Enable Errors support for any multi-error (#75)

Starting Go 1.20, any multi-error should conform to the standard unwrap
method: Unwrap() []error.

This changes multierr.Errors() method to support any error that complies
to that interface.

Fix #70 / GO-1883

CHANGELOG.md

@@ -1,6 +1,11 @@
 Releases
 ========
 
+Unreleased
+====================
+-   `Errors` now supports any error that implements multiple-error
+    interface.
+
 v1.10.0 (2023-03-08)
 ====================
 

error.go

@@ -194,23 +194,7 @@ type errorGroup interface {
 //
 // Callers of this function are free to modify the returned slice.
 func Errors(err error) []error {
-	if err == nil {
-		return nil
-	}
-
-	// Note that we're casting to multiError, not errorGroup. Our contract is
-	// that returned errors MAY implement errorGroup. Errors, however, only
-	// has special behavior for multierr-specific error objects.
-	//
-	// This behavior can be expanded in the future but I think it's prudent to
-	// start with as little as possible in terms of contract and possibility
-	// of misuse.
-	eg, ok := err.(*multiError)
-	if !ok {
-		return []error{err}
-	}
-
-	return append(([]error)(nil), eg.Errors()...)
+	return extractErrors(err)
 }
 
 // multiError is an error that holds one or more errors.
@@ -225,8 +209,6 @@ type multiError struct {
 	errors     []error
 }
 
-var _ errorGroup = (*multiError)(nil)
-
 // Errors returns the list of underlying errors.
 //
 // This slice MUST NOT be modified.

error_post_go120.go

@@ -27,3 +27,22 @@ package multierr
 func (merr *multiError) Unwrap() []error {
 	return merr.Errors()
 }
+
+type multipleErrors interface {
+	Unwrap() []error
+}
+
+func extractErrors(err error) []error {
+	if err == nil {
+		return nil
+	}
+
+	// check if the given err is an Unwrapable error that
+	// implements multipleErrors interface.
+	eg, ok := err.(multipleErrors)
+	if !ok {
+		return []error{err}
+	}
+
+	return append(([]error)(nil), eg.Unwrap()...)
+}

error_post_go120_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2023 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+//go:build go1.20
+// +build go1.20
+
+package multierr
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestErrorsOnErrorsJoin(t *testing.T) {
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+	err := errors.Join(err1, err2)
+
+	errs := Errors(err)
+	assert.Equal(t, 2, len(errs))
+	assert.Equal(t, err1, errs[0])
+	assert.Equal(t, err2, errs[1])
+}

error_pre_go120.go

@@ -57,3 +57,23 @@ func (merr *multiError) Is(target error) bool {
 	}
 	return false
 }
+
+func extractErrors(err error) []error {
+	if err == nil {
+		return nil
+	}
+
+	// Note that we're casting to multiError, not errorGroup. Our contract is
+	// that returned errors MAY implement errorGroup. Errors, however, only
+	// has special behavior for multierr-specific error objects.
+	//
+	// This behavior can be expanded in the future but I think it's prudent to
+	// start with as little as possible in terms of contract and possibility
+	// of misuse.
+	eg, ok := err.(*multiError)
+	if !ok {
+		return []error{err}
+	}
+
+	return append(([]error)(nil), eg.Errors()...)
+}

error_test.go

@@ -386,7 +386,8 @@ func TestErrors(t *testing.T) {
 			dontCast: true,
 		},
 		{
-			// We don't yet support non-multierr errors.
+			// We don't yet support non-multierr errors that do
+			// not implement Unwrap() []error.
 			give:     notMultiErr{},
 			want:     []error{notMultiErr{}},
 			dontCast: true,