errors: add Unwrap, Is, and As

Unwrap, Is and As are as defined in proposal
Issue #29934.

Also add Opaque for enforcing an error cannot
be unwrapped.

Change-Id: I4f3feaa42e3ee7477b588164ac622ba4d5e77cad
Reviewed-on: https://go-review.googlesource.com/c/163558
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

1 files changed, 15 insertions, 0 deletions

diff --git a/src/errors/example_test.go b/src/errors/example_test.go
index 5dc8841237..7724c16cdf 100644
--- a/src/errors/example_test.go
+++ b/src/errors/example_test.go
@@ -5,7 +5,9 @@
 package errors_test
 
 import (
+	"errors"
 	"fmt"
+	"os"
 	"time"
 )
 
@@ -32,3 +34,16 @@ func Example() {
 	}
 	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
 }
+
+func ExampleAs() {
+	_, err := os.Open("non-existing")
+	if err != nil {
+		var pathError *os.PathError
+		if errors.As(err, &pathError) {
+			fmt.Println("Failed at path:", pathError.Path)
+		}
+	}
+
+	// Output:
+	// Failed at path: non-existing
+}