github.com/spf13/pflag v1.0.10: Parsing flags with NoOptDefVal assigns the no-option default and preserves subsequent space-separated arguments as positional parameters unless explicit equals delimiter syntax is used

# Flag Parsing with NoOptDefVal

## Search Results
`search_known_solution` returned no prior samples for `github.com/spf13/pflag`.

## Failure Mode
Silently with incorrect flag values and corrupted positional arguments: when passing `--flag value` to a flag configured with `NoOptDefVal`, parsing succeeds without returning any error, but the flag silently receives `NoOptDefVal` instead of `value`, and `value` leaks into positional arguments (`FlagSet.Args()`), displacing positional parameter ordering.

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module example.com/pflagsample

go 1.26.5

require github.com/spf13/pflag v1.0.10

github.com/spf13/pflag v1.0.10 h1:4EBh2KAYBwaONj6b2Ye1GiHfwjqyROoF4RwYO+vPwFk=
github.com/spf13/pflag v1.0.10/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=

package pflagsample

import (
	"reflect"
	"testing"

	"github.com/spf13/pflag"
)

func newTestFlagSet() (*pflag.FlagSet, *string) {
	fs := pflag.NewFlagSet("sample", pflag.ContinueOnError)
	var format string
	fs.StringVarP(&format, "format", "f", "text", "output format")
	fs.Lookup("format").NoOptDefVal = "json"
	return fs, &format
}

func TestNoOptDefValSpaceSeparatedArgumentLeakedToPositional(t *testing.T) {
	fs, format := newTestFlagSet()

	// Naive expectation: "--format", "yaml" consumes "yaml" as the flag value.
	// Actual pflag behavior: "--format" is treated as zero-arg, takes NoOptDefVal ("json"),
	// and "yaml" spills into positional arguments alongside "pos1".
	err := fs.Parse([]string{"--format", "yaml", "pos1"})
	if err != nil {
		t.Fatalf("unexpected parse error: %v", err)
	}

	if *format != "json" {
		t.Fatalf("expected format to be NoOptDefVal 'json', got '%s'", *format)
	}
	expectedArgs := []string{"yaml", "pos1"}
	if !reflect.DeepEqual(fs.Args(), expectedArgs) {
		t.Fatalf("expected args %v, got %v", expectedArgs, fs.Args())
	}
	if !fs.Changed("format") {
		t.Fatalf("expected format flag to be marked as changed")
	}
}

func TestNoOptDefValExplicitEqualsSyntax(t *testing.T) {
	fs, format := newTestFlagSet()

	// To pass a custom value to a flag with NoOptDefVal, caller must use "--flag=value".
	err := fs.Parse([]string{"--format=yaml", "pos1"})
	if err != nil {
		t.Fatalf("unexpected parse error: %v", err)
	}

	if *format != "yaml" {
		t.Fatalf("expected format 'yaml', got '%s'", *format)
	}
	expectedArgs := []string{"pos1"}
	if !reflect.DeepEqual(fs.Args(), expectedArgs) {
		t.Fatalf("expected args %v, got %v", expectedArgs, fs.Args())
	}
	if !fs.Changed("format") {
		t.Fatalf("expected format flag to be marked as changed")
	}
}

func TestNoOptDefValShorthandSpaceSeparatedArgument(t *testing.T) {
	fs, format := newTestFlagSet()

	// Shorthand "-f", "yaml" also treats "-f" as zero-arg, assigning NoOptDefVal ("json")
	// and leaving "yaml" and "pos1" in positional arguments.
	err := fs.Parse([]string{"-f", "yaml", "pos1"})
	if err != nil {
		t.Fatalf("unexpected parse error: %v", err)
	}

	if *format != "json" {
		t.Fatalf("expected format 'json', got '%s'", *format)
	}
	expectedArgs := []string{"yaml", "pos1"}
	if !reflect.DeepEqual(fs.Args(), expectedArgs) {
		t.Fatalf("expected args %v, got %v", expectedArgs, fs.Args())
	}
}

func TestNoOptDefValShorthandExplicitEqualsSyntax(t *testing.T) {
	fs, format := newTestFlagSet()

	// Shorthand with equals "-f=yaml" assigns custom value "yaml".
	err := fs.Parse([]string{"-f=yaml", "pos1"})
	if err != nil {
		t.Fatalf("unexpected parse error: %v", err)
	}

	if *format != "yaml" {
		t.Fatalf("expected format 'yaml', got '%s'", *format)
	}
	expectedArgs := []string{"pos1"}
	if !reflect.DeepEqual(fs.Args(), expectedArgs) {
		t.Fatalf("expected args %v, got %v", expectedArgs, fs.Args())
	}
}

func TestNoOptDefValOmittedFlagRetainsBaseDefault(t *testing.T) {
	fs, format := newTestFlagSet()

	// When flag is omitted entirely, it retains base default ("text"), NOT NoOptDefVal ("json").
	err := fs.Parse([]string{"pos1"})
	if err != nil {
		t.Fatalf("unexpected parse error: %v", err)
	}

	if *format != "text" {
		t.Fatalf("expected format 'text', got '%s'", *format)
	}
	expectedArgs := []string{"pos1"}
	if !reflect.DeepEqual(fs.Args(), expectedArgs) {
		t.Fatalf("expected args %v, got %v", expectedArgs, fs.Args())
	}
	if fs.Changed("format") {
		t.Fatalf("expected omitted flag to NOT be marked as changed")
	}
}

func TestNoOptDefValStandaloneFlagReceivesNoOptDefVal(t *testing.T) {
	fs, format := newTestFlagSet()

	// When passed alone without arguments, flag receives NoOptDefVal ("json").
	err := fs.Parse([]string{"--format"})
	if err != nil {
		t.Fatalf("unexpected parse error: %v", err)
	}

	if *format != "json" {
		t.Fatalf("expected format 'json', got '%s'", *format)
	}
	if len(fs.Args()) != 0 {
		t.Fatalf("expected empty args, got %v", fs.Args())
	}
	if !fs.Changed("format") {
		t.Fatalf("expected standalone flag to be marked as changed")
	}
}