Add support for Go 1.13 error chains

errors.go

@@ -159,6 +159,9 @@ type withStack struct {
 
 func (w *withStack) Cause() error { return w.error }
 
+// Unwrap provides compatibility for Go 1.13 error chains.
+func (w *withStack) Unwrap() error { return w.error }
+
 func (w *withStack) Format(s fmt.State, verb rune) {
 	switch verb {
 	case 'v':
@@ -241,6 +244,9 @@ type withMessage struct {
 func (w *withMessage) Error() string { return w.msg + ": " + w.cause.Error() }
 func (w *withMessage) Cause() error  { return w.cause }
 
+// Unwrap provides compatibility for Go 1.13 error chains.
+func (w *withMessage) Unwrap() error { return w.cause }
+
 func (w *withMessage) Format(s fmt.State, verb rune) {
 	switch verb {
 	case 'v':

go113_test.go

@@ -0,0 +1,16 @@
+// +build go1.13
+
+package errors
+
+import (
+	stdlib_errors "errors"
+	"testing"
+)
+
+func TestErrorChainCompat(t *testing.T) {
+	err := stdlib_errors.New("error that gets wrapped")
+	wrapped := Wrap(err, "wrapped up")
+	if !stdlib_errors.Is(wrapped, err) {
+		t.Errorf("Wrap does not support Go 1.13 error chains")
+	}
+}