text: // Copyright 2018 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.
  
  package xerrors
  
  import (
  	"reflect"
  )
  
  // A Wrapper provides context around another error.
  type Wrapper interface {
  	// Unwrap returns the next error in the error chain.
  	// If there is no next error, Unwrap returns nil.
  	Unwrap() error
  }
  
  // Opaque returns an error with the same error formatting as err
  // but that does not match err and cannot be unwrapped.
  func Opaque(err error) error {
  	return noWrapper{err}
  }
  
  type noWrapper struct {
  	error
  }
  
  func (e noWrapper) FormatError(p Printer) (next error) {
  	if f, ok := e.error.(Formatter); ok {
  		return f.FormatError(p)
  	}
  	p.Print(e.error)
  	return nil
  }
  
  // Unwrap returns the result of calling the Unwrap method on err, if err implements
  // Unwrap. Otherwise, Unwrap returns nil.
  func Unwrap(err error) error {
  	u, ok := err.(Wrapper)
  	if !ok {
  		return nil
  	}
  	return u.Unwrap()
  }
  
  // Is reports whether any error in err's chain matches target.
  //
  // An error is considered to match a target if it is equal to that target or if
  // it implements a method Is(error) bool such that Is(target) returns true.
  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
  		}
  	}
  }
  
  // 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 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 not a non-nil pointer to a type which implements error or is of interface type.
  //
  // The As method should set the target to its value and return true if err
  // matches the type to which target points.
  func As(err error, target interface{}) bool {
  	if target == nil {
  		panic("errors: target cannot be nil")
  	}
  	val := reflect.ValueOf(target)
  	typ := val.Type()
  	if typ.Kind() != reflect.Ptr || val.IsNil() {
  		panic("errors: target must be a non-nil pointer")
  	}
  	if e := typ.Elem(); e.Kind() != reflect.Interface && !e.Implements(errorType) {
  		panic("errors: *target must be interface or implement error")
  	}
  	targetType := typ.Elem()
  	for err != nil {
  		if reflect.TypeOf(err).AssignableTo(targetType) {
  			val.Elem().Set(reflect.ValueOf(err))
  			return true
  		}
  		if x, ok := err.(interface{ As(interface{}) bool }); ok && x.As(target) {
  			return true
  		}
  		err = Unwrap(err)
  	}
  	return false
  }
  
  var errorType = reflect.TypeOf((*error)(nil)).Elem()
  