Adding upstream version 1.22.1.upstream/1.22.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

7 files changed, 2334 insertions, 0 deletions

diff --git a/src/flag/example_func_test.go b/src/flag/example_func_test.go
new file mode 100644
index 0000000..ac9f985
--- /dev/null
+++ b/src/flag/example_func_test.go
@@ -0,0 +1,57 @@
+// Copyright 2020 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 (
+	"errors"
+	"flag"
+	"fmt"
+	"net"
+	"os"
+)
+
+func ExampleFunc() {
+	fs := flag.NewFlagSet("ExampleFunc", flag.ContinueOnError)
+	fs.SetOutput(os.Stdout)
+	var ip net.IP
+	fs.Func("ip", "`IP address` to parse", func(s string) error {
+		ip = net.ParseIP(s)
+		if ip == nil {
+			return errors.New("could not parse IP")
+		}
+		return nil
+	})
+	fs.Parse([]string{"-ip", "127.0.0.1"})
+	fmt.Printf("{ip: %v, loopback: %t}\n\n", ip, ip.IsLoopback())
+
+	// 256 is not a valid IPv4 component
+	fs.Parse([]string{"-ip", "256.0.0.1"})
+	fmt.Printf("{ip: %v, loopback: %t}\n\n", ip, ip.IsLoopback())
+
+	// Output:
+	// {ip: 127.0.0.1, loopback: true}
+	//
+	// invalid value "256.0.0.1" for flag -ip: could not parse IP
+	// Usage of ExampleFunc:
+	//   -ip IP address
+	//     	IP address to parse
+	// {ip: <nil>, loopback: false}
+}
+
+func ExampleBoolFunc() {
+	fs := flag.NewFlagSet("ExampleBoolFunc", flag.ContinueOnError)
+	fs.SetOutput(os.Stdout)
+
+	fs.BoolFunc("log", "logs a dummy message", func(s string) error {
+		fmt.Println("dummy message:", s)
+		return nil
+	})
+	fs.Parse([]string{"-log"})
+	fs.Parse([]string{"-log=0"})
+
+	// Output:
+	// dummy message: true
+	// dummy message: 0
+}
diff --git a/src/flag/example_test.go b/src/flag/example_test.go
new file mode 100644
index 0000000..088447d
--- /dev/null
+++ b/src/flag/example_test.go
@@ -0,0 +1,85 @@
+// Copyright 2012 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.
+
+// These examples demonstrate more intricate uses of the flag package.
+package flag_test
+
+import (
+	"errors"
+	"flag"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// Example 1: A single string flag called "species" with default value "gopher".
+var species = flag.String("species", "gopher", "the species we are studying")
+
+// Example 2: Two flags sharing a variable, so we can have a shorthand.
+// The order of initialization is undefined, so make sure both use the
+// same default value. They must be set up with an init function.
+var gopherType string
+
+func init() {
+	const (
+		defaultGopher = "pocket"
+		usage         = "the variety of gopher"
+	)
+	flag.StringVar(&gopherType, "gopher_type", defaultGopher, usage)
+	flag.StringVar(&gopherType, "g", defaultGopher, usage+" (shorthand)")
+}
+
+// Example 3: A user-defined flag type, a slice of durations.
+type interval []time.Duration
+
+// String is the method to format the flag's value, part of the flag.Value interface.
+// The String method's output will be used in diagnostics.
+func (i *interval) String() string {
+	return fmt.Sprint(*i)
+}
+
+// Set is the method to set the flag value, part of the flag.Value interface.
+// Set's argument is a string to be parsed to set the flag.
+// It's a comma-separated list, so we split it.
+func (i *interval) Set(value string) error {
+	// If we wanted to allow the flag to be set multiple times,
+	// accumulating values, we would delete this if statement.
+	// That would permit usages such as
+	//	-deltaT 10s -deltaT 15s
+	// and other combinations.
+	if len(*i) > 0 {
+		return errors.New("interval flag already set")
+	}
+	for _, dt := range strings.Split(value, ",") {
+		duration, err := time.ParseDuration(dt)
+		if err != nil {
+			return err
+		}
+		*i = append(*i, duration)
+	}
+	return nil
+}
+
+// Define a flag to accumulate durations. Because it has a special type,
+// we need to use the Var function and therefore create the flag during
+// init.
+
+var intervalFlag interval
+
+func init() {
+	// Tie the command-line flag to the intervalFlag variable and
+	// set a usage message.
+	flag.Var(&intervalFlag, "deltaT", "comma-separated list of intervals to use between events")
+}
+
+func Example() {
+	// All the interesting pieces are with the variables declared above, but
+	// to enable the flag package to see the flags defined there, one must
+	// execute, typically at the start of main (not init!):
+	//	flag.Parse()
+	// We don't call it here because this code is a function called "Example"
+	// that is part of the testing suite for the package, which has already
+	// parsed the flags. When viewed at pkg.go.dev, however, the function is
+	// renamed to "main" and it could be run as a standalone example.
