src/flag/flag_test.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package flag_test

import (
	"bytes"
	. "flag"
	"fmt"
	"internal/testenv"
	"io"
	"os"
	"os/exec"
	"regexp"
	"runtime"
	"slices"
	"strconv"
	"strings"
	"testing"
	"time"
)

func boolString(s string) string {
	if s == "0" {
		return "false"
	}
	return "true"
}

func TestEverything(t *testing.T) {
	ResetForTesting(nil)
	Bool("test_bool", false, "bool value")
	Int("test_int", 0, "int value")
	Int64("test_int64", 0, "int64 value")
	Uint("test_uint", 0, "uint value")
	Uint64("test_uint64", 0, "uint64 value")
	String("test_string", "0", "string value")
	Float64("test_float64", 0, "float64 value")
	Duration("test_duration", 0, "time.Duration value")
	Func("test_func", "func value", func(string) error { return nil })
	BoolFunc("test_boolfunc", "func", func(string) error { return nil })

	m := make(map[string]*Flag)
	desired := "0"
	visitor := func(f *Flag) {
		if len(f.Name) > 5 && f.Name[0:5] == "test_" {
			m[f.Name] = f
			ok := false
			switch {
			case f.Value.String() == desired:
				ok = true
			case f.Name == "test_bool" && f.Value.String() == boolString(desired):
				ok = true
			case f.Name == "test_duration" && f.Value.String() == desired+"s":
				ok = true
			case f.Name == "test_func" && f.Value.String() == "":
				ok = true
			case f.Name == "test_boolfunc" && f.Value.String() == "":
				ok = true
			}
			if !ok {
				t.Error("Visit: bad value", f.Value.String(), "for", f.Name)
			}
		}
	}
	VisitAll(visitor)
	if len(m) != 10 {
		t.Error("VisitAll misses some flags")
		for k, v := range m {
			t.Log(k, *v)
		}
	}
	m = make(map[string]*Flag)
	Visit(visitor)
	if len(m) != 0 {
		t.Errorf("Visit sees unset flags")
		for k, v := range m {
			t.Log(k, *v)
		}
	}
	// Now set all flags
	Set("test_bool", "true")
	Set("test_int", "1")
	Set("test_int64", "1")
	Set("test_uint", "1")
	Set("test_uint64", "1")
	Set("test_string", "1")
	Set("test_float64", "1")
	Set("test_duration", "1s")
	Set("test_func", "1")
	Set("test_boolfunc", "")
	desired = "1"
	Visit(visitor)
	if len(m) != 10 {
		t.Error("Visit fails after set")
		for k, v := range m {
			t.Log(k, *v)
		}
	}
	// Now test they're visited in sort order.
	var flagNames []string
	Visit(func(f *Flag) { flagNames = append(flagNames, f.Name) })
	if !slices.IsSorted(flagNames) {
		t.Errorf("flag names not sorted: %v", flagNames)
	}
}

func TestGet(t *testing.T) {
	ResetForTesting(nil)
	Bool("test_bool", true, "bool value")
	Int("test_int", 1, "int value")
	Int64("test_int64", 2, "int64 value")
	Uint("test_uint", 3, "uint value")
	Uint64("test_uint64", 4, "uint64 value")
	String("test_string", "5", "string value")
	Float64("test_float64", 6, "float64 value")
	Duration("test_duration", 7, "time.Duration value")

	visitor := func(f *Flag) {
		if len(f.Name) > 5 && f.Name[0:5] == "test_" {
			g, ok := f.Value.(Getter)
			if !ok {
				t.Errorf("Visit: value does not satisfy Getter: %T", f.Value)
				return
			}
			switch f.Name {
			case "test_bool":
				ok = g.Get() == true
			case "test_int":
				ok = g.Get() == int(1)
			case "test_int64":
				ok = g.Get() == int64(2)
			case "test_uint":
				ok = g.Get() == uint(3)
			case "test_uint64":
				ok = g.Get() == uint64(4)
			case "test_string":
				ok = g.Get() == "5"
			case "test_float64":
				ok = g.Get() == float64(6)
			case "test_duration":
				ok = g.Get() == time.Duration(7)
			}
			if !ok {
				t.Errorf("Visit: bad value %T(%v) for %s", g.Get(), g.Get(), f.Name)
			}
		}
	}
	VisitAll(visitor)
}

