design/29934-error-values.md: add os package changes
Describe proposed changes to the `os` package to permit use of the
`os.ErrExist` et al. sentinels with `errors.Is`.
Change-Id: I8c24e2e1b930436e4dc3141acb89b5ed85dd48d5
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/162877
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
diff --git a/design/29934-error-values.md b/design/29934-error-values.md
index d773eee..b6bef15 100644
--- a/design/29934-error-values.md
+++ b/design/29934-error-values.md
@@ -232,6 +232,16 @@
the case of the new verb `%w`, the returned error will also implement
`errors.Wrapper` with an `Unwrap` method returning `err`.
+### Changes to the `os` package
+
+The `os` package contains a several predicate functions which test
+an error against a condition: `IsExist`, `IsNotExist`, `IsPermission`,
+and `IsTimeout`. For each of these conditions, we modify the `os`
+package so that `errors.Is(err, os.ErrX)` returns true when
+`os.IsX(err)` is true for any error in `err`'s chain. The `os` package
+already contains `ErrExist`, `ErrIsNotExist`, and `ErrPermission`
+sentinel values; we will add `ErrTimeout`.
+
### Transition
If we add this functionality to the standard library in Go 1.13, code that needs