+}
diff --git a/src/flag/example_textvar_test.go b/src/flag/example_textvar_test.go
new file mode 100644
index 0000000..8b8cbf6
--- /dev/null
+++ b/src/flag/example_textvar_test.go
@@ -0,0 +1,35 @@
+// Copyright 2022 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 (
+	"flag"
+	"fmt"
+	"net"
+	"os"
+)
+
+func ExampleTextVar() {
+	fs := flag.NewFlagSet("ExampleTextVar", flag.ContinueOnError)
+	fs.SetOutput(os.Stdout)
+	var ip net.IP
+	fs.TextVar(&ip, "ip", net.IPv4(192, 168, 0, 100), "`IP address` to parse")
+	fs.Parse([]string{"-ip", "127.0.0.1"})
+	fmt.Printf("{ip: %v}\n\n", ip)
+
+	// 256 is not a valid IPv4 component
+	ip = nil
+	fs.Parse([]string{"-ip", "256.0.0.1"})
+	fmt.Printf("{ip: %v}\n\n", ip)
+
+	// Output:
+	// {ip: 127.0.0.1}
+	//
+	// invalid value "256.0.0.1" for flag -ip: invalid IP address: 256.0.0.1
+	// Usage of ExampleTextVar:
+	//   -ip IP address
+	//     	IP address to parse (default 192.168.0.100)
+	// {ip: <nil>}
+}
diff --git a/src/flag/example_value_test.go b/src/flag/example_value_test.go
new file mode 100644
index 0000000..9d464c6
--- /dev/null
+++ b/src/flag/example_value_test.go
@@ -0,0 +1,44 @@
+// Copyright 2018 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 (
+	"flag"
+	"fmt"
+	"net/url"
+)
+
+type URLValue struct {
+	URL *url.URL
+}
+
+func (v URLValue) String() string {
+	if v.URL != nil {
+		return v.URL.String()
+	}
+	return ""
+}
+
+func (v URLValue) Set(s string) error {
+	if u, err := url.Parse(s); err != nil {
+		return err
+	} else {
+		*v.URL = *u
+	}
+	return nil
+}
+
+var u = &url.URL{}
+
+func ExampleValue() {
+	fs := flag.NewFlagSet("ExampleValue", flag.ExitOnError)
+	fs.Var(&URLValue{u}, "url", "URL to parse")
+
+	fs.Parse([]string{"-url", "https://golang.org/pkg/flag/"})
+	fmt.Printf(`{scheme: %q, host: %q, path: %q}`, u.Scheme, u.Host, u.Path)
+
+	// Output:
+	// {scheme: "https", host: "golang.org", path: "/pkg/flag/"}
+}
diff --git a/src/flag/export_test.go b/src/flag/export_test.go
new file mode 100644
index 0000000..9ef93ed
--- /dev/null
+++ b/src/flag/export_test.go
@@ -0,0 +1,24 @@
+// Copyright 2010 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
+
+import (
+	"io"
+	"os"
+)
+
+// Additional routines compiled into the package only during testing.
+
+var DefaultUsage = Usage
+
+// ResetForTesting clears all flag state and sets the usage function as directed.
+// After calling ResetForTesting, parse errors in flag handling will not
+// exit the program.
+func ResetForTesting(usage func()) {
+	CommandLine = NewFlagSet(os.Args[0], ContinueOnError)
+	CommandLine.SetOutput(io.Discard)
+	CommandLine.Usage = commandLineUsage
+	Usage = usage
+}
diff --git a/src/flag/flag.go b/src/flag/flag.go
new file mode 100644
index 0000000..1669e9a
--- /dev/null
+++ b/src/flag/flag.go
@@ -0,0 +1,1231 @@
+// 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 implements command-line flag parsing.
+
+# Usage
+
+Define flags using [flag.String], [Bool], [Int], etc.
+
+This declares an integer flag, -n, stored in the pointer nFlag, with type *int:
+
+	import "flag"
+	var nFlag = flag.Int("n", 1234, "help message for flag n")
+
+If you like, you can bind the flag to a variable using the Var() functions.
+
+	var flagvar int
+	func init() {
+		flag.IntVar(&flagvar, "flagname", 1234, "help message for flagname")
+	}
+
+Or you can create custom flags that satisfy the Value interface (with
+pointer receivers) and couple them to flag parsing by
+
+	flag.Var(&flagVal, "name", "help message for flagname")
+
+For such flags, the default value is just the initial value of the variable.
+
+After all flags are defined, call
+
+	flag.Parse()
+
+to parse the command line into the defined flags.
+
+Flags may then be used directly. If you're using the flags themselves,
+they are all pointers; if you bind to variables, they're values.
+
+	fmt.Println("ip has value ", *ip)
+	fmt.Println("flagvar has value ", flagvar)
+
+After parsing, the arguments following the flags are available as the
+slice [flag.Args] or individually as [flag.Arg](i).
+The arguments are indexed from 0 through [flag.NArg]-1.
+
+# Command line flag syntax
+
+The following forms are permitted:
+
+	-flag
+	--flag   // double dashes are also permitted
+	-flag=x
+	-flag x  // non-boolean flags only
+
+One or two dashes may be used; they are equivalent.
+The last form is not permitted for boolean flags because the
+meaning of the command
+
+	cmd -x *
+
+where * is a Unix shell wildcard, will change if there is a file
+called 0, false, etc. You must use the -flag=false form to turn
+off a boolean flag.
+
+Flag parsing stops just before the first non-flag argument
+("-" is a non-flag argument) or after the terminator "--".
+
+Integer flags accept 1234, 0664, 0x1234 and may be negative.
+Boolean flags may be:
+
+	1, 0, t, f, T, F, true, false, TRUE, FALSE, True, False
+
+Duration flags accept any input valid for time.ParseDuration.
+
+The default set of command-line flags is controlled by
+top-level functions.  The [FlagSet] type allows one to define
+independent sets of flags, such as to implement subcommands
+in a command-line interface. The methods of [FlagSet] are
+analogous to the top-level functions for the command-line
+flag set.
+*/
+package flag
+
+import (
+	"encoding"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"reflect"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// ErrHelp is the error returned if the -help or -h flag is invoked
+// but no such flag is defined.
+var ErrHelp = errors.New("flag: help requested")
+
+// errParse is returned by Set if a flag's value fails to parse, such as with an invalid integer for Int.
+// It then gets wrapped through failf to provide more information.
+var errParse = errors.New("parse error")
+
+// errRange is returned by Set if a flag's value is out of range.
+// It then gets wrapped through failf to provide more information.
