1 files changed, 15 insertions, 0 deletions
diff --git a/mysql-test/t/select.test b/mysql-test/t/select.test
index c4fe1906cbc..8e3c5847846 100644
--- a/mysql-test/t/select.test
+++ b/mysql-test/t/select.test
@@ -2871,3 +2871,18 @@ SELECT t2.sku, t2.sppr, t2.name, t1.sku, t1.pr
 
 
 DROP TABLE t1,t2;
+
+#
+# Bug#18712: Truncation problem (needs just documenting and test
+# cases to prevent fixing this accidently. It is intended behaviour)
+#
+
+CREATE TABLE t1 (i TINYINT UNSIGNED NOT NULL);
+INSERT t1 SET i = 0;
+UPDATE t1 SET i = -1;
+SELECT * FROM t1;
+UPDATE t1 SET i = CAST(i - 1 AS SIGNED);
+SELECT * FROM t1;
+UPDATE t1 SET i = i - 1;
+SELECT * FROM t1;
+DROP TABLE t1;