Fix over-optimization that could result in an unsigned double field being

set to a negative value. (Bug #7700)

mysql-test/r/type_float.result

@@ -120,3 +120,10 @@ drop table t1;
 create table t1 (f float(54));
 Incorrect column specifier for column 'f'
 drop table if exists t1;
+create table t1 (d1 double, d2 double unsigned);
+insert into t1 set d1 = -1.0;
+update t1 set d2 = d1;
+select * from t1;
+d1	d2
+-1	0
+drop table t1;

mysql-test/t/type_float.test

@@ -67,3 +67,9 @@ drop table t1;
 create table t1 (f float(54));	# Should give an error
 drop table if exists t1;
 
+# Don't allow 'double unsigned' to be set to a negative value (Bug #7700)
+create table t1 (d1 double, d2 double unsigned);
+insert into t1 set d1 = -1.0;
+update t1 set d2 = d1;
+select * from t1;
+drop table t1;

sql/field_conv.cc

@@ -537,6 +537,7 @@ void field_conv(Field *to,Field *from)
   if (to->real_type() == from->real_type())
   {
     if (to->pack_length() == from->pack_length() &&
+        !(to->flags & UNSIGNED_FLAG && !(from->flags & UNSIGNED_FLAG)) &&
 	to->real_type() != FIELD_TYPE_ENUM &&
 	to->real_type() != FIELD_TYPE_SET &&
 	to->table->db_low_byte_first == from->table->db_low_byte_first)