func TestUsage(t *testing.T) {
	called := false
	ResetForTesting(func() { called = true })
	if CommandLine.Parse([]string{"-x"}) == nil {
		t.Error("parse did not fail for unknown flag")
	}
	if !called {
		t.Error("did not call Usage for unknown flag")
	}
}

func testParse(f *FlagSet, t *testing.T) {
	if f.Parsed() {
		t.Error("f.Parse() = true before Parse")
	}
	boolFlag := f.Bool("bool", false, "bool value")
	bool2Flag := f.Bool("bool2", false, "bool2 value")
	intFlag := f.Int("int", 0, "int value")
	int64Flag := f.Int64("int64", 0, "int64 value")
	uintFlag := f.Uint("uint", 0, "uint value")
	uint64Flag := f.Uint64("uint64", 0, "uint64 value")
	stringFlag := f.String("string", "0", "string value")
	float64Flag := f.Float64("float64", 0, "float64 value")
	durationFlag := f.Duration("duration", 5*time.Second, "time.Duration value")
	extra := "one-extra-argument"
	args := []string{
		"-bool",
		"-bool2=true",
		"--int", "22",
		"--int64", "0x23",
		"-uint", "24",
		"--uint64", "25",
		"-string", "hello",
		"-float64", "2718e28",
		"-duration", "2m",
		extra,
	}
	if err := f.Parse(args); err != nil {
		t.Fatal(err)
	}
	if !f.Parsed() {
		t.Error("f.Parse() = false after Parse")
	}
	if *boolFlag != true {
		t.Error("bool flag should be true, is ", *boolFlag)
	}
	if *bool2Flag != true {
		t.Error("bool2 flag should be true, is ", *bool2Flag)
	}
	if *intFlag != 22 {
		t.Error("int flag should be 22, is ", *intFlag)
	}
	if *int64Flag != 0x23 {
		t.Error("int64 flag should be 0x23, is ", *int64Flag)
	}
	if *uintFlag != 24 {
		t.Error("uint flag should be 24, is ", *uintFlag)
	}
	if *uint64Flag != 25 {
		t.Error("uint64 flag should be 25, is ", *uint64Flag)
	}
	if *stringFlag != "hello" {
		t.Error("string flag should be `hello`, is ", *stringFlag)
	}
	if *float64Flag != 2718e28 {
		t.Error("float64 flag should be 2718e28, is ", *float64Flag)
	}
	if *durationFlag != 2*time.Minute {
		t.Error("duration flag should be 2m, is ", *durationFlag)
	}
	if len(f.Args()) != 1 {
		t.Error("expected one argument, got", len(f.Args()))
	} else if f.Args()[0] != extra {
		t.Errorf("expected argument %q got %q", extra, f.Args()[0])
	}
}

func TestParse(t *testing.T) {
	ResetForTesting(func() { t.Error("bad parse") })
	testParse(CommandLine, t)
}

func TestFlagSetParse(t *testing.T) {
	testParse(NewFlagSet("test", ContinueOnError), t)
}

