remove showFlagError, we can use the help printer assertion to accomplish the same goal

app.go

@@ -23,8 +23,6 @@ var (
 	errInvalidActionType = NewExitError("ERROR invalid Action type. "+
 		fmt.Sprintf("Must be `func(*Context`)` or `func(*Context) error).  %s", contactSysadmin)+
 		fmt.Sprintf("See %s", appActionDeprecationURL), 2)
-
-	showFlagError printerFunc = fmt.Fprintln
 )
 
 // App is the main structure of a cli application. It is recommended that
@@ -235,7 +233,7 @@ func (a *App) Run(arguments []string) (err error) {
 
 	cerr := checkRequiredFlags(a.Flags, set)
 	if cerr != nil {
-		showFlagError(a.Writer, cerr)
+		fmt.Fprintln(a.Writer, cerr)
 		ShowAppHelp(context)
 		return cerr
 	}
@@ -366,7 +364,7 @@ func (a *App) RunAsSubcommand(ctx *Context) (err error) {
 
 	cerr := checkRequiredFlags(a.Flags, set)
 	if cerr != nil {
-		showFlagError(a.Writer, cerr)
+		fmt.Fprintln(a.Writer, cerr)
 		ShowSubcommandHelp(context)
 		return cerr
 	}

app_test.go

@@ -913,17 +913,6 @@ func TestRequiredFlagAppRunBehavior(t *testing.T) {
 	}
 	for _, test := range tdata {
 		t.Run(test.testCase, func(t *testing.T) {
-			// setup - undo showFlagError mock when finished
-			oldShowError := showFlagError
-			defer func() {
-				showFlagError = oldShowError
-			}()
-			// setup - mock showFlagError
-			var showFlagErrorWasCalled = false
-			showFlagError = func(writer io.Writer, err ...interface{}) (int, error) {
-				showFlagErrorWasCalled = true
-				return 0, nil
-			}
 			// setup - undo HelpPrinter mock when finished
 			oldPrinter := HelpPrinter
 			defer func() {
@@ -948,9 +937,6 @@ func TestRequiredFlagAppRunBehavior(t *testing.T) {
 			if test.expectedAnError && err == nil {
 				t.Errorf("expected an error, but there was none")
 			}
-			if test.expectedAnError && showFlagErrorWasCalled == false {
-				t.Errorf("showFlagError expected to be called, but was not")
-			}
 			if !test.expectedAnError && err != nil {
 				t.Errorf("did not expected an error, but there was one: %s", err)
 			}

command.go

@@ -137,7 +137,7 @@ func (c Command) Run(ctx *Context) (err error) {
 
 	cerr := checkRequiredFlags(c.Flags, set)
 	if cerr != nil {
-		showFlagError(context.App.Writer, cerr)
+		fmt.Fprintln(context.App.Writer, cerr)
 		ShowCommandHelp(context, c.Name)
 		return cerr
 	}
@@ -273,7 +273,7 @@ func reorderArgs(args []string) []string {
 }
 
 func translateShortOptions(set *flag.FlagSet, flagArgs Args) []string {
-	allCharsFlags := func (s string) bool {
+	allCharsFlags := func(s string) bool {
 		for i := range s {
 			f := set.Lookup(string(s[i]))
 			if f == nil {