fix: honor custom IsBoolFlag compatibility

bool.go

@@ -12,6 +12,13 @@ type boolFlag interface {
 	IsBoolFlag() bool
 }
 
+func isNoOptBoolValue(v Value) bool {
+	if bf, ok := v.(boolFlag); ok {
+		return bf.IsBoolFlag()
+	}
+	return false
+}
+
 // -- bool Value
 type boolValue bool
 

bool_test.go

@@ -152,6 +152,34 @@ func TestImplicitFalse(t *testing.T) {
 	}
 }
 
+func TestCustomIsBoolFlagGetsNoOptDefaultAutomatically(t *testing.T) {
+	var tristate triStateValue
+	f := NewFlagSet("test", ContinueOnError)
+	tristate = triStateFalse
+	f.VarPF(&tristate, "tristate", "t", "tristate value (true, maybe or false)")
+
+	if err := f.Parse([]string{"--tristate"}); err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateTrue {
+		t.Fatal("expected", triStateTrue, "(triStateTrue) but got", tristate, "instead")
+	}
+}
+
+func TestCustomIsBoolFlagShortFormGetsNoOptDefaultAutomatically(t *testing.T) {
+	var tristate triStateValue
+	f := NewFlagSet("test", ContinueOnError)
+	tristate = triStateFalse
+	f.VarPF(&tristate, "tristate", "t", "tristate value (true, maybe or false)")
+
+	if err := f.Parse([]string{"-t"}); err != nil {
+		t.Fatal("expected no error; got", err)
+	}
+	if tristate != triStateTrue {
+		t.Fatal("expected", triStateTrue, "(triStateTrue) but got", tristate, "instead")
+	}
+}
+
 func TestInvalidValue(t *testing.T) {
 	var tristate triStateValue
 	f := setUpFlagSet(&tristate)

flag.go

@@ -614,9 +614,10 @@ func (f *FlagSet) PrintDefaults() {
 // defaultIsZeroValue returns true if the default value for this flag represents
 // a zero value.
 func (f *Flag) defaultIsZeroValue() bool {
-	switch f.Value.(type) {
-	case boolFlag:
+	if isNoOptBoolValue(f.Value) {
 		return f.DefValue == "false" || f.DefValue == ""
+	}
+	switch f.Value.(type) {
 	case *durationValue:
 		// Beginning in Go 1.7, duration zero values are "0s"
 		return f.DefValue == "0" || f.DefValue == "0s"
@@ -665,6 +666,9 @@ func UnquoteUsage(flag *Flag) (name string, usage string) {
 	}
 
 	name = flag.Value.Type()
+	if isNoOptBoolValue(flag.Value) {
+		name = ""
+	}
 	switch name {
 	case "bool", "boolfunc":
 		name = ""
@@ -779,7 +783,7 @@ func (f *FlagSet) FlagUsagesWrapped(cols int) string {
 		}
 
 		varname, usage := UnquoteUsage(flag)
-		if flag.Value.Type() == "bool" {
+		if isNoOptBoolValue(flag.Value) && flag.Value.Type() == "bool" {
 			line += "[=true|false]"
 		} else if varname != "" {
 			line += " " + varname
@@ -941,6 +945,9 @@ func (f *FlagSet) AddFlag(flag *Flag) {
 	}
 
 	flag.Name = string(normalizedFlagName)
+	if flag.NoOptDefVal == "" && isNoOptBoolValue(flag.Value) {
+		flag.NoOptDefVal = "true"
+	}
 	f.formal[normalizedFlagName] = flag
 	f.orderedFormal = append(f.orderedFormal, flag)
 

flag_test.go

@@ -1481,6 +1481,20 @@ func (cv *customValue) Set(s string) error {
 
 func (cv *customValue) Type() string { return "custom" }
 
+type customBoolFlagValue bool
+
+func (cv *customBoolFlagValue) String() string { return strconv.FormatBool(bool(*cv)) }
+
+func (cv *customBoolFlagValue) Set(s string) error {
+	v, err := strconv.ParseBool(s)
+	*cv = customBoolFlagValue(v)
+	return err
+}
+
+func (cv *customBoolFlagValue) Type() string { return "custom-bool" }
+
+func (cv *customBoolFlagValue) IsBoolFlag() bool { return true }
+
 func TestPrintDefaults(t *testing.T) {
 	fs := NewFlagSet("print defaults test", ContinueOnError)
 	var buf bytes.Buffer
@@ -1529,6 +1543,36 @@ func TestPrintDefaults(t *testing.T) {
 	}
 }
 
+func TestCustomIsBoolFlagDefaultIsZeroValue(t *testing.T) {
+	var v customBoolFlagValue
+	flag := &Flag{
+		Name:     "custom-bool",
+		Usage:    "custom bool",
+		Value:    &v,
+		DefValue: "",
+	}
+
+	if !flag.defaultIsZeroValue() {
+		t.Fatal("expected empty default for custom IsBoolFlag value to be treated as zero value")
+	}
+}
+
+func TestPrintDefaultsCustomIsBoolFlagOmitsDefault(t *testing.T) {
+	fs := NewFlagSet("print defaults custom bool", ContinueOnError)
+	var buf bytes.Buffer
+	fs.SetOutput(&buf)
+
+	var v customBoolFlagValue
+	fs.Var(&v, "custom-bool", "custom bool value implementation")
+
+	fs.PrintDefaults()
+	got := buf.String()
+	want := "      --custom-bool[=true]   custom bool value implementation\n"
+	if got != want {
+		t.Errorf("\n--- Got:\n%s--- Wanted:\n%s\n", got, want)
+	}
+}
+
 func TestVisitAllFlagOrder(t *testing.T) {
 	fs := NewFlagSet("TestVisitAllFlagOrder", ContinueOnError)
 	fs.SortFlags = false