// Declare a user-defined flag type.
type flagVar []string

func (f *flagVar) String() string {
	return fmt.Sprint([]string(*f))
}

func (f *flagVar) Set(value string) error {
	*f = append(*f, value)
	return nil
}

func TestUserDefined(t *testing.T) {
	var flags FlagSet
	flags.Init("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	var v flagVar
	flags.Var(&v, "v", "usage")
	if err := flags.Parse([]string{"-v", "1", "-v", "2", "-v=3"}); err != nil {
		t.Error(err)
	}
	if len(v) != 3 {
		t.Fatal("expected 3 args; got ", len(v))
	}
	expect := "[1 2 3]"
	if v.String() != expect {
		t.Errorf("expected value %q got %q", expect, v.String())
	}
}

func TestUserDefinedFunc(t *testing.T) {
	flags := NewFlagSet("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	var ss []string
	flags.Func("v", "usage", func(s string) error {
		ss = append(ss, s)
		return nil
	})
	if err := flags.Parse([]string{"-v", "1", "-v", "2", "-v=3"}); err != nil {
		t.Error(err)
	}
	if len(ss) != 3 {
		t.Fatal("expected 3 args; got ", len(ss))
	}
	expect := "[1 2 3]"
	if got := fmt.Sprint(ss); got != expect {
		t.Errorf("expected value %q got %q", expect, got)
	}
	// test usage
	var buf strings.Builder
	flags.SetOutput(&buf)
	flags.Parse([]string{"-h"})
	if usage := buf.String(); !strings.Contains(usage, "usage") {
		t.Errorf("usage string not included: %q", usage)
	}
	// test Func error
	flags = NewFlagSet("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	flags.Func("v", "usage", func(s string) error {
		return fmt.Errorf("test error")
	})
	// flag not set, so no error
	if err := flags.Parse(nil); err != nil {
		t.Error(err)
	}
	// flag set, expect error
	if err := flags.Parse([]string{"-v", "1"}); err == nil {
		t.Error("expected error; got none")
	} else if errMsg := err.Error(); !strings.Contains(errMsg, "test error") {
		t.Errorf(`error should contain "test error"; got %q`, errMsg)
	}
}

func TestUserDefinedForCommandLine(t *testing.T) {
	const help = "HELP"
	var result string
	ResetForTesting(func() { result = help })
	Usage()
	if result != help {
		t.Fatalf("got %q; expected %q", result, help)
	}
}

// Declare a user-defined boolean flag type.
type boolFlagVar struct {
	count int
}

func (b *boolFlagVar) String() string {
	return fmt.Sprintf("%d", b.count)
}

func (b *boolFlagVar) Set(value string) error {
	if value == "true" {
		b.count++
	}
	return nil
}

func (b *boolFlagVar) IsBoolFlag() bool {
	return b.count < 4
}

func TestUserDefinedBool(t *testing.T) {
	var flags FlagSet
	flags.Init("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	var b boolFlagVar
	var err error
	flags.Var(&b, "b", "usage")
	if err = flags.Parse([]string{"-b", "-b", "-b", "-b=true", "-b=false", "-b", "barg", "-b"}); err != nil {
		if b.count < 4 {
			t.Error(err)
		}
	}

	if b.count != 4 {
		t.Errorf("want: %d; got: %d", 4, b.count)
	}

	if err == nil {
		t.Error("expected error; got none")
	}
}

func TestUserDefinedBoolUsage(t *testing.T) {
	var flags FlagSet
	flags.Init("test", ContinueOnError)
	var buf bytes.Buffer
	flags.SetOutput(&buf)
	var b boolFlagVar
	flags.Var(&b, "b", "X")
	b.count = 0
	// b.IsBoolFlag() will return true and usage will look boolean.
	flags.PrintDefaults()
	got := buf.String()
	want := "  -b\tX\n"
	if got != want {
		t.Errorf("false: want %q; got %q", want, got)
	}
	b.count = 4
	// b.IsBoolFlag() will return false and usage will look non-boolean.
	flags.PrintDefaults()
	got = buf.String()
	want = "  -b\tX\n  -b value\n    \tX\n"
	if got != want {
		t.Errorf("false: want %q; got %q", want, got)
	}
}

func TestSetOutput(t *testing.T) {
	var flags FlagSet
	var buf strings.Builder
	flags.SetOutput(&buf)
	flags.Init("test", ContinueOnError)
	flags.Parse([]string{"-unknown"})
	if out := buf.String(); !strings.Contains(out, "-unknown") {
		t.Logf("expected output mentioning unknown; got %q", out)
	}
}

