use errors.Is() to check for errors

Since go 1.13 you can wrap errors. This make it no longer possible to
compare with `==`, instead you have to compare with `errors.Is()`.

I noticed this problem because -h was no longer working after I stared
wrapping the errors in my custom FlagErrorFunc function.
Note that this is only a problem when a custom help flag is defined.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

Merge spf13/cobra#1730

command.go

@@ -1122,7 +1122,7 @@ func (c *Command) ExecuteC() (cmd *Command, err error) {
 
 		// Always show help if requested, even if SilenceErrors is in
 		// effect
-		if err == zflag.ErrHelp {
+		if errors.Is(err, zflag.ErrHelp) {
 			cmd.HelpFunc()(cmd, args)
 			return cmd, nil
 		}

command_test.go

@@ -1824,6 +1824,26 @@ func TestFlagErrorFunc(t *testing.T) {
 	}
 }
 
+func TestFlagErrorFuncHelp(t *testing.T) {
+	t.Parallel()
+
+	c := &zulu.Command{Use: "c", RunE: emptyRun}
+	c.PersistentFlags().Bool("help", false, "help for c")
+	c.SetFlagErrorFunc(func(_ *zulu.Command, err error) error {
+		return fmt.Errorf("wrap error: %w", err)
+	})
+
+	expected := "Usage:\n  c [flags]\n\nFlags:\n      --help   help for c\n"
+
+	out, err := executeCommand(c, "--help")
+	assertNoErr(t, err)
+	assertEqual(t, expected, out)
+
+	out, err = executeCommand(c, "-h")
+	assertNoErr(t, err)
+	assertEqual(t, expected, out)
+}
+
 // TestSortedFlags checks,
 // if cmd.LocalFlags() is unsorted when cmd.Flags().SortFlags set to false.
 // Related to https://github.com/spf13/cobra/issues/404.

zulu_test.go

@@ -7,6 +7,18 @@ import (
 	"github.com/gowarden/zulu"
 )
 
+func assertEqual(t *testing.T, expected, actual interface{}) {
+	t.Helper()
+	assertEqualf(t, expected, actual, "expected %[1]v with type %[1]T but got %[2]v with type %[2]T", expected, actual)
+}
+
+func assertEqualf(t *testing.T, expected, actual interface{}, msg string, fmt ...interface{}) {
+	t.Helper()
+	if expected != actual {
+		t.Errorf(msg, fmt...)
+	}
+}
+
 func assertNoErr(t *testing.T, e error) {
 	if e != nil {
 		t.Error(e)