The wrong result set returned by the left join query  from
the bug test case happened due to several inconsistencies 
and bugs of the legacy mysql code.

The bug test case uses an execution plan that employs a scan
of a materialized IN subquery from the WHERE condition.
When materializing such an IN- subquery the optimizer injects
additional equalities  into the WHERE clause. These equalities
express the constraints imposed by the subquery predicate.
The injected equality of the query in the  test case happens
to belong to the same equality class, and a new equality 
imposing a condition on the rows of the materialized subquery
is inferred from this class. Simultaneously the multiple
equality is added to the ON expression of the LEFT JOIN
used in the main query.
  
The inferred equality of the form f1=f2 is taken into account
when optimizing the scan of  the rows the temporary table 
that is the result of the subquery materialization: only the 
values of the field f1 are read from the table into the record 
buffer. Meanwhile the inferred equality is removed from the
WHERE conditions altogether as a constraint on the fields
of the temporary table that has been used when filling this table. 
This equality is supposed to be removed from the ON expression
when the multiple equalities of the ON expression are converted
into an optimal set of equality predicates. It supposed to be
removed from the ON expression as an equality inferred from only
equalities of the WHERE condition. Yet, it did not happened
due to the following bug in the code.

Erroneously the code tried to build multiple equality for ON
expression twice: the first time, when it called optimize_cond()
for the WHERE condition, the second time, when it called
this function for the HAVING condition. When executing
optimize_con() for the WHERE condition  a reference
to the multiple equality of the WHERE condition is set
in the multiple equality of the  ON expression. This reference
would allow later to convert multiple equalities of the
ON expression into equality predicates. However the 
the second call of build_equal_items() for the ON expression
that happened when optimize_cond() was called for the
HAVING condition reset this reference to NULL.

This bug fix blocks calling build_equal_items() for ON
expressions for the second time. In general, it will be
beneficial for many queries as it removes from ON 
expressions any equalities that are to be checked for the
WHERE condition.
The patch also fixes two bugs in the list manipulation
operations and a bug in the function  
substitute_for_best_equal_field() that resulted
in passing wrong reference to the multiple equalities
of where conditions when processing multiple
equalities  of ON expressions.

The code of substitute_for_best_equal_field() and
the code the helper function eliminate_item_equal()
were also streamlined and cleaned up.
Now the conversion of the multiple equalities into
an optimal set of equality predicates first produces
the sequence of the all equalities processing multiple
equalities one by one, and, only after this, it inserts
the equalities at the beginning of the other conditions.

The multiple changes in the output of EXPLAIN
EXTENDED are mainly the result of this streamlining,
but in some cases is the result of the removal of
unneeded equalities from ON expressions. In
some test cases this removal were reflected in the
output of EXPLAIN resulted in disappearance of 
“Using where” in some rows of the execution plans.