// This tests that one can reset the flags. This still works but not well, and is
// superseded by FlagSet.
func TestChangingArgs(t *testing.T) {
	ResetForTesting(func() { t.Fatal("bad parse") })
	oldArgs := os.Args
	defer func() { os.Args = oldArgs }()
	os.Args = []string{"cmd", "-before", "subcmd", "-after", "args"}
	before := Bool("before", false, "")
	if err := CommandLine.Parse(os.Args[1:]); err != nil {
		t.Fatal(err)
	}
	cmd := Arg(0)
	os.Args = Args()
	after := Bool("after", false, "")
	Parse()
	args := Args()

	if !*before || cmd != "subcmd" || !*after || len(args) != 1 || args[0] != "args" {
		t.Fatalf("expected true subcmd true [args] got %v %v %v %v", *before, cmd, *after, args)
	}
}

// Test that -help invokes the usage message and returns ErrHelp.
func TestHelp(t *testing.T) {
	var helpCalled = false
	fs := NewFlagSet("help test", ContinueOnError)
	fs.Usage = func() { helpCalled = true }
	var flag bool
	fs.BoolVar(&flag, "flag", false, "regular flag")
	// Regular flag invocation should work
	err := fs.Parse([]string{"-flag=true"})
	if err != nil {
		t.Fatal("expected no error; got ", err)
	}
	if !flag {
		t.Error("flag was not set by -flag")
	}
	if helpCalled {
		t.Error("help called for regular flag")
		helpCalled = false // reset for next test
	}
	// Help flag should work as expected.
	err = fs.Parse([]string{"-help"})
	if err == nil {
		t.Fatal("error expected")
	}
	if err != ErrHelp {
		t.Fatal("expected ErrHelp; got ", err)
	}
	if !helpCalled {
		t.Fatal("help was not called")
	}
	// If we define a help flag, that should override.
	var help bool
	fs.BoolVar(&help, "help", false, "help flag")
	helpCalled = false
	err = fs.Parse([]string{"-help"})
	if err != nil {
		t.Fatal("expected no error for defined -help; got ", err)
	}
	if helpCalled {
		t.Fatal("help was called; should not have been for defined help flag")
	}
}

// zeroPanicker is a flag.Value whose String method panics if its dontPanic
// field is false.
type zeroPanicker struct {
	dontPanic bool
	v         string
}

func (f *zeroPanicker) Set(s string) error {
	f.v = s
	return nil
}

func (f *zeroPanicker) String() string {
	if !f.dontPanic {
		panic("panic!")
	}
	return f.v
}

const defaultOutput = `  -A	for bootstrapping, allow 'any' type
  -Alongflagname
    	disable bounds checking
  -C	a boolean defaulting to true (default true)
  -D path
    	set relative path for local imports
  -E string
    	issue 23543 (default "0")
  -F number
    	a non-zero number (default 2.7)
  -G float
    	a float that defaults to zero
  -M string
    	a multiline
    	help
    	string
  -N int
    	a non-zero int (default 27)
  -O	a flag
    	multiline help string (default true)
  -V list
    	a list of strings (default [a b])
  -Z int
    	an int that defaults to zero
  -ZP0 value
    	a flag whose String method panics when it is zero
  -ZP1 value
    	a flag whose String method panics when it is zero
  -maxT timeout
    	set timeout for dial

panic calling String method on zero flag_test.zeroPanicker for flag ZP0: panic!
panic calling String method on zero flag_test.zeroPanicker for flag ZP1: panic!
`

