1 // Copyright 2020 The Go Authors. All rights reserved.

     2 // Use of this source code is governed by a BSD-style

     3 // license that can be found in the LICENSE file.

     4 

     5 // +build !go1.13

     6 

     7 package errors

     8 

     9 import "reflect"

    10 

    11 // Is is a copy of Go 1.13's errors.Is for use with older Go versions.

    12 func Is(err, target error) bool {

    13 	if target == nil {

    14 		return err == target

    15 	}

    16 

    17 	isComparable := reflect.TypeOf(target).Comparable()

    18 	for {

    19 		if isComparable && err == target {

    20 			return true

    21 		}

    22 		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {

    23 			return true

    24 		}

    25 		if err = unwrap(err); err == nil {

    26 			return false

    27 		}

    28 	}

    29 }

    30 

    31 func unwrap(err error) error {

    32 	u, ok := err.(interface {

    33 		Unwrap() error

    34 	})

    35 	if !ok {

    36 		return nil

    37 	}

    38 	return u.Unwrap()

    39 }