Backport of the fix for bug #50335 to 5.0.

The problem was in an incorrect debug assertion. The expression
used in the failing assertion states that when finding
references matching ORDER BY expressions, there can be only one
reference to a single table. But that does not make any sense,
all test cases for this bug are valid examples with multiple
identical WHERE expressions referencing the same table which
are also present in the ORDER BY list.

Fixed by removing the failing assertion. We also have to take
care of the 'found' counter so that we count multiple
references only once. We rely on this fact later in
eq_ref_table().

mysql-test/r/join.result:
  Added a test case for bug #50335.
mysql-test/t/join.test:
  Added a test case for bug #50335.
sql/sql_select.cc:
  Removing the assertion in eq_ref_table() as it does not make
  any sense. We also have to take care of the 'found' counter so
  that we count multiple references only once. We rely on this
  fact later in eq_ref_table().

mysql-test/r/join.result

@@ -942,4 +942,14 @@ ROW(t1.b, 1111.11) <=> ROW('','');
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 DROP TABLE t1;
+#
+# Bug #50335: Assertion `!(order->used & map)' in eq_ref_table
+# 
+CREATE TABLE t1 (a INT NOT NULL, b INT NOT NULL, PRIMARY KEY (a,b));
+INSERT INTO t1 VALUES (0,0), (1,1);
+SELECT * FROM t1 STRAIGHT_JOIN t1 t2 ON t1.a=t2.a AND t1.a=t2.b ORDER BY t2.a, t1.a;
+a	b	a	b
+0	0	0	0
+1	1	1	1
+DROP TABLE t1;
 End of 5.0 tests.

mysql-test/t/join.test

@@ -736,4 +736,15 @@ WHERE ROW(t1.a, 1111.11) = ROW(1111.11, 1111.11) AND
 ROW(t1.b, 1111.11) <=> ROW('','');
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #50335: Assertion `!(order->used & map)' in eq_ref_table
+--echo # 
+
+CREATE TABLE t1 (a INT NOT NULL, b INT NOT NULL, PRIMARY KEY (a,b));
+INSERT INTO t1 VALUES (0,0), (1,1);
+
+SELECT * FROM t1 STRAIGHT_JOIN t1 t2 ON t1.a=t2.a AND t1.a=t2.b ORDER BY t2.a, t1.a;
+
+DROP TABLE t1;
+
 --echo End of 5.0 tests.

sql/sql_select.cc

@@ -6719,10 +6719,12 @@ eq_ref_table(JOIN *join, ORDER *start_order, JOIN_TAB *tab)
 	  break;
       }
       if (order)
+      {
+        if (!(order->used & map))
         {
           found++;
-	DBUG_ASSERT(!(order->used & map));
-	order->used|=map;
+          order->used|= map;
+        }
 	continue;				// Used in ORDER BY
       }
       if (!only_eq_ref_tables(join,start_order, (*ref_item)->used_tables()))