func TestPrintDefaults(t *testing.T) {
	fs := NewFlagSet("print defaults test", ContinueOnError)
	var buf strings.Builder
	fs.SetOutput(&buf)
	fs.Bool("A", false, "for bootstrapping, allow 'any' type")
	fs.Bool("Alongflagname", false, "disable bounds checking")
	fs.Bool("C", true, "a boolean defaulting to true")
	fs.String("D", "", "set relative `path` for local imports")
	fs.String("E", "0", "issue 23543")
	fs.Float64("F", 2.7, "a non-zero `number`")
	fs.Float64("G", 0, "a float that defaults to zero")
	fs.String("M", "", "a multiline\nhelp\nstring")
	fs.Int("N", 27, "a non-zero int")
	fs.Bool("O", true, "a flag\nmultiline help string")
	fs.Var(&flagVar{"a", "b"}, "V", "a `list` of strings")
	fs.Int("Z", 0, "an int that defaults to zero")
	fs.Var(&zeroPanicker{true, ""}, "ZP0", "a flag whose String method panics when it is zero")
	fs.Var(&zeroPanicker{true, "something"}, "ZP1", "a flag whose String method panics when it is zero")
	fs.Duration("maxT", 0, "set `timeout` for dial")
	fs.PrintDefaults()
	got := buf.String()
	if got != defaultOutput {
		t.Errorf("got:\n%q\nwant:\n%q", got, defaultOutput)
	}
}

// Issue 19230: validate range of Int and Uint flag values.
func TestIntFlagOverflow(t *testing.T) {
	if strconv.IntSize != 32 {
		return
	}
	ResetForTesting(nil)
	Int("i", 0, "")
	Uint("u", 0, "")
	if err := Set("i", "2147483648"); err == nil {
		t.Error("unexpected success setting Int")
	}
	if err := Set("u", "4294967296"); err == nil {
		t.Error("unexpected success setting Uint")
	}
}

// Issue 20998: Usage should respect CommandLine.output.
func TestUsageOutput(t *testing.T) {
	ResetForTesting(DefaultUsage)
	var buf strings.Builder
	CommandLine.SetOutput(&buf)
	defer func(old []string) { os.Args = old }(os.Args)
	os.Args = []string{"app", "-i=1", "-unknown"}
	Parse()
	const want = "flag provided but not defined: -i\nUsage of app:\n"
	if got := buf.String(); got != want {
		t.Errorf("output = %q; want %q", got, want)
	}
}

func TestGetters(t *testing.T) {
	expectedName := "flag set"
	expectedErrorHandling := ContinueOnError
	expectedOutput := io.Writer(os.Stderr)
	fs := NewFlagSet(expectedName, expectedErrorHandling)

	if fs.Name() != expectedName {
		t.Errorf("unexpected name: got %s, expected %s", fs.Name(), expectedName)
	}
	if fs.ErrorHandling() != expectedErrorHandling {
		t.Errorf("unexpected ErrorHandling: got %d, expected %d", fs.ErrorHandling(), expectedErrorHandling)
	}
	if fs.Output() != expectedOutput {
		t.Errorf("unexpected output: got %#v, expected %#v", fs.Output(), expectedOutput)
	}

	expectedName = "gopher"
	expectedErrorHandling = ExitOnError
	expectedOutput = os.Stdout
	fs.Init(expectedName, expectedErrorHandling)
	fs.SetOutput(expectedOutput)

	if fs.Name() != expectedName {
		t.Errorf("unexpected name: got %s, expected %s", fs.Name(), expectedName)
	}
	if fs.ErrorHandling() != expectedErrorHandling {
		t.Errorf("unexpected ErrorHandling: got %d, expected %d", fs.ErrorHandling(), expectedErrorHandling)
	}
	if fs.Output() != expectedOutput {
		t.Errorf("unexpected output: got %v, expected %v", fs.Output(), expectedOutput)
	}
}

