If a primary key is defined over column c of enum type then 
the EXPLAIN command for a look-up query of the form
  SELECT * FROM t WHERE c=0
said that the query was with an impossible where condition though the
query correctly returned non-empty result set when the table indeed 
contained rows with error empty strings for column c. 

This kind of misbehavior was due to a bug in the function 
Field_enum::store(longlong,bool) that erroneously returned 1 if
the the value to be stored was equal to 0. 
Note that the method 
Field_enum::store(const char *from,uint length,CHARSET_INFO *cs)
correctly returned 0 if a value of the error empty string 
was stored. 


mysql-test/r/type_enum.result:
  Added a test case for bug #29661.
mysql-test/t/type_enum.test:
  Added a test case for bug #29661.
sql/field.cc:
  Fixed bug #29611.
  If a primary key was defined over column c of enum type then 
  the EXPLAIN command for a look-up query of the form
    SELECT * FROM t WHERE c=0
  said that the query was with an impossible where condition though the
  query correctly returned non-empty result set when the table indeed 
  contained rows with error empty strings for column c. 
  
  This kind of misbehavior was due to a bug in the function 
  Field_enum::store(longlong,bool) that erroneously returned 1 if
  the the value to be stored was equal to 0. 
  Note that the method 
  Field_enum::store(const char *from,uint length,CHARSET_INFO *cs)
  correctly returned 0 if a value of the error empty string 
  was stored.