+var errRange = errors.New("value out of range")
+
+func numError(err error) error {
+	ne, ok := err.(*strconv.NumError)
+	if !ok {
+		return err
+	}
+	if ne.Err == strconv.ErrSyntax {
+		return errParse
+	}
+	if ne.Err == strconv.ErrRange {
+		return errRange
+	}
+	return err
+}
+
+// -- bool Value
+type boolValue bool
+
+func newBoolValue(val bool, p *bool) *boolValue {
+	*p = val
+	return (*boolValue)(p)
+}
+
+func (b *boolValue) Set(s string) error {
+	v, err := strconv.ParseBool(s)
+	if err != nil {
+		err = errParse
+	}
+	*b = boolValue(v)
+	return err
+}
+
+func (b *boolValue) Get() any { return bool(*b) }
+
+func (b *boolValue) String() string { return strconv.FormatBool(bool(*b)) }
+
+func (b *boolValue) IsBoolFlag() bool { return true }
+
+// optional interface to indicate boolean flags that can be
+// supplied without "=value" text
+type boolFlag interface {
+	Value
+	IsBoolFlag() bool
+}
+
+// -- int Value
+type intValue int
+
+func newIntValue(val int, p *int) *intValue {
+	*p = val
+	return (*intValue)(p)
+}
+
+func (i *intValue) Set(s string) error {
+	v, err := strconv.ParseInt(s, 0, strconv.IntSize)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = intValue(v)
+	return err
+}
+
+func (i *intValue) Get() any { return int(*i) }
+
+func (i *intValue) String() string { return strconv.Itoa(int(*i)) }
+
+// -- int64 Value
+type int64Value int64
+
+func newInt64Value(val int64, p *int64) *int64Value {
+	*p = val
+	return (*int64Value)(p)
+}
+
+func (i *int64Value) Set(s string) error {
+	v, err := strconv.ParseInt(s, 0, 64)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = int64Value(v)
+	return err
+}
+
+func (i *int64Value) Get() any { return int64(*i) }
+
+func (i *int64Value) String() string { return strconv.FormatInt(int64(*i), 10) }
+
+// -- uint Value
+type uintValue uint
+
+func newUintValue(val uint, p *uint) *uintValue {
+	*p = val
+	return (*uintValue)(p)
+}
+
+func (i *uintValue) Set(s string) error {
+	v, err := strconv.ParseUint(s, 0, strconv.IntSize)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = uintValue(v)
+	return err
+}
+
+func (i *uintValue) Get() any { return uint(*i) }
+
+func (i *uintValue) String() string { return strconv.FormatUint(uint64(*i), 10) }
+
+// -- uint64 Value
+type uint64Value uint64
+
+func newUint64Value(val uint64, p *uint64) *uint64Value {
+	*p = val
+	return (*uint64Value)(p)
+}
+
+func (i *uint64Value) Set(s string) error {
+	v, err := strconv.ParseUint(s, 0, 64)
+	if err != nil {
+		err = numError(err)
+	}
+	*i = uint64Value(v)
+	return err
+}
+
+func (i *uint64Value) Get() any { return uint64(*i) }
+
+func (i *uint64Value) String() string { return strconv.FormatUint(uint64(*i), 10) }
+
+// -- string Value
+type stringValue string
+
+func newStringValue(val string, p *string) *stringValue {
+	*p = val
+	return (*stringValue)(p)
+}
+
+func (s *stringValue) Set(val string) error {
+	*s = stringValue(val)
+	return nil
+}
+
+func (s *stringValue) Get() any { return string(*s) }
+
+func (s *stringValue) String() string { return string(*s) }
+
+// -- float64 Value
+type float64Value float64
+
+func newFloat64Value(val float64, p *float64) *float64Value {
+	*p = val
+	return (*float64Value)(p)
+}
+
+func (f *float64Value) Set(s string) error {
+	v, err := strconv.ParseFloat(s, 64)
+	if err != nil {
+		err = numError(err)
+	}
+	*f = float64Value(v)
+	return err
+}
+
+func (f *float64Value) Get() any { return float64(*f) }
+
+func (f *float64Value) String() string { return strconv.FormatFloat(float64(*f), 'g', -1, 64) }
+
+// -- time.Duration Value
+type durationValue time.Duration
+
+func newDurationValue(val time.Duration, p *time.Duration) *durationValue {
+	*p = val
+	return (*durationValue)(p)
+}
+
+func (d *durationValue) Set(s string) error {
+	v, err := time.ParseDuration(s)
+	if err != nil {
+		err = errParse
+	}
+	*d = durationValue(v)
+	return err
+}
+
+func (d *durationValue) Get() any { return time.Duration(*d) }
+
+func (d *durationValue) String() string { return (*time.Duration)(d).String() }
+
+// -- encoding.TextUnmarshaler Value
+type textValue struct{ p encoding.TextUnmarshaler }
+
+func newTextValue(val encoding.TextMarshaler, p encoding.TextUnmarshaler) textValue {
+	ptrVal := reflect.ValueOf(p)
+	if ptrVal.Kind() != reflect.Ptr {
+		panic("variable value type must be a pointer")
+	}
+	defVal := reflect.ValueOf(val)
+	if defVal.Kind() == reflect.Ptr {
+		defVal = defVal.Elem()
+	}
+	if defVal.Type() != ptrVal.Type().Elem() {
+		panic(fmt.Sprintf("default type does not match variable type: %v != %v", defVal.Type(), ptrVal.Type().Elem()))
+	}
+	ptrVal.Elem().Set(defVal)
+	return textValue{p}
+}
+
+func (v textValue) Set(s string) error {
+	return v.p.UnmarshalText([]byte(s))
+}
+
+func (v textValue) Get() interface{} {
+	return v.p
+}
+
+func (v textValue) String() string {
+	if m, ok := v.p.(encoding.TextMarshaler); ok {
+		if b, err := m.MarshalText(); err == nil {
+			return string(b)
+		}
+	}
+	return ""
+}
+
+// -- func Value
+type funcValue func(string) error
+
+func (f funcValue) Set(s string) error { return f(s) }
+
+func (f funcValue) String() string { return "" }
+
+// -- boolFunc Value
+type boolFuncValue func(string) error
+
+func (f boolFuncValue) Set(s string) error { return f(s) }
+
+func (f boolFuncValue) String() string { return "" }
+
+func (f boolFuncValue) IsBoolFlag() bool { return true }
+
+// Value is the interface to the dynamic value stored in a flag.
+// (The default value is represented as a string.)
+//
+// If a Value has an IsBoolFlag() bool method returning true,
+// the command-line parser makes -name equivalent to -name=true
+// rather than using the next command-line argument.
+//
+// Set is called once, in command line order, for each flag present.
+// The flag package may call the [String] method with a zero-valued receiver,
+// such as a nil pointer.
+type Value interface {
+	String() string
+	Set(string) error
+}
+
+// Getter is an interface that allows the contents of a [Value] to be retrieved.
+// It wraps the [Value] interface, rather than being part of it, because it
+// appeared after Go 1 and its compatibility rules. All [Value] types provided
+// by this package satisfy the [Getter] interface, except the type used by [Func].
+type Getter interface {
+	Value
+	Get() any
+}
+
+// ErrorHandling defines how [FlagSet.Parse] behaves if the parse fails.
