Added test for bug #7884 "Able to add invalid unique index on TIMESTAMP

prefix", which roots were fixed in 4.0 tree.

mysql-test/r/alter_table.result

@@ -493,3 +493,7 @@ select hex(a) from t1;
 hex(a)
 F2E5F1F2
 drop table t1;
+create table t1 ( a timestamp );
+alter table t1 add unique ( a(1) );
+ERROR HY000: Incorrect sub part key; the used key part isn't a string, the used length is longer than the key part, or the storage engine doesn't support unique sub keys
+drop table t1;

mysql-test/t/alter_table.test

@@ -336,3 +336,14 @@ select hex(a) from t1;
 alter table t1 convert to character set cp1251;
 select hex(a) from t1;
 drop table t1;
+
+#
+# Test for bug #7884 "Able to add invalid unique index on TIMESTAMP prefix"
+# MySQL should not think that packed field with non-zero decimals is
+# geometry field and allow to create prefix index which is
+# shorter than packed field length.
+#
+create table t1 ( a timestamp );
+--error 1089
+alter table t1 add unique ( a(1) );
+drop table t1;