xerrors: make As match on assignability

Change As to consider assignability rather than type equivalence, allowing
it to convert an error to an interface type. e.g.,

	var to interface{ Timeout() bool }
	if errors.As(err, &to) && to.Timeout() { ... }j

Change-Id: Ia3ecaefb6c27f878f81f1d467b07b69fee9db976
Reviewed-on: https://go-review.googlesource.com/c/161017
Reviewed-by: Marcel van Lohuizen <[email protected]>

wrap.go

@@ -68,9 +68,10 @@ func Is(err, target error) bool {
 }
 
 // As finds the first error in err's chain that matches the type to which target
-// points, and if so, sets the target to its value and and returns true. An error
-// matches a type if it is of the same type, or if it has a method As(interface{}) bool
-// such that As(target) returns true. As will panic if target is nil or not a pointer.
+// points, and if so, sets the target to its value and returns true. An error
+// matches a type if it is assignable to the target type, or if it has a method
+// As(interface{}) bool such that As(target) returns true. As will panic if target
+// is nil or not a pointer.
 //
 // The As method should set the target to its value and return true if err
 // matches the type to which target points.
@@ -84,7 +85,7 @@ func As(err error, target interface{}) bool {
 	}
 	targetType := typ.Elem()
 	for {
-		if reflect.TypeOf(err) == targetType {
+		if reflect.TypeOf(err).AssignableTo(targetType) {
 			reflect.ValueOf(target).Elem().Set(reflect.ValueOf(err))
 			return true
 		}

wrap_test.go

@@ -80,6 +80,7 @@ func (p *poser) As(err interface{}) bool {
 func TestAs(t *testing.T) {
 	var errT errorT
 	var errP *os.PathError
+	var timeout interface{ Timeout() bool }
 	var p *poser
 	_, errF := os.Open("non-existing")
 
@@ -120,16 +121,24 @@ func TestAs(t *testing.T) {
 		&p,
 		true,
 	}, {
-		&poser{"oo", nil},
-		&errF,
+		xerrors.New("err"),
+		&timeout,
 		false,
+	}, {
+		errF,
+		&timeout,
+		true,
+	}, {
+		xerrors.Errorf("path error: %w", errF),
+		&timeout,
+		true,
 	}}
-	for _, tc := range testCases {
-		name := fmt.Sprintf("As(Errorf(..., %v), %v)", tc.err, tc.target)
+	for i, tc := range testCases {
+		name := fmt.Sprintf("%d:As(Errorf(..., %v), %v)", i, tc.err, tc.target)
 		t.Run(name, func(t *testing.T) {
 			match := xerrors.As(tc.err, tc.target)
 			if match != tc.match {
-				t.Fatalf("match: got %v; want %v", match, tc.match)
+				t.Fatalf("xerrors.As(%T, %T): got %v; want %v", tc.err, tc.target, match, tc.match)
 			}
 			if !match {
 				return