+type ErrorHandling int
+
+// These constants cause [FlagSet.Parse] to behave as described if the parse fails.
+const (
+	ContinueOnError ErrorHandling = iota // Return a descriptive error.
+	ExitOnError                          // Call os.Exit(2) or for -h/-help Exit(0).
+	PanicOnError                         // Call panic with a descriptive error.
+)
+
+// A FlagSet represents a set of defined flags. The zero value of a FlagSet
+// has no name and has [ContinueOnError] error handling.
+//
+// [Flag] names must be unique within a FlagSet. An attempt to define a flag whose
+// name is already in use will cause a panic.
+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. What happens after Usage is called depends
+	// on the ErrorHandling setting; for the command line, this defaults
+	// to ExitOnError, which exits the program after calling Usage.
+	Usage func()
+
+	name          string
+	parsed        bool
+	actual        map[string]*Flag
+	formal        map[string]*Flag
+	args          []string // arguments after flags
+	errorHandling ErrorHandling
+	output        io.Writer         // nil means stderr; use Output() accessor
+	undef         map[string]string // flags which didn't exist at the time of Set
+}
+
+// A Flag represents the state of a flag.
+type Flag struct {
+	Name     string // name as it appears on command line
+	Usage    string // help message
+	Value    Value  // value as set
+	DefValue string // default value (as text); for usage message
+}
+
+// sortFlags returns the flags as a slice in lexicographical sorted order.
+func sortFlags(flags map[string]*Flag) []*Flag {
+	result := make([]*Flag, len(flags))
+	i := 0
+	for _, f := range flags {
+		result[i] = f
+		i++
+	}
+	sort.Slice(result, func(i, j int) bool {
+		return result[i].Name < result[j].Name
+	})
+	return result
+}
+
+// Output returns the destination for usage and error messages. [os.Stderr] is returned if
+// output was not set or was set to nil.
+func (f *FlagSet) Output() io.Writer {
+	if f.output == nil {
+		return os.Stderr
+	}
+	return f.output
+}
+
+// Name returns the name of the flag set.
+func (f *FlagSet) Name() string {
+	return f.name
+}
+
+// ErrorHandling returns the error handling behavior of the flag set.
+func (f *FlagSet) ErrorHandling() ErrorHandling {
+	return f.errorHandling
+}
+
+// SetOutput sets the destination for usage and error messages.
+// If output is nil, [os.Stderr] is used.
+func (f *FlagSet) SetOutput(output io.Writer) {
+	f.output = output
+}
+
+// VisitAll visits the flags in lexicographical order, calling fn for each.
+// It visits all flags, even those not set.
+func (f *FlagSet) VisitAll(fn func(*Flag)) {
+	for _, flag := range sortFlags(f.formal) {
+		fn(flag)
+	}
+}
+
+// VisitAll visits the command-line flags in lexicographical order, calling
+// fn for each. It visits all flags, even those not set.
+func VisitAll(fn func(*Flag)) {
+	CommandLine.VisitAll(fn)
+}
+
+// Visit visits the flags in lexicographical order, calling fn for each.
+// It visits only those flags that have been set.
+func (f *FlagSet) Visit(fn func(*Flag)) {
+	for _, flag := range sortFlags(f.actual) {
+		fn(flag)
+	}
+}
+
+// Visit visits the command-line flags in lexicographical order, calling fn
+// for each. It visits only those flags that have been set.
+func Visit(fn func(*Flag)) {
+	CommandLine.Visit(fn)
+}
+
+// Lookup returns the [Flag] structure of the named flag, returning nil if none exists.
+func (f *FlagSet) Lookup(name string) *Flag {
+	return f.formal[name]
+}
+
+// Lookup returns the [Flag] structure of the named command-line flag,
+// returning nil if none exists.
+func Lookup(name string) *Flag {
+	return CommandLine.formal[name]
+}
+
+// Set sets the value of the named flag.
+func (f *FlagSet) Set(name, value string) error {
+	return f.set(name, value)
+}
+func (f *FlagSet) set(name, value string) error {
+	flag, ok := f.formal[name]
+	if !ok {
+		// Remember that a flag that isn't defined is being set.
+		// We return an error in this case, but in addition if
+		// subsequently that flag is defined, we want to panic
+		// at the definition point.
+		// This is a problem which occurs if both the definition
+		// and the Set call are in init code and for whatever
+		// reason the init code changes evaluation order.
+		// See issue 57411.
+		_, file, line, ok := runtime.Caller(2)
+		if !ok {
+			file = "?"
+			line = 0
+		}
+		if f.undef == nil {
+			f.undef = map[string]string{}
+		}
+		f.undef[name] = fmt.Sprintf("%s:%d", file, line)
+
+		return fmt.Errorf("no such flag -%v", name)
+	}
+	err := flag.Value.Set(value)
+	if err != nil {
+		return err
+	}
+	if f.actual == nil {
+		f.actual = make(map[string]*Flag)
+	}
+	f.actual[name] = flag
+	return nil
+}
+
+// Set sets the value of the named command-line flag.
+func Set(name, value string) error {
+	return CommandLine.set(name, value)
+}
+
+// isZeroValue determines whether the string represents the zero
+// value for a flag.
+func isZeroValue(flag *Flag, value string) (ok bool, err error) {
+	// Build a zero value of the flag's Value type, and see if the
+	// result of calling its String method equals the value passed in.
+	// This works unless the Value type is itself an interface type.
+	typ := reflect.TypeOf(flag.Value)
+	var z reflect.Value
+	if typ.Kind() == reflect.Pointer {
+		z = reflect.New(typ.Elem())
+	} else {
+		z = reflect.Zero(typ)
+	}
+	// Catch panics calling the String method, which shouldn't prevent the
+	// usage message from being printed, but that we should report to the
+	// user so that they know to fix their code.
+	defer func() {
+		if e := recover(); e != nil {
+			if typ.Kind() == reflect.Pointer {
+				typ = typ.Elem()
+			}
+			err = fmt.Errorf("panic calling String method on zero %v for flag %s: %v", typ, flag.Name, e)
+		}
+	}()
+	return value == z.Interface().(Value).String(), nil
+}
+
+// UnquoteUsage extracts a back-quoted name from the usage
+// string for a flag and returns it and the un-quoted usage.
+// Given "a `name` to show" it returns ("name", "a name to show").
