The implicit casting rule for WEIGHT_STRING is not correct

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

mysql> SELECT WEIGHT_STRING(123);
+----------------------------------------+
| WEIGHT_STRING(123)                     |
+----------------------------------------+
| 0x800000000000007B                     |
+----------------------------------------+
1 row in set (0.01 sec)

mysql> SELECT WEIGHT_STRING(TRUE);
+------------------------------------------+
| WEIGHT_STRING(TRUE)                      |
+------------------------------------------+
| 0x8000000000000001                       |
+------------------------------------------+
1 row in set (0.01 sec)

mysql> SELECT WEIGHT_STRING('a' COLLATE utf8mb4_bin);
+--------------------------------------------------------------------------------+
| WEIGHT_STRING('a' COLLATE utf8mb4_bin)                                         |
+--------------------------------------------------------------------------------+
| 0x000061                                                                       |
+--------------------------------------------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

mysql> SELECT WEIGHT_STRING(123);
+----------------------------------------+
| WEIGHT_STRING(123)                     |
+----------------------------------------+
| NULL                                   |
+----------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT WEIGHT_STRING(TRUE);
+------------------------------------------+
| WEIGHT_STRING(TRUE)                      |
+------------------------------------------+
| NULL                                     |
+------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT WEIGHT_STRING('a' COLLATE utf8mb4_bin);
+--------------------------------------------------------------------------------+
| WEIGHT_STRING('a' COLLATE utf8mb4_bin)                                         |
+--------------------------------------------------------------------------------+
| 0x61                                                                           |
+--------------------------------------------------------------------------------+
1 row in set (0.00 sec)

4. What is your TiDB version? (Required)

[zanmato1984]

/severity moderate