Fix for BUG#12943.

The problem was that in the first production in rule 'join_table', that
processes simple cross joins, the parser was processing the second join operand
before the first one due to unspecified priorities of JOINs. As a result in the
case of cross joins the parser constructed a tree with incorrect nesting:
the expression "t1 join t2 join t3 on some_cond" was interpreted as
"t1 join (t2 join t3 on some_cond)" instead of
"(t1 join t2) join t3 on some_cond".
Because of this incorrect nesting the method make_join_on_context picked an
incorrect table as the first table of the name resolution context.

The solution assignes correct priorities to the related production.

mysql-test/r/select.result

@@ -2897,3 +2897,23 @@ select * from t1 natural join t2 where a = 'b';
 a
 b
 drop table t1, t2;
+create table t1 (a int, c int);
+create table t2 (b int);
+create table t3 (b int, a int);
+create table t4 (c int);
+insert into t1 values (1,1);
+insert into t2 values (1);
+insert into t3 values (1,1);
+insert into t4 values (1);
+select * from t1 join t2 join t3 on (t2.b = t3.b and t1.a = t3.a);
+a	c	b	b	a
+1	1	1	1	1
+select * from t1, t2 join t3 on (t2.b = t3.b and t1.a = t3.a);
+ERROR 42S22: Unknown column 't1.a' in 'on clause'
+select * from t1 join t2 join t3 join t4 on (t1.a = t4.c and t2.b = t4.c);
+a	c	b	b	a	c
+1	1	1	1	1	1
+select * from t1 join t2 join t4 using (c);
+c	a	b
+1	1	1
+drop table t1, t2, t3, t4;

mysql-test/t/select.test

@@ -2465,3 +2465,26 @@ insert into t2 values ('b'),('c'),('d');
 select a from t1 natural join t2;
 select * from t1 natural join t2 where a = 'b';
 drop table t1, t2;
+
+#
+# Bug #12943 Incorrect nesting of [INNER| CROSS] JOIN due to unspecified
+#            associativity in the parser.
+#
+
+create table t1 (a int, c int);
+create table t2 (b int);
+create table t3 (b int, a int);
+create table t4 (c int);
+insert into t1 values (1,1);
+insert into t2 values (1);
+insert into t3 values (1,1);
+insert into t4 values (1);
+
+select * from t1 join t2 join t3 on (t2.b = t3.b and t1.a = t3.a);
+# Notice that ',' has lower priority than 'join', thus we have that:
+# t1, t2 join t3 <==> t1, (t2 join t3).
+-- error 1054
+select * from t1, t2 join t3 on (t2.b = t3.b and t1.a = t3.a);
+select * from t1 join t2 join t3 join t4 on (t1.a = t4.c and t2.b = t4.c);
+select * from t1 join t2 join t4 using (c);
+drop table t1, t2, t3, t4;

sql/sql_parse.cc

@@ -6354,7 +6354,7 @@ void st_select_lex::set_lock_for_tables(thr_lock_type lock_type)
   SYNOPSIS
     make_join_on_context()
     thd       pointer to current thread
-    left_op   lefto operand of the JOIN
+    left_op   left  operand of the JOIN
     right_op  rigth operand of the JOIN
 
   DESCRIPTION

sql/sql_yacc.yy

@@ -660,6 +660,9 @@ bool my_yyoverflow(short **a, YYSTYPE **b, ulong *yystacksize);
 %token  YEAR_SYM
 %token  ZEROFILL
 
+%left   JOIN_SYM
+/* A dummy token to force the priority of table_ref production in a join. */
+%left   TABLE_REF_PRIORITY
 %left   SET_VAR
 %left	OR_OR_SYM OR_SYM OR2_SYM XOR
 %left	AND_SYM AND_AND_SYM
@@ -5189,7 +5192,13 @@ derived_table_list:
         ;
 
 join_table:
-        table_ref normal_join table_ref { YYERROR_UNLESS($1 && ($$=$3)); }
+        /*
+          Evaluate production 'table_ref' before 'normal_join' so that
+          [INNER | CROSS] JOIN is properly nested as other left-associative
+          joins.
+        */
+        table_ref %prec TABLE_REF_PRIORITY normal_join table_ref
+        { YYERROR_UNLESS($1 && ($$=$3)); }
 	| table_ref STRAIGHT_JOIN table_factor
 	  { YYERROR_UNLESS($1 && ($$=$3)); $3->straight=1; }
 	| table_ref normal_join table_ref