flag: UnquoteUsage checks for IsBoolFlag() method presence but doesn't actually call it

[ldemailly]

What version of Go are you using (go version)?

$ go version
go version go1.18.3 darwin/arm64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

not relevant (but apple silicon M1 macos)

What did you do?

Implemented a generic set of flag extension and part of the generic includes:

func (d *DynValue[T]) IsBoolFlag() bool {
	var v T
	switch any(v).(type) {
	case bool:
		return true
	default:
		return false
	}
}

Yet despite returning false for the non bool, flag.PrintDefaults through UnquoteUsage thinks all my types are booleans (don't require a value) and thus my usage output regressed from for instance

-  -loglevel value
+  -loglevel
         loglevel, one of [Debug Verbose Info Warning Error Critical Fatal]
 (default Info)

What did you expect to see?

value still output

What did you see instead?

nothing

ps: I can workaround by making the bool type special but it's unfortunate

[robpike]

Fix is easy, I think. Can you please provide a complete runnable example that shows the problem?

[ldemailly]

Thanks @robpike, here you go (simplified without all the generics etc):

package main

import "flag"

type NotBool struct{}

func (f *NotBool) IsBoolFlag() bool {
	return false
}

func (f *NotBool) Set(v string) error {
	return nil
}
func (f *NotBool) String() string {
	return ""
}

func main() {
	value := NotBool{}
	flag.Var(&value, "test", "test usage, line above should be -test value")
	flag.PrintDefaults()
}

output

% go run .
  -test
    	test usage, line above should be -test value

expected:

% go run .
  -test value
    	test usage, line above should be -test value

also on https://go.dev/play/p/URV2A7fhI4- and https://github.com/ldemailly/go-flag-bug/blob/main/bool_flag_issue.go

[gopherbot]

Change https://go.dev/cl/431102 mentions this issue: flag: test IsBoolFlag when creating the usage message

[ldemailly]

any chance to backport to 1.19?

[ianlancetaylor]

This doesn't seem to me to fit the backport requirements (https://github.com/golang/go/wiki/MinorReleases).

[ldemailly]

yes I guess it's not a big bug (even though the fix is really small and safe) - oh well... will wait then, thanks