errors: add ErrUnsupported

Fixes #41198

Change-Id: Ib33a11d0eb311f8e2b81de24d11df49e00b2fc81
Reviewed-on: https://go-review.googlesource.com/c/go/+/473935
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/errors/errors.go b/src/errors/errors.go
index 8436f812a6..26db2d2bbf 100644
--- a/src/errors/errors.go
+++ b/src/errors/errors.go
@@ -70,3 +70,18 @@ type errorString struct {
 func (e *errorString) Error() string {
 	return e.s
 }
+
+// ErrUnsupported indicates that a requested operation cannot be performed,
+// because it is unsupported. For example, a call to os.Link when using a
+// file system that does not support hard links.
+//
+// Functions and methods should not return this error but should instead
+// return an error including appropriate context that satisfies
+//
+//	errors.Is(err, errors.ErrUnsupported)
+//
+// either by directly wrapping ErrUnsupported or by implementing an Is method.
+//
+// Functions and methods should document the cases in which an error
+// wrapping this will be returned.
+var ErrUnsupported = New("unsupported operation")