internal/errors/is_go112.go

// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build !go1.13

package errors

import "reflect"

// Is is a copy of Go 1.13's errors.Is for use with older Go versions.
func Is(err, target error) bool {
	if target == nil {
		return err == target
	}

	isComparable := reflect.TypeOf(target).Comparable()
	for {
		if isComparable && err == target {
			return true
		}
		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
			return true
		}
		// TODO: consider supporing target.Is(err). This would allow
		// user-definable predicates, but also may allow for coping with sloppy
		// APIs, thereby making it easier to get away with them.
		if err = unwrap(err); err == nil {
			return false
		}
	}
}

func unwrap(err error) error {
	u, ok := err.(interface {
		Unwrap() error
	})
	if !ok {
		return nil
	}
	return u.Unwrap()
}