WIP

after_go120.go

@@ -0,0 +1,22 @@
+//go:build go1.20
+// +build go1.20
+
+package multierror
+
+// Split returns the underlying list of errors wrapped in a multierror.
+//
+// A multierror is an error tha implements Unwrap() []error (see https://pkg.go.dev/errors)
+// Errors produced by multierror.Join implement Unwrap() []error.
+func Split(err error) []error {
+	u, ok := err.(interface {
+		Unwrap() []error
+	})
+	if !ok {
+		return []error{err}
+	}
+	return u.Unwrap()
+}
+
+func (e *Error) Unwrap() []error {
+	return e.errs
+}

after_go120_test.go

@@ -0,0 +1,17 @@
+package multierror_test
+
+import (
+	"errors"
+	"fmt"
+
+	"github.com/mkmik/multierror"
+)
+
+// multierror.Split works also with multierrors produced by the new stdlib errors.Join
+func ExampleErrorsSplit() {
+	err := errors.Join(fmt.Errorf("foo"), fmt.Errorf("bar"), fmt.Errorf("baz"))
+
+	fmt.Printf("%q", multierror.Split(err))
+	// Output:
+	// ["foo" "bar" "baz"]
+}

before_go120.go

@@ -0,0 +1,14 @@
+//go:build !go1.20
+// +build !go1.20
+
+package multierror
+
+// Split returns the underlying list of errors wrapped in a multierror.
+// If err is not a multierror, then a singleton list is returned.
+func Split(err error) []error {
+	if me, ok := err.(*Error); ok {
+		return me.errs
+	} else {
+		return []error{err}
+	}
+}

multierror.go

@@ -73,16 +73,6 @@ func Fold(errs []error) error {
 	return Join(errs)
 }
 
-// Split returns the underlying list of errors wrapped in a multierror.
-// If err is not a multierror, then a singleton list is returned.
-func Split(err error) []error {
-	if me, ok := err.(*Error); ok {
-		return me.errs
-	} else {
-		return []error{err}
-	}
-}
-
 // Unfold is deprecated, use Split instead.
 //
 // Unfold returns the underlying list of errors wrapped in a multierror.

multierror_test.go

@@ -47,6 +47,30 @@ func TestAppendNil(t *testing.T) {
 	}
 }
 
+func TestAppendNil2(t *testing.T) {
+	err := fmt.Errorf("foo")
+	nerr := multierror.Append(nil, err)
+	if got, want := nerr, err; got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+}
+
+func TestAppendNil3(t *testing.T) {
+	err1 := fmt.Errorf("foo1")
+	err2 := fmt.Errorf("foo2")
+	nerr := multierror.Append(nil, err1, err2)
+	serr := multierror.Split(nerr)
+	if got, want := len(serr), 2; got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+	if got, want := serr[0], err1; got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+	if got, want := serr[1], err2; got != want {
+		t.Errorf("got: %v, want: %v", got, want)
+	}
+}
+
 func TestAppendNilOnSomething(t *testing.T) {
 	err1 := fmt.Errorf("test")
 	errs := err1
@@ -152,6 +176,16 @@ func TestSplitSingleton(t *testing.T) {
 	}
 }
 
+func TestUnfoldSingleton(t *testing.T) {
+	errs := multierror.Unfold(fmt.Errorf("foo"))
+	if got, want := len(errs), 1; got != want {
+		t.Fatalf("got: %d, want: %d", got, want)
+	}
+	if got, want := errs[0].Error(), "foo"; got != want {
+		t.Fatalf("got: %q, want: %q", got, want)
+	}
+}
+
 func TestUniqEmpty(t *testing.T) {
 	errs := multierror.Uniq(nil)
 	if got, want := errs, []error(nil); got != nil {
@@ -234,3 +268,13 @@ func ExampleTransformer() {
 	// Output:
 	// foo (k1, k2); bar (k3)
 }
+
+func TestFormatLegacyError(t *testing.T) {
+	err := fmt.Errorf("foo")
+	formatted := multierror.Format(err, func(errs []string) string {
+		return strings.Join(errs, "; ")
+	})
+	if got, want := err, formatted; got != want {
+		t.Fatalf("got %v, wanted: %v", got, want)
+	}
+}