Added test case for Bug#18712: Truncation problem. The test

is only to make sure that this will not be fixed, as it is
intended behaviour. Documentation will be improved accordingly.

mysql-test/r/select.result

@@ -3390,3 +3390,24 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	const	PRIMARY	PRIMARY	4	const	1	
 1	SIMPLE	t1	range	PRIMARY	PRIMARY	4	NULL	2	Using where
 DROP TABLE t1,t2;
+CREATE TABLE t1 (i TINYINT UNSIGNED NOT NULL);
+INSERT t1 SET i = 0;
+UPDATE t1 SET i = -1;
+Warnings:
+Warning	1264	Out of range value adjusted for column 'i' at row 1
+SELECT * FROM t1;
+i
+0
+UPDATE t1 SET i = CAST(i - 1 AS SIGNED);
+Warnings:
+Warning	1264	Out of range value adjusted for column 'i' at row 1
+SELECT * FROM t1;
+i
+0
+UPDATE t1 SET i = i - 1;
+Warnings:
+Warning	1264	Out of range value adjusted for column 'i' at row 1
+SELECT * FROM t1;
+i
+255
+DROP TABLE t1;

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;