fixed system.delete (#18507)

changelog.md

@@ -4,7 +4,18 @@
 
 ## Changes affecting backward compatibility
 
-- Deprecated `std/mersenne`
+- Deprecated `std/mersenne`.
+
+- `system.delete` had a most surprising behavior when the index passed to it was out of
+  bounds (it would delete the last entry then). Compile with `-d:nimStrictDelete` so
+  that an index error is produced instead. But be aware that your code might depend on
+  this quirky behavior so a review process is required on your part before you can
+  use `-d:nimStrictDelete`. To make this review easier, use the `-d:nimAuditDelete`
+  switch, it pretends that `system.delete` is deprecated so that it is easier to see
+  where it was used in your code.
+
+  `-d:nimStrictDelete` will become the default in upcoming versions.
+
 
 - `cuchar` is now deprecated as it aliased `char` where arguably it should have aliased `uint8`.
   Please use `char` or `uint8` instead.