/DISTINCT
+  
+  SYNOPSIS
+    test_if_equality_guarantees_uniqueness()
+      l          the left comparison argument (a field if any)
+      r          the right comparison argument (a const of any)
+  
+  DESCRIPTION    
+    Checks if an equality predicate can be used to take away 
+    DISTINCT/GROUP BY because it is known to be true for exactly one 
+    distinct value (e.g. <expr> == <const>).
+    Arguments must be of the same type because e.g. 
+    <string_field> = <int_const> may match more than 1 distinct value from 
+    the column. 
+    We must take into consideration and the optimization done for various 
+    string constants when compared to dates etc (see Item_int_with_ref) as
+    well as the collation of the arguments.
+  
+  RETURN VALUE  
+    TRUE    can be used
+    FALSE   cannot be used
+*/
+static bool
+test_if_equality_guarantees_uniqueness(Item *l, Item *r)
+{
+  return r->const_item() &&
+    /* elements must be of the same result type */
+    (r->result_type() == l->result_type() ||
+    /* or dates compared to longs */
+     (((l->type() == Item::FIELD_ITEM &&
+        ((Item_field *)l)->field->can_be_compared_as_longlong()) ||
+       (l->type() == Item::FUNC_ITEM &&
+        ((Item_func *)l)->result_as_longlong())) &&
+      r->result_type() == INT_RESULT))
+    /* and must have the same collation if compared as strings */
+    && (l->result_type() != STRING_RESULT ||
+        l->collation.collation == r->collation.collation);
+}
+
 /*
   Return 1 if the item is a const value in all the WHERE clause
 */
@@ -8468,7 +8508,7 @@ const_expression_in_where(COND *cond, Item *comp_item, Item **const_item)
     Item *right_item= ((Item_func*) cond)->arguments()[1];
     if (left_item->eq(comp_item,1))
     {
-      if (right_item->const_item())
+      if (test_if_equality_guarantees_uniqueness (left_item, right_item))
       {
 	if (*const_item)
 	  return right_item->eq(*const_item, 1);
@@ -8478,7 +8518,7 @@ const_expression_in_where(COND *cond, Item *comp_item, Item **const_item)
     }
     else if (right_item->eq(comp_item,1))
     {
-      if (left_item->const_item())
+      if (test_if_equality_guarantees_uniqueness (right_item, left_item))
       {
 	if (*const_item)
 	  return left_item->eq(*const_item, 1);