fix: initialize default Usage function on NewFlagSet and Init

flag.go

@@ -175,7 +175,7 @@ type NormalizedName string
 type FlagSet struct {
 	// Usage is the function called when an error occurs while parsing flags.
 	// The field is a function (not a method) that may be changed to point to
-	// a custom error handler.
+	// a custom error handler. By default, it prints the default usage message.
 	Usage func()
 
 	// SortFlags is used to indicate, if user wants to have sorted flags in
@@ -1388,7 +1388,8 @@ func Parsed() bool {
 var CommandLine = NewFlagSet(os.Args[0], ExitOnError)
 
 // NewFlagSet returns a new, empty flag set with the specified name,
-// error handling property and SortFlags set to true.
+// error handling property, SortFlags set to true, and Usage set to
+// print the default usage message.
 func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
 	f := &FlagSet{
 		name:          name,
@@ -1397,6 +1398,7 @@ func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
 		interspersed:  true,
 		SortFlags:     true,
 	}
+	f.Usage = func() { defaultUsage(f) }
 	return f
 }
 
@@ -1407,9 +1409,11 @@ func (f *FlagSet) SetInterspersed(interspersed bool) {
 
 // Init sets the name and error handling property for a flag set.
 // By default, the zero FlagSet uses an empty name and the
-// ContinueOnError error handling policy.
+// ContinueOnError error handling policy. It also sets Usage to
+// print the default usage message.
 func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
 	f.name = name
 	f.errorHandling = errorHandling
 	f.argsLenAtDash = -1
+	f.Usage = func() { defaultUsage(f) }
 }

flag_test.go

@@ -1626,3 +1626,40 @@ func TestVisitFlagOrder(t *testing.T) {
 		i++
 	})
 }
+
+func TestNewFlagSetDefaultUsage(t *testing.T) {
+	var buf bytes.Buffer
+	flags := NewFlagSet("test", ContinueOnError)
+	flags.SetOutput(&buf)
+	if flags.Usage == nil {
+		t.Fatal("expected flags.Usage to be defined, got nil")
+	}
+	flags.Bool("testflag", false, "a test flag")
+	flags.Usage()
+	got := buf.String()
+	if !strings.Contains(got, "Usage of test:\n") {
+		t.Errorf("expected usage output to contain 'Usage of test:\n', got %q", got)
+	}
+	if !strings.Contains(got, "--testflag") {
+		t.Errorf("expected usage output to contain '--testflag', got %q", got)
+	}
+}
+
+func TestFlagSetInitDefaultUsage(t *testing.T) {
+	var buf bytes.Buffer
+	var flags FlagSet
+	flags.Init("test-init", ContinueOnError)
+	flags.SetOutput(&buf)
+	if flags.Usage == nil {
+		t.Fatal("expected flags.Usage to be defined after Init, got nil")
+	}
+	flags.Bool("testflag", false, "a test flag")
+	flags.Usage()
+	got := buf.String()
+	if !strings.Contains(got, "Usage of test-init:\n") {
+		t.Errorf("expected usage output to contain 'Usage of test-init:\n', got %q", got)
+	}
+	if !strings.Contains(got, "--testflag") {
+		t.Errorf("expected usage output to contain '--testflag', got %q", got)
+	}
+}