BoolWithInverseFlag.String() panics when FlagStringer returns string without tab #2303

Summary

BoolWithInverseFlag.String() panics with slice bounds out of range [-1:] when the FlagStringer function returns a string that does not contain a tab character. Since FlagStringer is a public variable that users are encouraged to override, this panic is reachable through documented API usage.

Version: cli/v3 (commit ef08e2e)
Go: 1.22

Affected Code

// flag_bool_with_inverse.go:170-183
func (bif *BoolWithInverseFlag) String() string {
    out := FlagStringer(bif)

    i := strings.Index(out, "\t")

    prefix := "--"

    // single character flags are prefixed with `-` instead of `--`
    if len(bif.Name) == 1 {
        prefix = "-"
    }

    return fmt.Sprintf("%s[%s]%s%s", prefix, bif.inversePrefix(), bif.Name, out[i:])
}

When FlagStringer returns a string without \t, strings.Index returns -1, and out[-1:] panics. The default stringifyFlag can also return "" (no tab) when the flag doesn't satisfy DocGenerationFlag.

Reproduction

package main

import (
    "fmt"
    "github.com/urfave/cli/v3"
)

func main() {
    cli.FlagStringer = func(f cli.Flag) string {
        return "custom output without tab"
    }

    flag := &cli.BoolWithInverseFlag{
        BoolFlag: cli.BoolFlag{
            Name: "verbose",
        },
    }

    fmt.Println(flag.String())
    // panic: runtime error: slice bounds out of range [-1:]
}

Why This Matters

• BoolWithInverseFlag.String() is an exported method implementing fmt.Stringer — called implicitly by fmt.Print, logging, error messages
• FlagStringer is a documented public variable that users are encouraged to override for custom help output
• The panic is undocumented and cannot be anticipated by users setting a custom FlagStringer
• Note: FlagBase[T].String() in flag_impl.go:226 calls FlagStringer(f) directly without tab parsing, so it is not affected — only BoolWithInverseFlag has this issue

Suggested Fix

Guard against strings.Index returning -1:

func (bif *BoolWithInverseFlag) String() string {
    out := FlagStringer(bif)

    i := strings.Index(out, "\t")
    if i == -1 {
        return out
    }

    prefix := "--"
    if len(bif.Name) == 1 {
        prefix = "-"
    }

    return fmt.Sprintf("%s[%s]%s%s", prefix, bif.inversePrefix(), bif.Name, out[i:])
}