+// If there are no back quotes, the name is an educated guess of the
+// type of the flag's value, or the empty string if the flag is boolean.
+func UnquoteUsage(flag *Flag) (name string, usage string) {
+	// Look for a back-quoted name, but avoid the strings package.
+	usage = flag.Usage
+	for i := 0; i < len(usage); i++ {
+		if usage[i] == '`' {
+			for j := i + 1; j < len(usage); j++ {
+				if usage[j] == '`' {
+					name = usage[i+1 : j]
+					usage = usage[:i] + name + usage[j+1:]
+					return name, usage
+				}
+			}
+			break // Only one back quote; use type name.
+		}
+	}
+	// No explicit name, so use type if we can find one.
+	name = "value"
+	switch fv := flag.Value.(type) {
+	case boolFlag:
+		if fv.IsBoolFlag() {
+			name = ""
+		}
+	case *durationValue:
+		name = "duration"
+	case *float64Value:
+		name = "float"
+	case *intValue, *int64Value:
+		name = "int"
+	case *stringValue:
+		name = "string"
+	case *uintValue, *uint64Value:
+		name = "uint"
+	}
+	return
+}
+
+// PrintDefaults prints, to standard error unless configured otherwise, the
+// default values of all defined command-line flags in the set. See the
+// documentation for the global function PrintDefaults for more information.
+func (f *FlagSet) PrintDefaults() {
+	var isZeroValueErrs []error
+	f.VisitAll(func(flag *Flag) {
+		var b strings.Builder
+		fmt.Fprintf(&b, "  -%s", flag.Name) // Two spaces before -; see next two comments.
+		name, usage := UnquoteUsage(flag)
+		if len(name) > 0 {
+			b.WriteString(" ")
+			b.WriteString(name)
+		}
+		// Boolean flags of one ASCII letter are so common we
+		// treat them specially, putting their usage on the same line.
+		if b.Len() <= 4 { // space, space, '-', 'x'.
+			b.WriteString("\t")
+		} else {
+			// Four spaces before the tab triggers good alignment
+			// for both 4- and 8-space tab stops.
+			b.WriteString("\n    \t")
+		}
+		b.WriteString(strings.ReplaceAll(usage, "\n", "\n    \t"))
+
+		// Print the default value only if it differs to the zero value
+		// for this flag type.
+		if isZero, err := isZeroValue(flag, flag.DefValue); err != nil {
+			isZeroValueErrs = append(isZeroValueErrs, err)
+		} else if !isZero {
+			if _, ok := flag.Value.(*stringValue); ok {
+				// put quotes on the value
+				fmt.Fprintf(&b, " (default %q)", flag.DefValue)
+			} else {
+				fmt.Fprintf(&b, " (default %v)", flag.DefValue)
+			}
+		}
+		fmt.Fprint(f.Output(), b.String(), "\n")
+	})
+	// If calling String on any zero flag.Values triggered a panic, print
+	// the messages after the full set of defaults so that the programmer
+	// knows to fix the panic.
+	if errs := isZeroValueErrs; len(errs) > 0 {
+		fmt.Fprintln(f.Output())
+		for _, err := range errs {
+			fmt.Fprintln(f.Output(), err)
+		}
+	}
+}
+
+// PrintDefaults prints, to standard error unless configured otherwise,
+// a usage message showing the default settings of all defined
+// command-line flags.
+// For an integer valued flag x, the default output has the form
+//
+//	-x int
+//		usage-message-for-x (default 7)
+//
+// The usage message will appear on a separate line for anything but
+// a bool flag with a one-byte name. For bool flags, the type is
+// omitted and if the flag name is one byte the usage message appears
+// on the same line. The parenthetical default is omitted if the
+// default is the zero value for the type. The listed type, here int,
+// can be changed by placing a back-quoted name in the flag's usage
+// string; the first such item in the message is taken to be a parameter
+// name to show in the message and the back quotes are stripped from
+// the message when displayed. For instance, given
+//
+//	flag.String("I", "", "search `directory` for include files")
+//
+// the output will be
+//
+//	-I directory
+//		search directory for include files.
+//
+// To change the destination for flag messages, call [CommandLine].SetOutput.
+func PrintDefaults() {
+	CommandLine.PrintDefaults()
+}
+
+// defaultUsage is the default function to print a usage message.
+func (f *FlagSet) defaultUsage() {
+	if f.name == "" {
+		fmt.Fprintf(f.Output(), "Usage:\n")
+	} else {
+		fmt.Fprintf(f.Output(), "Usage of %s:\n", f.name)
+	}
+	f.PrintDefaults()
+}
+
+// NOTE: Usage is not just defaultUsage(CommandLine)
+// because it serves (via godoc flag Usage) as the example
+// for how to write your own usage function.
+
+// Usage prints a usage message documenting all defined command-line flags
+// to [CommandLine]'s output, which by default is [os.Stderr].
+// It is called when an error occurs while parsing flags.
+// The function is a variable that may be changed to point to a custom function.
+// By default it prints a simple header and calls [PrintDefaults]; for details about the
+// format of the output and how to control it, see the documentation for [PrintDefaults].
+// Custom usage functions may choose to exit the program; by default exiting
+// happens anyway as the command line's error handling strategy is set to
+// [ExitOnError].
+var Usage = func() {
+	fmt.Fprintf(CommandLine.Output(), "Usage of %s:\n", os.Args[0])
+	PrintDefaults()
+}
+
+// NFlag returns the number of flags that have been set.
+func (f *FlagSet) NFlag() int { return len(f.actual) }
+
+// NFlag returns the number of command-line flags that have been set.
+func NFlag() int { return len(CommandLine.actual) }
+
+// Arg returns the i'th argument. Arg(0) is the first remaining argument
+// after flags have been processed. Arg returns an empty string if the
+// requested element does not exist.
+func (f *FlagSet) Arg(i int) string {
+	if i < 0 || i >= len(f.args) {
+		return ""
+	}
+	return f.args[i]
+}
+
+// Arg returns the i'th command-line argument. Arg(0) is the first remaining argument
+// after flags have been processed. Arg returns an empty string if the
+// requested element does not exist.
+func Arg(i int) string {
+	return CommandLine.Arg(i)
+}
+
+// NArg is the number of arguments remaining after flags have been processed.
