allow FlattenMultierror to accept standard error

helper/funcs.go

@@ -528,7 +528,12 @@ func Merge[T comparable](a, b T) T {
 
 // FlattenMultierror takes a multierror and unwraps it if there's only one error
 // in the output, otherwise returning the multierror or nil.
-func FlattenMultierror(mErr *multierror.Error) error {
+func FlattenMultierror(err error) error {
+	mErr, ok := err.(*multierror.Error)
+	if !ok {
+		return err
+	}
+	// note: mErr is a pointer so we still need to nil-check even after the cast
 	if mErr == nil {
 		return nil
 	}

helper/funcs_test.go

@@ -488,11 +488,21 @@ func Test_SliceSetEq(t *testing.T) {
 
 func TestFlattenMultiError(t *testing.T) {
 
+	err := FlattenMultierror(nil)
+	must.Nil(t, err)
+
+	err0 := errors.New("oh no!")
+	err = FlattenMultierror(err0)
+	must.Eq(t, `oh no!`, err.Error())
+
 	var mErr0 *multierror.Error
+	err = FlattenMultierror(mErr0)
+	must.Nil(t, err)
+
 	mErr0 = multierror.Append(mErr0, func() error {
 		return nil
 	}())
-	err := FlattenMultierror(mErr0)
+	err = FlattenMultierror(mErr0)
 	must.Nil(t, err)
 
 	var mErr1 *multierror.Error