multierr without multierr

errs.go

// You can edit this code!
// Click here and start typing.
package main

import (
	"errors"
	"fmt"
)

var myerr = errors.New("error!")

func main() {
	errs := []error{
		myerr,
		nil, nil, nil,
		myerr,
		nil, nil,
		myerr,
		myerr,
		nil,
	}

	// Filter errs in-place.
	n := 0
	for _, e := range errs {
		if e != nil {
			errs[n] = e
			n++
		}
	}
	errs = errs[:n]

	// Collapse into a single error.
	var err error
	if len(errs) == 0 {
		err = nil
	} else if len(errs) == 1 {
		err = errs[0]
	} else {
		err = fmt.Errorf("%s; %w", errs[:len(errs)-1], errs[len(errs)-1])
	}

	fmt.Println(err)
	fmt.Println(errors.Is(err, myerr))
}