fix: reject single-dash long flag names

errors.go

@@ -127,6 +127,22 @@ func (e *InvalidValueError) GetValue() string {
 	return e.value
 }
 
+// LongFlagSingleDashError is returned when a registered long flag name is
+// passed with a single leading dash (e.g. -name instead of --name).
+type LongFlagSingleDashError struct {
+	name string
+}
+
+// Error implements error.
+func (e *LongFlagSingleDashError) Error() string {
+	return fmt.Sprintf("flag %q must be specified as --%s (not -%s)", e.name, e.name, e.name)
+}
+
+// GetName returns the long flag name that was mistyped.
+func (e *LongFlagSingleDashError) GetName() string {
+	return e.name
+}
+
 // InvalidSyntaxError is the error returned when a bad flag name is passed on
 // the command line.
 type InvalidSyntaxError struct {

flag.go

@@ -1128,6 +1128,18 @@ func (f *FlagSet) parseShortArg(s string, args []string, fn parseFunc) (a []stri
 	a = args
 	shorthands := s[1:]
 
+	if len(shorthands) > 1 {
+		flagName := shorthands
+		if eq := strings.IndexByte(flagName, '='); eq >= 0 {
+			flagName = flagName[:eq]
+		}
+		if !isGotestShorthandFlag(flagName) {
+			if _, exists := f.formal[f.normalizeFlagName(flagName)]; exists {
+				return a, f.fail(&LongFlagSingleDashError{name: flagName})
+			}
+		}
+	}
+
 	// "shorthands" can be a series of shorthand letters of flags (e.g. "-vvv").
 	for len(shorthands) > 0 {
 		shorthands, a, err = f.parseSingleShortArg(shorthands, args, fn)

flag_test.go

@@ -6,6 +6,7 @@ package pflag
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -1461,3 +1462,38 @@ func TestVisitFlagOrder(t *testing.T) {
 		i++
 	})
 }
+
+func TestParseRejectsSingleDashLongFlag(t *testing.T) {
+	f := NewFlagSet("test", ContinueOnError)
+	var name string
+	f.StringVarP(&name, "name", "n", "", "name of configuration")
+
+	err := f.Parse([]string{"-name", "wrong"})
+	if err == nil {
+		t.Fatal("expected error for -name mistyped long flag")
+	}
+	var longDash *LongFlagSingleDashError
+	if !errors.As(err, &longDash) {
+		t.Fatalf("expected LongFlagSingleDashError, got %T: %v", err, err)
+	}
+	if name != "" {
+		t.Fatalf("expected name to stay empty, got %q", name)
+	}
+
+	err = f.Parse([]string{"-name=wrong"})
+	if err == nil {
+		t.Fatal("expected error for -name= mistyped long flag")
+	}
+	if !errors.As(err, &longDash) {
+		t.Fatalf("expected LongFlagSingleDashError, got %T: %v", err, err)
+	}
+
+	f = NewFlagSet("test", ContinueOnError)
+	f.StringVarP(&name, "name", "n", "", "name of configuration")
+	if err := f.Parse([]string{"-n", "value"}); err != nil {
+		t.Fatalf("expected -n value to parse: %v", err)
+	}
+	if name != "value" {
+		t.Fatalf("expected name=value, got %q", name)
+	}
+}