+func (f *FlagSet) NArg() int { return len(f.args) }
+
+// NArg is the number of arguments remaining after flags have been processed.
+func NArg() int { return len(CommandLine.args) }
+
+// Args returns the non-flag arguments.
+func (f *FlagSet) Args() []string { return f.args }
+
+// Args returns the non-flag command-line arguments.
+func Args() []string { return CommandLine.args }
+
+// BoolVar defines a bool flag with specified name, default value, and usage string.
+// The argument p points to a bool variable in which to store the value of the flag.
+func (f *FlagSet) BoolVar(p *bool, name string, value bool, usage string) {
+	f.Var(newBoolValue(value, p), name, usage)
+}
+
+// BoolVar defines a bool flag with specified name, default value, and usage string.
+// The argument p points to a bool variable in which to store the value of the flag.
+func BoolVar(p *bool, name string, value bool, usage string) {
+	CommandLine.Var(newBoolValue(value, p), name, usage)
+}
+
+// Bool defines a bool flag with specified name, default value, and usage string.
+// The return value is the address of a bool variable that stores the value of the flag.
+func (f *FlagSet) Bool(name string, value bool, usage string) *bool {
+	p := new(bool)
+	f.BoolVar(p, name, value, usage)
+	return p
+}
+
+// Bool defines a bool flag with specified name, default value, and usage string.
+// The return value is the address of a bool variable that stores the value of the flag.
+func Bool(name string, value bool, usage string) *bool {
+	return CommandLine.Bool(name, value, usage)
+}
+
+// IntVar defines an int flag with specified name, default value, and usage string.
+// The argument p points to an int variable in which to store the value of the flag.
+func (f *FlagSet) IntVar(p *int, name string, value int, usage string) {
+	f.Var(newIntValue(value, p), name, usage)
+}
+
+// IntVar defines an int flag with specified name, default value, and usage string.
+// The argument p points to an int variable in which to store the value of the flag.
+func IntVar(p *int, name string, value int, usage string) {
+	CommandLine.Var(newIntValue(value, p), name, usage)
+}
+
+// Int defines an int flag with specified name, default value, and usage string.
+// The return value is the address of an int variable that stores the value of the flag.
+func (f *FlagSet) Int(name string, value int, usage string) *int {
+	p := new(int)
+	f.IntVar(p, name, value, usage)
+	return p
+}
+
+// Int defines an int flag with specified name, default value, and usage string.
+// The return value is the address of an int variable that stores the value of the flag.
+func Int(name string, value int, usage string) *int {
+	return CommandLine.Int(name, value, usage)
+}
+
+// Int64Var defines an int64 flag with specified name, default value, and usage string.
+// The argument p points to an int64 variable in which to store the value of the flag.
+func (f *FlagSet) Int64Var(p *int64, name string, value int64, usage string) {
+	f.Var(newInt64Value(value, p), name, usage)
+}
+
+// Int64Var defines an int64 flag with specified name, default value, and usage string.
+// The argument p points to an int64 variable in which to store the value of the flag.
+func Int64Var(p *int64, name string, value int64, usage string) {
+	CommandLine.Var(newInt64Value(value, p), name, usage)
+}
+
+// Int64 defines an int64 flag with specified name, default value, and usage string.
+// The return value is the address of an int64 variable that stores the value of the flag.
+func (f *FlagSet) Int64(name string, value int64, usage string) *int64 {
+	p := new(int64)
+	f.Int64Var(p, name, value, usage)
+	return p
+}
+
+// Int64 defines an int64 flag with specified name, default value, and usage string.
+// The return value is the address of an int64 variable that stores the value of the flag.
+func Int64(name string, value int64, usage string) *int64 {
+	return CommandLine.Int64(name, value, usage)
+}
+
+// UintVar defines a uint flag with specified name, default value, and usage string.
+// The argument p points to a uint variable in which to store the value of the flag.
+func (f *FlagSet) UintVar(p *uint, name string, value uint, usage string) {
+	f.Var(newUintValue(value, p), name, usage)
+}
+
+// UintVar defines a uint flag with specified name, default value, and usage string.
+// The argument p points to a uint variable in which to store the value of the flag.
+func UintVar(p *uint, name string, value uint, usage string) {
+	CommandLine.Var(newUintValue(value, p), name, usage)
+}
+
+// Uint defines a uint flag with specified name, default value, and usage string.
+// The return value is the address of a uint variable that stores the value of the flag.
+func (f *FlagSet) Uint(name string, value uint, usage string) *uint {
+	p := new(uint)
+	f.UintVar(p, name, value, usage)
+	return p
+}
+
+// Uint defines a uint flag with specified name, default value, and usage string.
+// The return value is the address of a uint variable that stores the value of the flag.
+func Uint(name string, value uint, usage string) *uint {
+	return CommandLine.Uint(name, value, usage)
+}
+
+// Uint64Var defines a uint64 flag with specified name, default value, and usage string.
+// The argument p points to a uint64 variable in which to store the value of the flag.
+func (f *FlagSet) Uint64Var(p *uint64, name string, value uint64, usage string) {
+	f.Var(newUint64Value(value, p), name, usage)
+}
+
+// Uint64Var defines a uint64 flag with specified name, default value, and usage string.
+// The argument p points to a uint64 variable in which to store the value of the flag.
+func Uint64Var(p *uint64, name string, value uint64, usage string) {
+	CommandLine.Var(newUint64Value(value, p), name, usage)
+}
+
+// Uint64 defines a uint64 flag with specified name, default value, and usage string.
+// The return value is the address of a uint64 variable that stores the value of the flag.
+func (f *FlagSet) Uint64(name string, value uint64, usage string) *uint64 {
+	p := new(uint64)
+	f.Uint64Var(p, name, value, usage)
+	return p
+}
+
+// Uint64 defines a uint64 flag with specified name, default value, and usage string.
+// The return value is the address of a uint64 variable that stores the value of the flag.
+func Uint64(name string, value uint64, usage string) *uint64 {
+	return CommandLine.Uint64(name, value, usage)
+}
+
+// StringVar defines a string flag with specified name, default value, and usage string.
+// The argument p points to a string variable in which to store the value of the flag.
