Fix errors.Wrap()/errors.Is() (#106)

errors/errors.go

@@ -169,7 +169,8 @@ type withStack struct {
 	*stack.CallStack
 }
 
-func (w *withStack) Cause() error { return w.error }
+func (w *withStack) Cause() error  { return w.error }
+func (w *withStack) Unwrap() error { return w.error }
 func (w *withStack) Context() map[string]interface{} {
 	if child, ok := w.error.(HasContext); ok {
 		return child.Context()
@@ -263,6 +264,7 @@ type withMessage struct {
 
 func (w *withMessage) Error() string { return w.msg + ": " + w.cause.Error() }
 func (w *withMessage) Cause() error  { return w.cause }
+func (w *withMessage) Unwrap() error { return w.cause }
 
 func (w *withMessage) Format(s fmt.State, verb rune) {
 	switch verb {

errors/go113.go

@@ -1,3 +1,4 @@
+//go:build go1.13
 // +build go1.13
 
 package errors

errors/go113_test.go

@@ -0,0 +1,59 @@
+//go:build go1.13
+// +build go1.13
+
+package errors
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIs(t *testing.T) {
+	target := New("wrapped")
+
+	tests := []struct {
+		name   string
+		target error
+		err    error
+	}{
+		{
+			name:   "holster_wrap",
+			target: target,
+			err:    Wrap(target, "some reason"),
+		},
+		{
+			name:   "holster_double_wrap",
+			target: target,
+			err:    Wrap(Wrap(target, "reason 2"), "reason 1"),
+		},
+		{
+			name:   "holster_triple_wrap",
+			target: target,
+			err:    Wrap(Wrap(Wrap(target, "reason 3"), "reason 2"), "reason 1"),
+		},
+		{
+			name:   "holster_triple_wrapf",
+			target: target,
+			err:    Wrapf(Wrapf(Wrapf(target, "reason %d", 3), "reason %d", 2), "reason %d", 1),
+		},
+		{
+			name:   "std_wrap",
+			target: target,
+			err:    fmt.Errorf("some reason: %w", target),
+		},
+		{
+			name:   "std_double_wrap",
+			target: target,
+			err:    fmt.Errorf("reason 1: %w", fmt.Errorf("reason 2: %w", target)),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.True(t, Is(tt.err, tt.target))
+			assert.ErrorIs(t, tt.err, tt.target)
+		})
+	}
+}