func TestParseError(t *testing.T) {
	for _, typ := range []string{"bool", "int", "int64", "uint", "uint64", "float64", "duration"} {
		fs := NewFlagSet("parse error test", ContinueOnError)
		fs.SetOutput(io.Discard)
		_ = fs.Bool("bool", false, "")
		_ = fs.Int("int", 0, "")
		_ = fs.Int64("int64", 0, "")
		_ = fs.Uint("uint", 0, "")
		_ = fs.Uint64("uint64", 0, "")
		_ = fs.Float64("float64", 0, "")
		_ = fs.Duration("duration", 0, "")
		// Strings cannot give errors.
		args := []string{"-" + typ + "=x"}
		err := fs.Parse(args) // x is not a valid setting for any flag.
		if err == nil {
			t.Errorf("Parse(%q)=%v; expected parse error", args, err)
			continue
		}
		if !strings.Contains(err.Error(), "invalid") || !strings.Contains(err.Error(), "parse error") {
			t.Errorf("Parse(%q)=%v; expected parse error", args, err)
		}
	}
}

func TestRangeError(t *testing.T) {
	bad := []string{
		"-int=123456789012345678901",
		"-int64=123456789012345678901",
		"-uint=123456789012345678901",
		"-uint64=123456789012345678901",
		"-float64=1e1000",
	}
	for _, arg := range bad {
		fs := NewFlagSet("parse error test", ContinueOnError)
		fs.SetOutput(io.Discard)
		_ = fs.Int("int", 0, "")
		_ = fs.Int64("int64", 0, "")
		_ = fs.Uint("uint", 0, "")
		_ = fs.Uint64("uint64", 0, "")
		_ = fs.Float64("float64", 0, "")
		// Strings cannot give errors, and bools and durations do not return strconv.NumError.
		err := fs.Parse([]string{arg})
		if err == nil {
			t.Errorf("Parse(%q)=%v; expected range error", arg, err)
			continue
		}
		if !strings.Contains(err.Error(), "invalid") || !strings.Contains(err.Error(), "value out of range") {
			t.Errorf("Parse(%q)=%v; expected range error", arg, err)
		}
	}
}

func TestExitCode(t *testing.T) {
	testenv.MustHaveExec(t)

	magic := 123
	if os.Getenv("GO_CHILD_FLAG") != "" {
		fs := NewFlagSet("test", ExitOnError)
		if os.Getenv("GO_CHILD_FLAG_HANDLE") != "" {
			var b bool
			fs.BoolVar(&b, os.Getenv("GO_CHILD_FLAG_HANDLE"), false, "")
		}
		fs.Parse([]string{os.Getenv("GO_CHILD_FLAG")})
		os.Exit(magic)
	}

	tests := []struct {
		flag       string
		flagHandle string
		expectExit int
	}{
		{
			flag:       "-h",
			expectExit: 0,
		},
		{
			flag:       "-help",
			expectExit: 0,
		},
		{
			flag:       "-undefined",
			expectExit: 2,
		},
		{
			flag:       "-h",
			flagHandle: "h",
			expectExit: magic,
		},
		{
			flag:       "-help",
			flagHandle: "help",
			expectExit: magic,
		},
	}

	for _, test := range tests {
		cmd := exec.Command(testenv.Executable(t), "-test.run=^TestExitCode$")
		cmd.Env = append(
			os.Environ(),
			"GO_CHILD_FLAG="+test.flag,
			"GO_CHILD_FLAG_HANDLE="+test.flagHandle,
		)
		cmd.Run()
		got := cmd.ProcessState.ExitCode()
		// ExitCode is either 0 or 1 on Plan 9.
		if runtime.GOOS == "plan9" && test.expectExit != 0 {
			test.expectExit = 1
		}
		if got != test.expectExit {
			t.Errorf("unexpected exit code for test case %+v \n: got %d, expect %d",
				test, got, test.expectExit)
		}
	}
}

func mustPanic(t *testing.T, testName string, expected string, f func()) {
	t.Helper()
	defer func() {
		switch msg := recover().(type) {
		case nil:
			t.Errorf("%s\n: expected panic(%q), but did not panic", testName, expected)
		case string:
			if ok, _ := regexp.MatchString(expected, msg); !ok {
				t.Errorf("%s\n: expected panic(%q), but got panic(%q)", testName, expected, msg)
			}
		default:
			t.Errorf("%s\n: expected panic(%q), but got panic(%T%v)", testName, expected, msg, msg)
		}
	}()
	f()
}