+func (f *FlagSet) StringVar(p *string, name string, value string, usage string) {
+	f.Var(newStringValue(value, p), name, usage)
+}
+
+// StringVar defines a string flag with specified name, default value, and usage string.
+// The argument p points to a string variable in which to store the value of the flag.
+func StringVar(p *string, name string, value string, usage string) {
+	CommandLine.Var(newStringValue(value, p), name, usage)
+}
+
+// String defines a string flag with specified name, default value, and usage string.
+// The return value is the address of a string variable that stores the value of the flag.
+func (f *FlagSet) String(name string, value string, usage string) *string {
+	p := new(string)
+	f.StringVar(p, name, value, usage)
+	return p
+}
+
+// String defines a string flag with specified name, default value, and usage string.
+// The return value is the address of a string variable that stores the value of the flag.
+func String(name string, value string, usage string) *string {
+	return CommandLine.String(name, value, usage)
+}
+
+// Float64Var defines a float64 flag with specified name, default value, and usage string.
+// The argument p points to a float64 variable in which to store the value of the flag.
+func (f *FlagSet) Float64Var(p *float64, name string, value float64, usage string) {
+	f.Var(newFloat64Value(value, p), name, usage)
+}
+
+// Float64Var defines a float64 flag with specified name, default value, and usage string.
+// The argument p points to a float64 variable in which to store the value of the flag.
+func Float64Var(p *float64, name string, value float64, usage string) {
+	CommandLine.Var(newFloat64Value(value, p), name, usage)
+}
+
+// Float64 defines a float64 flag with specified name, default value, and usage string.
+// The return value is the address of a float64 variable that stores the value of the flag.
+func (f *FlagSet) Float64(name string, value float64, usage string) *float64 {
+	p := new(float64)
+	f.Float64Var(p, name, value, usage)
+	return p
+}
+
+// Float64 defines a float64 flag with specified name, default value, and usage string.
+// The return value is the address of a float64 variable that stores the value of the flag.
+func Float64(name string, value float64, usage string) *float64 {
+	return CommandLine.Float64(name, value, usage)
+}
+
+// DurationVar defines a time.Duration flag with specified name, default value, and usage string.
+// The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
+	f.Var(newDurationValue(value, p), name, usage)
+}
+
+// DurationVar defines a time.Duration flag with specified name, default value, and usage string.
+// The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
+	CommandLine.Var(newDurationValue(value, p), name, usage)
+}
+
+// Duration defines a time.Duration flag with specified name, default value, and usage string.
+// The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration {
+	p := new(time.Duration)
+	f.DurationVar(p, name, value, usage)
+	return p
+}
+
+// Duration defines a time.Duration flag with specified name, default value, and usage string.
+// The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
+func Duration(name string, value time.Duration, usage string) *time.Duration {
+	return CommandLine.Duration(name, value, usage)
+}
+
+// TextVar defines a flag with a specified name, default value, and usage string.
+// The argument p must be a pointer to a variable that will hold the value
+// of the flag, and p must implement encoding.TextUnmarshaler.
+// If the flag is used, the flag value will be passed to p's UnmarshalText method.
+// The type of the default value must be the same as the type of p.
+func (f *FlagSet) TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string) {
+	f.Var(newTextValue(value, p), name, usage)
+}
+
+// TextVar defines a flag with a specified name, default value, and usage string.
+// The argument p must be a pointer to a variable that will hold the value
+// of the flag, and p must implement encoding.TextUnmarshaler.
+// If the flag is used, the flag value will be passed to p's UnmarshalText method.
+// The type of the default value must be the same as the type of p.
+func TextVar(p encoding.TextUnmarshaler, name string, value encoding.TextMarshaler, usage string) {
+	CommandLine.Var(newTextValue(value, p), name, usage)
+}
+
+// Func defines a flag with the specified name and usage string.
+// Each time the flag is seen, fn is called with the value of the flag.
+// If fn returns a non-nil error, it will be treated as a flag value parsing error.
+func (f *FlagSet) Func(name, usage string, fn func(string) error) {
+	f.Var(funcValue(fn), name, usage)
+}
+
+// Func defines a flag with the specified name and usage string.
+// Each time the flag is seen, fn is called with the value of the flag.
+// If fn returns a non-nil error, it will be treated as a flag value parsing error.
+func Func(name, usage string, fn func(string) error) {
+	CommandLine.Func(name, usage, fn)
+}
+
+// BoolFunc defines a flag with the specified name and usage string without requiring values.
+// Each time the flag is seen, fn is called with the value of the flag.
+// If fn returns a non-nil error, it will be treated as a flag value parsing error.
+func (f *FlagSet) BoolFunc(name, usage string, fn func(string) error) {
+	f.Var(boolFuncValue(fn), name, usage)
+}
+
+// BoolFunc defines a flag with the specified name and usage string without requiring values.
+// Each time the flag is seen, fn is called with the value of the flag.
+// If fn returns a non-nil error, it will be treated as a flag value parsing error.
+func BoolFunc(name, usage string, fn func(string) error) {
+	CommandLine.BoolFunc(name, usage, fn)
+}
+
+// Var defines a flag with the specified name and usage string. The type and
+// value of the flag are represented by the first argument, of type [Value], which
+// typically holds a user-defined implementation of [Value]. For instance, the
+// caller could create a flag that turns a comma-separated string into a slice
+// of strings by giving the slice the methods of [Value]; in particular, [Set] would
+// decompose the comma-separated string into the slice.
+func (f *FlagSet) Var(value Value, name string, usage string) {
+	// Flag must not begin "-" or contain "=".
+	if strings.HasPrefix(name, "-") {
+		panic(f.sprintf("flag %q begins with -", name))
+	} else if strings.Contains(name, "=") {
+		panic(f.sprintf("flag %q contains =", name))
+	}
+
+	// Remember the default value as a string; it won't change.
+	flag := &Flag{name, usage, value, value.String()}
+	_, alreadythere := f.formal[name]
+	if alreadythere {
+		var msg string
+		if f.name == "" {
+			msg = f.sprintf("flag redefined: %s", name)
+		} else {
+			msg = f.sprintf("%s flag redefined: %s", f.name, name)
+		}
+		panic(msg) // Happens only if flags are declared with identical names
+	}
+	if pos := f.undef[name]; pos != "" {
+		panic(fmt.Sprintf("flag %s set at %s before being defined", name, pos))
+	}
+	if f.formal == nil {
+		f.formal = make(map[string]*Flag)
+	}
+	f.formal[name] = flag
+}
+
+// Var defines a flag with the specified name and usage string. The type and
+// value of the flag are represented by the first argument, of type [Value], which
+// typically holds a user-defined implementation of [Value]. For instance, the
+// caller could create a flag that turns a comma-separated string into a slice
+// of strings by giving the slice the methods of [Value]; in particular, [Set] would
+// decompose the comma-separated string into the slice.
