type TestError struct {
	err error
}

func New(err error) *TestError {
	return &TestError{err: err}
}

func (err *TestError) Error() string {
	if err == nil {
		return ""
	}

	return err.err.Error()
}

func (err *TestError) Unwrap() error {
	return err.err
}

func doSomething() error {
	return nil
}

func doSomethingElse() *TestError {
	return nil
}

func main() {
	var err error = doSomething()
	var err2 error = doSomethingElse()
	customErr := doSomethingElse()
	customErr2 := doSomethingElse()

	if err != nil {
		fmt.Printf("err: Not nil, type: %T, address: %p\n", err, err)
	} else {
		fmt.Printf("err: Nil, type: %T, address: %p\n", err, err)
	}

	if err2 != nil {
		fmt.Printf("err2: Not nil, type: %T, address: %p\n", err2, err2)
	} else {
		fmt.Printf("err2: Nil, type: %T, address: %p\n", err2, err2)
	}

	if customErr != nil {
		fmt.Printf("customErr: Not nil, type: %T, address: %p\n", customErr, customErr)
	} else {
		fmt.Printf("customErr: Nil, type: %T, address: %p\n", customErr, customErr)
	}

	if customErr2 != nil {
		fmt.Printf("customErr2: Not nil, type: %T, address: %p\n", customErr2, customErr2)
	} else {
		fmt.Printf("customErr2: Nil, type: %T, address: %p\n", customErr2, customErr2)
	}

	// Output
	// err: Nil, type: <nil>, address: %!p(<nil>) - expected
	// err2: Not nil, type: *main.TestError, address: 0x0 - unexpected ???
	// customErr: Nil, type: *main.TestError, address: 0x0 - expected
	// customErr2: Nil, type: *main.TestError, address: 0x0 - expected
}