Fixed bug #29360.

The special `zero' enum value was coerced to the normal
empty string enum value during a field-to-field copy.
This bug affected CREATE ... SELECT statements and
SELECT aggregate GROUP BY enum field statements.

Also this bug made unnecessary warnings during
the execution of CREATE ... SELECT statements:
Warning       1265    Data truncated for column...

mysql-test/r/type_enum.result

@@ -1809,3 +1809,23 @@ f1
 
 
 drop table t1;
+CREATE TABLE t1 (c1 ENUM('a', '', 'b'));
+INSERT INTO t1 (c1) VALUES ('b');
+INSERT INTO t1 (c1) VALUES ('');
+INSERT INTO t1 (c1) VALUES (0);
+Warnings:
+Warning	1265	Data truncated for column 'c1' at row 1
+INSERT INTO t1 (c1) VALUES ('');
+SELECT c1 + 0, COUNT(c1) FROM t1 GROUP BY c1;
+c1 + 0	COUNT(c1)
+0	1
+2	2
+3	1
+CREATE TABLE t2 SELECT * FROM t1;
+SELECT c1 + 0 FROM t2;
+c1 + 0
+3
+2
+0
+2
+DROP TABLE t1,t2;

mysql-test/r/type_ranges.result

@@ -208,10 +208,6 @@ options	flags
 one	one
 drop table t2;
 create table t2 select * from t1;
-Warnings:
-Warning	1265	Data truncated for column 'options' at row 4
-Warning	1265	Data truncated for column 'options' at row 5
-Warning	1265	Data truncated for column 'options' at row 6
 update t2 set string="changed" where auto=16;
 show full columns from t1;
 Field	Type	Collation	Null	Key	Default	Extra	Privileges	Comment

mysql-test/t/type_enum.test

@@ -182,3 +182,21 @@ create table t1(f1 set('a','b'), index(f1));
 insert into t1 values(''),(''),('a'),('b');
 select * from t1 where f1='';
 drop table t1;
+
+#
+# Bug#29360: Confluence of the special 0 enum value with the normal empty string
+# value during field to field copy.
+#
+
+CREATE TABLE t1 (c1 ENUM('a', '', 'b'));
+INSERT INTO t1 (c1) VALUES ('b');
+INSERT INTO t1 (c1) VALUES ('');
+INSERT INTO t1 (c1) VALUES (0);
+INSERT INTO t1 (c1) VALUES ('');
+
+SELECT c1 + 0, COUNT(c1) FROM t1 GROUP BY c1;
+
+CREATE TABLE t2 SELECT * FROM t1;
+SELECT c1 + 0 FROM t2;
+
+DROP TABLE t1,t2;

sql/field_conv.cc

@@ -790,7 +790,13 @@ int field_conv(Field *to,Field *from)
       blob->value.copy();
     return blob->store(blob->value.ptr(),blob->value.length(),from->charset());
   }
-  if ((from->result_type() == STRING_RESULT &&
+  if (from->real_type() == FIELD_TYPE_ENUM &&
+      to->real_type() == FIELD_TYPE_ENUM &&
+      from->val_int() == 0)
+  {
+    ((Field_enum *)(to))->store_type(0);
+  }
+  else if ((from->result_type() == STRING_RESULT &&
             (to->result_type() == STRING_RESULT ||
              (from->real_type() != FIELD_TYPE_ENUM &&
               from->real_type() != FIELD_TYPE_SET))) ||