+func Var(value Value, name string, usage string) {
+	CommandLine.Var(value, name, usage)
+}
+
+// sprintf formats the message, prints it to output, and returns it.
+func (f *FlagSet) sprintf(format string, a ...any) string {
+	msg := fmt.Sprintf(format, a...)
+	fmt.Fprintln(f.Output(), msg)
+	return msg
+}
+
+// failf prints to standard error a formatted error and usage message and
+// returns the error.
+func (f *FlagSet) failf(format string, a ...any) error {
+	msg := f.sprintf(format, a...)
+	f.usage()
+	return errors.New(msg)
+}
+
+// usage calls the Usage method for the flag set if one is specified,
+// or the appropriate default usage function otherwise.
+func (f *FlagSet) usage() {
+	if f.Usage == nil {
+		f.defaultUsage()
+	} else {
+		f.Usage()
+	}
+}
+
+// parseOne parses one flag. It reports whether a flag was seen.
+func (f *FlagSet) parseOne() (bool, error) {
+	if len(f.args) == 0 {
+		return false, nil
+	}
+	s := f.args[0]
+	if len(s) < 2 || s[0] != '-' {
+		return false, nil
+	}
+	numMinuses := 1
+	if s[1] == '-' {
+		numMinuses++
+		if len(s) == 2 { // "--" terminates the flags
+			f.args = f.args[1:]
+			return false, nil
+		}
+	}
+	name := s[numMinuses:]
+	if len(name) == 0 || name[0] == '-' || name[0] == '=' {
+		return false, f.failf("bad flag syntax: %s", s)
+	}
+
+	// it's a flag. does it have an argument?
+	f.args = f.args[1:]
+	hasValue := false
+	value := ""
+	for i := 1; i < len(name); i++ { // equals cannot be first
+		if name[i] == '=' {
+			value = name[i+1:]
+			hasValue = true
+			name = name[0:i]
+			break
+		}
+	}
+
+	flag, ok := f.formal[name]
+	if !ok {
+		if name == "help" || name == "h" { // special case for nice help message.
+			f.usage()
+			return false, ErrHelp
+		}
+		return false, f.failf("flag provided but not defined: -%s", name)
+	}
+
+	if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
+		if hasValue {
+			if err := fv.Set(value); err != nil {
+				return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
+			}
+		} else {
+			if err := fv.Set("true"); err != nil {
+				return false, f.failf("invalid boolean flag %s: %v", name, err)
+			}
+		}
+	} else {
+		// It must have a value, which might be the next argument.
+		if !hasValue && len(f.args) > 0 {
+			// value is the next arg
+			hasValue = true
+			value, f.args = f.args[0], f.args[1:]
+		}
+		if !hasValue {
+			return false, f.failf("flag needs an argument: -%s", name)
+		}
+		if err := flag.Value.Set(value); err != nil {
+			return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)
+		}
+	}
+	if f.actual == nil {
+		f.actual = make(map[string]*Flag)
+	}
+	f.actual[name] = flag
+	return true, nil
+}
+
+// Parse parses flag definitions from the argument list, which should not
+// include the command name. Must be called after all flags in the [FlagSet]
+// are defined and before flags are accessed by the program.
+// The return value will be [ErrHelp] if -help or -h were set but not defined.
+func (f *FlagSet) Parse(arguments []string) error {
+	f.parsed = true
+	f.args = arguments
+	for {
+		seen, err := f.parseOne()
+		if seen {
+			continue
+		}
+		if err == nil {
+			break
+		}
+		switch f.errorHandling {
+		case ContinueOnError:
+			return err
+		case ExitOnError:
+			if err == ErrHelp {
+				os.Exit(0)
+			}
+			os.Exit(2)
+		case PanicOnError:
+			panic(err)
+		}
+	}
+	return nil
+}
+
+// Parsed reports whether f.Parse has been called.
+func (f *FlagSet) Parsed() bool {
+	return f.parsed
+}
+
+// Parse parses the command-line flags from [os.Args][1:]. Must be called
+// after all flags are defined and before flags are accessed by the program.
+func Parse() {
+	// Ignore errors; CommandLine is set for ExitOnError.
+	CommandLine.Parse(os.Args[1:])
+}
+
+// Parsed reports whether the command-line flags have been parsed.
+func Parsed() bool {
+	return CommandLine.Parsed()
+}
+
+// CommandLine is the default set of command-line flags, parsed from [os.Args].
+// The top-level functions such as [BoolVar], [Arg], and so on are wrappers for the
+// methods of CommandLine.
+var CommandLine = NewFlagSet(os.Args[0], ExitOnError)
+
+func init() {
+	// Override generic FlagSet default Usage with call to global Usage.
+	// Note: This is not CommandLine.Usage = Usage,
+	// because we want any eventual call to use any updated value of Usage,
+	// not the value it has when this line is run.
+	CommandLine.Usage = commandLineUsage
+}
+
+func commandLineUsage() {
+	Usage()
+}
+
+// NewFlagSet returns a new, empty flag set with the specified name and
+// error handling property. If the name is not empty, it will be printed
+// in the default usage message and in error messages.
+func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
+	f := &FlagSet{
+		name:          name,
+		errorHandling: errorHandling,
+	}
+	f.Usage = f.defaultUsage
+	return f
+}
+
+// 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.
+func (f *FlagSet) Init(name string, errorHandling ErrorHandling) {
+	f.name = name
+	f.errorHandling = errorHandling
+}
diff --git a/src/flag/flag_test.go b/src/flag/flag_test.go
new file mode 100644
index 0000000..8e9ae31
--- /dev/null
+++ b/src/flag/flag_test.go
@@ -0,0 +1,858 @@
+// 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"
+	"sort"
+	"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 !sort.StringsAreSorted(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(os.Args[0], "-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")
+	})
+}