func TestInvalidFlags(t *testing.T) {
	tests := []struct {
		flag     string
		errorMsg string
	}{
		{
			flag:     "-foo",
			errorMsg: "flag \"-foo\" begins with -",
		},
		{
			flag:     "foo=bar",
			errorMsg: "flag \"foo=bar\" contains =",
		},
	}

	for _, test := range tests {
		testName := fmt.Sprintf("FlagSet.Var(&v, %q, \"\")", test.flag)

		fs := NewFlagSet("", ContinueOnError)
		buf := &strings.Builder{}
		fs.SetOutput(buf)

		mustPanic(t, testName, test.errorMsg, func() {
			var v flagVar
			fs.Var(&v, test.flag, "")
		})
		if msg := test.errorMsg + "\n"; msg != buf.String() {
			t.Errorf("%s\n: unexpected output: expected %q, bug got %q", testName, msg, buf)
		}
	}
}

func TestRedefinedFlags(t *testing.T) {
	tests := []struct {
		flagSetName string
		errorMsg    string
	}{
		{
			flagSetName: "",
			errorMsg:    "flag redefined: foo",
		},
		{
			flagSetName: "fs",
			errorMsg:    "fs flag redefined: foo",
		},
	}

	for _, test := range tests {
		testName := fmt.Sprintf("flag redefined in FlagSet(%q)", test.flagSetName)

		fs := NewFlagSet(test.flagSetName, ContinueOnError)
		buf := &strings.Builder{}
		fs.SetOutput(buf)

		var v flagVar
		fs.Var(&v, "foo", "")

		mustPanic(t, testName, test.errorMsg, func() {
			fs.Var(&v, "foo", "")
		})
		if msg := test.errorMsg + "\n"; msg != buf.String() {
			t.Errorf("%s\n: unexpected output: expected %q, bug got %q", testName, msg, buf)
		}
	}
}

func TestUserDefinedBoolFunc(t *testing.T) {
	flags := NewFlagSet("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	var ss []string
	flags.BoolFunc("v", "usage", func(s string) error {
		ss = append(ss, s)
		return nil
	})
	if err := flags.Parse([]string{"-v", "", "-v", "1", "-v=2"}); err != nil {
		t.Error(err)
	}
	if len(ss) != 1 {
		t.Fatalf("got %d args; want 1 arg", len(ss))
	}
	want := "[true]"
	if got := fmt.Sprint(ss); got != want {
		t.Errorf("got %q; want %q", got, want)
	}
	// test usage
	var buf strings.Builder
	flags.SetOutput(&buf)
	flags.Parse([]string{"-h"})
	if usage := buf.String(); !strings.Contains(usage, "usage") {
		t.Errorf("usage string not included: %q", usage)
	}
	// test BoolFunc error
	flags = NewFlagSet("test", ContinueOnError)
	flags.SetOutput(io.Discard)
	flags.BoolFunc("v", "usage", func(s string) error {
		return fmt.Errorf("test error")
	})
	// flag not set, so no error
	if err := flags.Parse(nil); err != nil {
		t.Error(err)
	}
	// flag set, expect error
	if err := flags.Parse([]string{"-v", ""}); err == nil {
		t.Error("got err == nil; want err != nil")
	} else if errMsg := err.Error(); !strings.Contains(errMsg, "test error") {
		t.Errorf(`got %q; error should contain "test error"`, errMsg)
	}
}

func TestDefineAfterSet(t *testing.T) {
	flags := NewFlagSet("test", ContinueOnError)
	// Set by itself doesn't panic.
	flags.Set("myFlag", "value")

	// Define-after-set panics.
	mustPanic(t, "DefineAfterSet", "flag myFlag set at .*/flag_test.go:.* before being defined", func() {
		_ = flags.String("myFlag", "default", "usage")
	})
}