fix: treat "-vn=" in a short option group as an empty value

[kevin9327]

What type of PR is this?

• bug

What this PR does / why we need it:

#2297 (for #2293) made --name= set the flag to an empty string instead of reading the next argument as its value. The short option group path in parseFlags was not changed, so with UseShortOptionHandling the same = behaves differently once the flag is the last member of a group:

cmd := &cli.Command{
	UseShortOptionHandling: true,
	Flags: []cli.Flag{
		&cli.BoolFlag{Name: "v"},
		&cli.StringFlag{Name: "n", Value: "default"},
	},
}

args	n before	n after
app -n= positional	"", args [positional]	unchanged
app -vn= positional	"positional", args []	"", args [positional]
app -vn=	error flag needs an argument: n	""

• command_parse.go: the last flag of a short option group only reads the next argument when there was no = in the token, which is the same !valFromEqual check the single flag path already has.
• command_test.go: two subtests in TestFlagEqualsEmptyValue, one for -vn= positional and one for -vn= at the end of the args.

Which issue(s) this PR fixes:

None filed. I found this while reading the short option code next to the #2297 change and couldn't find an existing issue or PR about it.

Testing

• The two new subtests fail on main (the positional is consumed as the value; flag needs an argument: n) and pass with the change.
• make generate vet test gfmrun passes. make generate rewrites a few doc links in godoc-current.txt with my local Go 1.24, on main too, so I left that file alone.

Release Notes

With short option handling, `-vn=` in a short option group now sets `n` to an empty string, as `-n=` and `--name=` do, instead of reading the next argument as its value.

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Fixes command-line flag parsing for empty values in option groups.

The PR appears safe to merge.

Summary

The PR makes an explicit empty value on the last flag in a short-option group behave like an explicit empty value on a single flag.

• Adds tests for -vn= with and without a following positional argument.

_{Reviews (1) · Last reviewed commit: "fix: treat "-vn=" in a short option grou..."}

[coilysiren]

👍🏼 love it

[coilysiren]

still approved - just housekeeping

[Juneezee]

LGTM, thanks! This is the same !valFromEqual check the single flag path already has:

cli/command_parse.go

Line 176 in 335d72b

if flagVal == "" && !valFromEqual {

I checked the table in the description against main and this branch, and the two new subtests fail on main.

While testing this I found a related bug that is already on main: the value after = is passed to every bool flag in the group, not only to the last flag. With the command from the description, app -vn=foo fails with invalid value "foo" for flag -v: parse error:

cli/command_parse.go

Lines 238 to 245 in bb59353

	if fb, ok := sf.(boolFlag); ok && fb.IsBoolFlag() {
	fv := flagVal
	if index == lastIndex && flagVal == "" {
	fv = "true"
	}
	if fv == "" {
	fv = "true"
	}

It is in the same loop, so I pushed a fix with a test to this branch in 0cd8330.